OpenEXR File Layout
###################

Document Purpose and Audience
=============================

This document gives an overview of the layout of OpenEXR 2.0 image files
as byte sequences. It covers both single and multi-part formats, and how
deep data is handled.

The text assumes that the reader is familiar with OpenEXR terms such as
"channel", "attribute", "data window" or "chunk". For an explanation of
those terms see the `Technical Introduction to OpenEXR`_.

**Note:** This document does not define the OpenEXR file format.  OpenEXR is
defined as the file format that is read and written by the IlmImf open-source
C++ library. If this document and the IlmImf library disagree, then the library
takes precedence.

Backwards Compatibility and New or Changed Functionality
========================================================

OpenEXR Backwards Compatibility (1.7 and 2.0)
---------------------------------------------

OpenEXR 1.7 and earlier format files are fully supported by OpenEXR 2.0.
You can still use the 1.7 file format with the 2.0 library. If you use
the 2.0 format for single-part scan line image and tile image data, your
data will be stored in the same way as the 1.7 files. You can recompile
your 1.7 files to take advantage of the new format EXRs (multiple-part
files, and/or deep scan line and deep tile data).

New Features for OpenEXR 2.0: Multi-Part and Deep Data
------------------------------------------------------

The multi-part format is an extension of the OpenEXR 1.7 single-part
file format. In addition to supporting the OpenEXR 1.7 data storage (a
single scan line or tiled image), OpenEXR 2.0 files can be used to store
multiple views and/or deep data (deep scan line or deep tiles).

While you can continue to use the 1.7 format for files, these changes to
the file layout are required to support the new multi-part and deep data
features:

.. list-table::

   * - Feature
     - Description
     - See
   * - Version field
     - Bits 11 and 12 indicate whether the file contains deep data
       (bit 11), or more than one part (bit 12).
     - `Deep Data`_
   * - Header
     - To store more than one part in the file, you need to have a
       header for each part.
     - `Structure`_
   * - Header attributes
     - There are a number of attributes which have been defined to
       store data which is relevant to deep data and multi-part
       files. These include: name (one for each part), data type (you
       can have different types of data in different views), and the
       maximum number of samples to take in a deep data channel.
     - `Multi-Part and Deep Data Header Attributes`_
   * - Offset tables and chunks
     - To store more than one part in the file, you need to have an
       offset table for each part, and chunks for each part.

       The chunks must begin with a part number.
     - `Component Four: Offset Tables`_ and `Chunk Layout`_.
       
   * - Deep Data
     - Deep data has a unique storage format.
     - `Deep Data`_.

Basic Data Types
================

An OpenEXR file is a sequence of 8-bit bytes. Groups of bytes represent
basic objects such as integral numbers, floating-point numbers and text.
Those objects are grouped together to form compound objects such as
attributes or scan lines.

Integers
--------

Binary integral numbers with 8, 16, 32 or 64 bits are stored as 1, 2, 4
or 8 bytes. Integral numbers can be signed or unsigned. Signed numbers
are represented using two's complement. Integral numbers are
little-endian (that is, the least significant byte is closest to the
start of the file).

OpenEXR uses the following six integer data types:

.. list-table::

   * - name
     - signed
     - size in bytes
   * - ``unsigned char``
     - no
     - 1
   * - ``short``
     - yes
     - 2
   * - ``unsigned short``
     - no
     - 2
   * - ``int``
     - yes
     - 4
   * - ``unsigned int``
     - no
     - 4
   * - ``unsigned long``
     - no
     - 8

Floating-Point Numbers
----------------------

Binary floating-point numbers with 16, 32 or 64 bits are stored as 2, 4
or 8 bytes. The representation of 32-bit and 64-bit floating-point
numbers conforms to the IEEE 754 standard. The representation of 16-bit
floating-point numbers is analogous to IEEE 754, but with 5 exponent
bits and 10 bits for the fraction. The exponent bias is 15.
Floating-point numbers are little-endian (that is: the least significant
bits of the fraction are in the byte closest to the beginning of the
file, while the sign bit and the most significant bits of the exponent
are in the byte closest to the end of the file).

The following table lists the names and sizes of OpenEXR's
floating-point data types:

.. list-table::

   * - name
     - size in bytes
   * - ``half``
     - 2
   * - ``float``
     - 4
   * - ``double``
     - 8

Text
----

Text strings are represented as sequences of 1-byte characters of type
``char``. Depending on the context, either the end of a string is
indicated by a null character (0x00), or the length of the string is
indicated by an ``int`` that precedes the string.

Packing
-------

Data in an OpenEXR file are densely packed; the file contains no
"padding". For example, consider the following C struct:

.. code-block::

    struct SI
    {
        short s;
        int i;
    };

On most computers, the in-memory representation of an ``SI`` object
occupies 8 bytes: 2 bytes for ``s``, 2 padding bytes to ensure four-byte
alignment of ``i``, and 4 bytes for ``i``. In an OpenEXR file the same
object would consume only 6 bytes: 2 bytes for ``s`` and 4 bytes for ``i``.
The 2 padding bytes are not stored in the file.

File Layout
===========

High-Level Layout
-----------------

Depending on whether the pixels in an OpenEXR file are stored as scan
lines or as tiles, the file consists of the following components:

+-----------+-----------------------------------------------+-----------------------------------+
| Component | single-part file with...                      |  multi-part file:                 |
+===========+=======================+=======================+===================================+
|           | scan-lines:           | tiles:                |                                   |
+-----------+-----------------------+-----------------------+-----------------------------------+
| one       | ``magic number``      | ``magic number``      | ``magic number``                  |
+-----------+-----------------------+-----------------------+-----------------------------------+
| two       | ``version field``     | ``version field``     | ``version field``                 | 
+-----------+-----------------------+-----------------------+-----------------------------------+
| three     | ``header``            | ``header``            | * ``part 0 header``               | 
|           |                       |                       | * ``[part 1 header]``             | 
|           |                       |                       | * ``...``                         | 
|           |                       |                       | * ``[<empty header>]``            | 
+-----------+-----------------------+-----------------------+-----------------------------------+
| four      | ``line offset table`` | ``line offset table`` | * ``part 0 chunk offset table``   |
|           |                       |                       | * ``[part 0 chunk offset table]`` |
|           |                       |                       | * ``...``                         |
+-----------+-----------------------+-----------------------+-----------------------------------+
| five      | ``scan line blocks``  | ``tiles``             | ``chunks``                        |
+-----------+-----------------------+-----------------------+-----------------------------------+

It is the version field part which indicates whether the file is single
or multi-part and whether the file contains deep data. “Chunk” is a
general term to describe blocks of pixel data. A chunk can be a scan
line block, a tile or deep data (scan line or tile).

Deep data has no unique component structure of its own, but uses the
structure that the file would have if it did not have deep data in it.

Comparison between Single-Part and Multi-Part File Layouts
----------------------------------------------------------

Multi-part files have the same high level structure as single-part
OpenEXR files, except the header, offset table and chunk components can
have any number (two or more) parts. There must be the same number of
headers as offset tables, and they must be in the same order. In
addition, the header component of a multi-part file must end with a null
byte (0x00). In multi-part files, each chunk contains a field that
indicates which part's data it contains.

Components One and Two: Magic Number and Version Field
======================================================

Magic Number
------------

The magic number, of type ``int``, is always 20000630 (decimal). It allows
file readers to distinguish OpenEXR files from other files, since the
first four bytes of an OpenEXR file are always 0x76, 0x2f, 0x31 and
0x01.

Version Field
-------------

The version field, of type ``int``, is the four-byte group following the
magic number, and it is treated as two separate bit fields.

+---------------------+-------------------------------------------------------------------------------------------------------------------------+
| Byte/bit position   | Description and notes                                                                                                   |
+=====================+=========================================================================================================================+
| first byte          | The 8 least significant bits, they                                                                                      |
| (bits 0 through 7)  | contain the file format version number.                                                                                 |
|                     |                                                                                                                         |
|                     | The current OpenEXR version number is version 2.                                                                        |
+---------------------+---------------------------------------+---------------------------------------------------------------------------------+
| second, third and   | The 24 most significant bits, these are treated as a set of boolean flags.                                              |
| fourth bytes (bits  |                                                                                                                         |
| 8 through 31)       +-----------------------------+---------------------------------------+---------------------------------------------------+
|                     | Bit 9 (the single tile bit) | Indicates that this is a single-part  | If bit 9 is 1:                                    |
|                     | bit mask: 0x200             | file which is in tiled format.        | * this is a regular single-part image and the     |
|                     |                             |                                       | pixels are stored as tiles, and                   |
|                     |                             |                                       | * bits 11 and 12 must be 0.                       |
|                     |                             |                                       |                                                   |
|                     |                             |                                       | If bit 9 is 0, and bits 11 and 12 are also 0:     |
|                     |                             |                                       | the data is stored as regular single-part scan    |
|                     |                             |                                       | line file.                                        |
|                     |                             |                                       |                                                   |
|                     |                             |                                       | This bit is for backwards compatibility with      |
|                     |                             |                                       | older libraries: it is only set when there is     |
|                     |                             |                                       | one "normal" tiled image in the file.             |
|                     +-----------------------------+---------------------------------------+---------------------------------------------------+
|                     | Bit 10 (the long name bit)  | Indicates whether the file contains   |                                                   |
|                     | bit mask: 0x400             | “long names”.                         |                                                   |
|                     |                             |                                       | If bit 10 is 1, the maximum length is 255 bytes.  |
|                     |                             |                                       |                                                   |
|                     |                             |                                       | If bit 10 is 0, the maximum length of attribute   |
|                     |                             |                                       | names, attribute type names and channel names     |
|                     |                             |                                       | is 31 bytes.                                      |
|                     +-----------------------------+---------------------------------------+---------------------------------------------------+
|                     | Bit 11 (the non-image bit)  | Indicates whether the file contains   | If bit 11 is 1, there is at least one             |
|                     | bit mask: 0x800             | any “non-image parts” (deep data).    | part which is not a regular scan line             |
|                     |                             |                                       | image or regular tiled image (that is, it         |
|                     |                             |                                       | is a deep format).                                |
|                     |                             |                                       |                                                   |
|                     |                             |                                       | If bit 11 is 0, all parts are entirely            |
|                     |                             |                                       | single or multiple scan line or tiled images.     |
|                     |                             |                                       |                                                   |
|                     |                             |                                       | New in 2.0.                                       |
|                     +-----------------------------+---------------------------------------+---------------------------------------------------+
|                     | Bit 12 (the multipart bit)  | Indicates the file is a               | If bit 12 is 1:                                   |
|                     | bit mask: 0x1000            | multi-part file.                      | * the file does not contain exactly 1             |
|                     |                             |                                       | part and the 'end of header' byte                 |
|                     |                             |                                       | must be included at the end of each               |
|                     |                             |                                       | header part, and                                  |
|                     |                             |                                       | * the part number fields must be added            |
|                     |                             |                                       | to the chunks.                                    |
|                     |                             |                                       |                                                   |
|                     |                             |                                       | If bit 12 is 0, this is not a multi-part          |
|                     |                             |                                       | file and the 'end of header' byte and             |
|                     |                             |                                       | part number fields in chunks must                 |
|                     |                             |                                       | be omitted.                                       |
|                     |                             |                                       |                                                   |
|                     |                             |                                       | New in 2.0.                                       |
|                     +-----------------------------+---------------------------------------+---------------------------------------------------+
|                     | The remaining 19 flags in the version field are currently unused and should be set to 0.                                |
+---------------------+-------------------------------------------------------------------------------------------------------------------------+

Version field, valid values
~~~~~~~~~~~~~~~~~~~~~~~~~~~

All valid combinations of the version field bits are as follows:

.. list-table::
   :header-rows: 1
                 
   * - Description
     - Compatible with
     - bit 9
     - bit 11
     - bit 12
   * - Single-part scan line.
       One normal scan line image.
     - All versions of OpenEXR.
     - 0
     - 0
     - 0
   * - Single-part tile.
     - All versions of OpenEXR.
       One normal tiled image.
     - 1
     - 0
     - 0
   * - Multi-part (new in 2.0).
       Multiple normal images (scan line and/or tiled).
     - OpenEXR 2.0.
     - 0
     - 0
     - 1
   * - Single-part deep data (new in 2.0).
       One deep tile or deep scan line part.
     - OpenEXR 2.0.
     - 0
     - 1
     - 0
   * - Multi-part deep data (new in 2.0).
       Multiple parts (any combination of: tiles, scan lines,
       deep tiles and/or deep scan lines).
     - OpenEXR 2.0.
     - 0
     - 1
     - 1

**Note:** The version field bits define what capabilities must be
available in the software so it can handle the file, rather than the
exact format of the file. While the 9 and 11 bit settings must agree
with the type attributes of all parts, in OpenEXR 2.0 the data format of
each type is definitively set by the type attribute in that part's
header alone.

Component Three: Header
=======================

Structure
---------

Single-part file
~~~~~~~~~~~~~~~~

The header component of the single-part file holds a single header (for
single-part files).

Each header is a sequence of attributes ended by a null byte.

The file has the same structure as a 1.7 file. That is, the multi-part
bit (bit 12) must be 0, and the single null byte that signals the end of
the headers must be omitted. This structure also applies to single-part
deep data files.

Multi-part file (new in 2.0)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The header component of a multi-part file holds a set of headers, with a
separate header for each part (in multi-part files) and a null byte
signalling the end of the header component:

===================== =
``part 0 header``
``[part 1 header]``
``...``
``[<empty header>]``
===================== =

Each header is a sequence of attributes ended by a null byte.

The multipart bit (bit 12) must be set to 1, and the list of headers
must be followed by a single null byte (0x00) (that is, an empty
header).

Attribute Layout
----------------

The layout of an attribute is as follows:

===================== =
attribute type
attribute size
attribute value
===================== =

The ``attribute name`` and the ``attribute type`` are null-terminated
text strings. Excluding the null byte, the name and type must each be as
least 1 byte and at most :

*  31 bytes long (if bit 10 is set to 0), or
*  255 bytes long (if bit 10 is set to 1).

Both single-part and multi-part files use the same attribute types.

The ``attribute size``, of type ``int``, indicates the size (in bytes) of
the attribute value.

The layout of the ``attribute value`` depends on the attribute
type. The IlmImf library predefines several different attribute
types. Application programs can define and store additional attribute
types.

Header Attributes (All Files)
-----------------------------

The header of every OpenEXR file must contain at least the following
attributes:

.. list-table::
   :header-rows: 1

   * - attribute name
     - attribute type
   * - ``channels``
     - ``chlist``
   * - ``compression``
     - ``compression``
   * - ``dataWindow``
     - ``box2i``
   * - ``displayWindow``
     - ``box2i``
   * - ``lineOrder``
     - ``lineOrder``
   * - ``pixelAspectRatio``
     - ``float``
   * - ``screenWindowCenter``
     - ``v2f``
   * - ``screenWindowWidth``
     - ``float``

For descriptions of what these attributes are for, see the _`Technical
Introduction to OpenEXR`.

Tile Header Attribute
---------------------

This attributes is required in the header for all files which contain
one or more tiles:

.. list-table::
   :header-rows: 1

   * - attribute name
     - attribute type
     - notes
   * - ``tiles``
     - ``tiledesc``
     - Determines the size of the tiles and the number of resolution levels
       in the file. 

       **Note:** The IlmImf library ignores tile description attributes in
       scan line based files. The decision whether the file contains scan
       lines or tiles is based on the value of bit 9 in the file's version
       field, not on the presence of a tile description attribute.

Multi-View Header Attribute
---------------------------

This attribute can be used in the header for multi-part files:

.. list-table::
   :header-rows: 1

   * - attribute name
     - attribute type
     - notes
   * - ``view``
     - ``text``
     -

Multi-Part and Deep Data Header Attributes (New in 2.0)
-------------------------------------------------------

These attributes are required in the header for all multi-part and/or
deep data OpenEXR files.

.. list-table::
   :header-rows: 1

   * - attribute name
     - attribute type
     - notes
   * - ``name``
     - ``string``
     - Required if either the multipart bit (12) or the non-image bit (11) is set.
       
   * - ``type``
     - ``string``
     - Required if either the multipart bit (12) or the non-image bit (11) is set.
       Set to one of:
       * ``scanlineimage``
       * ``tiledimage``
       * ``deepscanline``, or
       * ``deeptile``
       **Note:** This value must agree with the version field's tile bit (9) and
       non-image (deep data) bit (11) settings.  
   * - ``version``
     - ``int``
     - This document describes version 1 data for all part types.
       version is required for deep data (deepscanline and deeptile) parts. If
       not specified for other parts, assume version=1.  
   * - ``chunkCount``
     - ``int``
     - Required if either the multipart bit (12) or the non-image bit (11) is set.
   * - ``tiles``
     - ``tileDesc``
     - Required for parts of type ``tiledimage`` and ``deeptile``.

Deep Data Header Attributes (New in 2.0)
----------------------------------------

These attributes are required in the header for all files which contain
deep data (deepscanline or deeptile):

.. list-table::
   :header-rows: 1

   * - attribute name
     - attribute type
     - notes
   * - ``tiles``
     - ``tileDesc``
     - Required for parts of type tiledimage and deeptile.
   * - ``maxSamplesPerPixel``
     - ``int``
     - Required for deep data (deepscanline and deeptile) parts.
       **Note:** Since the value of ``maxSamplesPerPixel`` maybe be unknown at
       the time of opening the file, the value “\ *-1*\ ” is written to the file
       to indicate an unknown value. When the file is closed, this will be
       overwritten with the correct value. If file writing does not complete
       correctly due to an error, the value *-1* will remain. In this case, the
       value must be derived by decoding each chunk in the part. 
   * - ``version``
     - ``int``
     - Should be set to ``1``. It will be changed if the format is updated. 
   * - ``type``
     - ``string``
     - Must be set to ``deepscanline`` or ``deeptile``.

For information about channel layout and a list of reserved channel names, see
the _`Technical Introduction to OpenEXR` document, _`Channel Names` section.

Component Four: Offset Tables
=============================

Offset Tables
-------------

An offset table allows random access to pixel data chunks. An offset
table is a sequence of offsets, with one offset per chunk. Each offset
(of type ``unsigned long``) indicates the distance, in bytes, between the
start of the file and the start of the chunk.

Chunks can be of any of the four data types.

Offset Table Size
-----------------

The number of entries in an offset table is defined in one of two ways:

1. If the multipart (12) bit is unset and the chunkCount is not present,
   the number of entries in the chunk table is computed using the
   dataWindow and tileDesc attributes and the compression format.
2. If the multipart (12) bit is set, the header must contain a
   chunkCount attribute (which indicates the size of the table and the
   number of chunks).

Scan Lines
----------

For scan line blocks, the line offset table is a sequence of scan line
offsets, with one offset per scan line block. In the table, scan line
offsets are ordered according to increasing scan line y coordinates.

Tiles
-----

For tiles, the offset table is a sequence of tile offsets, one offset
per tile. In the table, scan line offsets are sorted the same way as
tiles in ``INCREASING_Y`` order.

Multi-part (New in 2.0)
-----------------------

For multi-part files, each part defined in the header component has a
corresponding chunk offset table.

Component Five: Pixel data
==========================

Chunk Layout (New in 2.0)
-------------------------

A “chunk” is a general term for a pixel data block. The scan line and
tile images have the same format that they did in OpenEXR 1.7. OpenEXR
2.0 introduces two new types (deep scan line and deep tile).

The layout of each chunk is as follows:

============================================== =
``[part number]`` (if multi-part bit is set)
``chunk data``
============================================== =

The ``part number`` (of type ``unsigned long``) is only present in
multi-part files. It indicates which part this chunk belongs to. 0
indicates the chunk belongs to the part defined by the first header and
the first chunk offset table. The part number is omitted if the
multi-part bit (12) is not set (this saves space and enforces backwards
compatibility to software which does not support multi-part files).

The ``chunk data`` is dependent on the type attribute - but (other than
the part number) has the same structure as a single-part file of the
same format:

.. list-table::
   :header-rows: 1

   * - part type
     - type attribute
     - notes
   * - scan line
     - indicated by a type attribute of “scanlineimage”
     - Each chunk stores a scan line block, with the minimum y coordinate of the
       scan line(s) within the chunk.
       See `Regular scan line image block layout`.
   * - tiled
     - indicated by a type attribute of “tiledimage”
     - See _`Regular image tile layout`.
   * - deep scan line
     - indicated by a type attribute of “deepscanline”
     - See _`Deep scan line layout`.
   * - deep tile
     - indicated by a type attribute of “deeptile”
     - See _`Deep tiled layout`.

For more information about data types, see XXX.

Regular Scan Line Blocks
------------------------

For scan line images and deep scan line images, one or more scan lines
may be stored together as a scan line block. The number of scan lines
per block depends on how the pixel data are compressed:

.. list-table::

   * - ``NO_COMPRESSION``
     - 1 
   * - ``RLE_COMPRESSION``
     - 1 
   * - ``ZIPS_COMPRESSION``
     - 1 
   * - ``ZIP_COMPRESSION``
     - 16
   * - ``PIZ_COMPRESSION``
     - 32
   * - ``PXR24_COMPRESSION``
     - 16
   * - ``B44_COMPRESSION``
     - 32
   * - ``B44A_COMPRESSION``
     - 32

Each scan line block has a y coordinate of type ``int``. The block's y
coordinate is equal to the pixel space y coordinate of the top scan line
in the block. The top scan line block in the image is aligned with the
top edge of the data window (that is, the y coordinate of the top scan
line block is equal to the data window's minimum y).

If the height of the image's data window is not a multiple of the number
of scan lines per block, then the block that contains the bottom scan
line contains fewer scan lines than the other blocks.

Regular scan line image block layout
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The layout of a regular image scan line block is as follows:

=================== =
y coordinate
pixel data size
pixel data
=================== =

The ``pixel data size``, of type ``int``, indicates the number of bytes
occupied by the actual pixel data.

Within the ``pixel data``, scan lines are stored top to bottom. Each
scan line is contiguous, and within a scan line the data for each
channel are contiguous. Channels are stored in alphabetical order,
according to channel names. Within a channel, pixels are stored left to
right.

Compressed data
~~~~~~~~~~~~~~~

If the file's compression method is ``NO_COMPRESSION``, then the original,
uncompressed pixel data are stored directly in the file. Otherwise, the
uncompressed pixels are fed to the appropriate compressor, and either
the compressed or the uncompressed data are stored in the file,
whichever is smaller.

The layout of the compressed data depends on which compression method
was applied. The compressed formats are not described here. For
information on the compressed data formats, see the source code for the
IlmImf library.

Regular ImageTiles
------------------

Regular image tile layout
~~~~~~~~~~~~~~~~~~~~~~~~~

The layout of a regular image tile is as follows:

=================== =
tile coordinates
pixel data size
pixel data
=================== =

The ``tile coordinates``, a sequence of four ``int`'s (``tileX``, ``tileY``,
``levelX``, ``levelY``) indicates the tile's position and resolution level. The
``pixel data size``, of type ``int``, indicates the number of bytes
occupied by the pixel data.

The ``pixel data`` in a tile are laid out in the same way as in a scan
line block, but the length of the scan lines is equal to the width of
the tile, and the number of scan lines is equal to the height of the
tile.

If the width of a resolution level is not a multiple of the file's tile
width, then the tiles at the right edge of that resolution level have
shorter scan lines. Similarly, if the height of a resolution level is
not a multiple of the file's tile height, then tiles at the bottom edge
of the resolution level have fewer scan lines.

Deep Data (New in 2.0)
----------------------

Deep images store an arbitrarily long list of data at each pixel
location (each pixel contains a list of samples, and each sample
contains a fixed number of channels).

Deep scan line layout
~~~~~~~~~~~~~~~~~~~~~

Deep scan line images are indicated by a type attribute of
“deepscanline”. Each chunk of deep scan line data is a single scan line
of data. The data in each chunk is laid out as follows:

======================================== =
[part number] (if multipart bit is set) 
y coordinate                            
packed size of pixel offset table       
packed size of sample data              
unpacked size of sample data            
compressed pixel offset table           
compressed sample data                  
======================================== =

The ``unpacked size of the sample data`` (an ``unsigned long``) is the
size of the deep sample data once it is unpacked. It is necessary to
specify the unpacked size since the data may be arbitrarily large (so
generally cannot otherwise be determined without decompressing the data
first).

Deep tiled layout
~~~~~~~~~~~~~~~~~

Tiled images are indicated by a type attribute of “deeptile”. Each chunk
of deep tile data is a single tile. The data in each chunk is laid out
as follows:

=========================================== =
[part number] (if multipart bit is set) 
tile coordinates                        
packed size of pixel offset table       
packed size of sample data              
unpacked size of sample data            
compressed pixel offset table           
compressed sample data                  
=========================================== =

The ``unpacked size of the sample data`` (an ``unsigned long``) is the
size of the deep data once it is unpacked. It is necessary to specify
the unpacked size since the data may be arbitrarily large (so generally
cannot otherwise be determined without decompressing the data first).

The ``pixel offset table`` is a list of ``int``, one for each column within
the dataWindow. Each entry ``n`` in the table indicates the total number of
samples required to store the pixel in ``n`` as well as all pixels to the left
of it. Thus, the first samples stored in each channel of the pixel data are for
the pixel in column 0, which contains ``table[1]`` samples. Each channel
contains ``table[width-1]`` samples in total.

Unpacked deep data chunks
~~~~~~~~~~~~~~~~~~~~~~~~~

When decompressed, the unpacked chunk consists of the channel data
stored in a non-interleaved fashion:

=========================================== =
pixel sample data for channel 0    
pixel sample data for channel 1    
pixel sample data for channel ...  
pixel sample data for channel n    
=========================================== =


**Exception:** For ``ZIP_COMPRESSION`` only there will be up to 16
scanlines in the packed sample data block:

================================================= =
pixel sample data for channel 0 for scanline 0   
pixel sample data for channel 1 for scanline 0   
pixel sample data for channel ... for scanline 0 
pixel sample data for channel n for scanline 0   
pixel sample data for channel 0 for scanline 1   
pixel sample data for channel 1 for scanline 1   
pixel sample data for channel ... for scanline 1 
pixel sample data for channel n for scanline 1
...                                              
================================================= =


Deep data compression
~~~~~~~~~~~~~~~~~~~~~

The following compression schemes are the only ones permitted for deep
data:

==================== ==
``NO_COMPRESSION``   1 
``RLE_COMPRESSION``  1 
``ZIPS_COMPRESSION`` 1 
``ZIP_COMPRESSION``  16
==================== ==

Predefined Attribute Types
==========================

The IlmImf library predefines the following attribute types:

+--------------------+----------------------------------------------------------------+
| type name          | data                                                           |
+====================+================================================================+
| ``box2i``          | Four ``int``\ 's: ``xMin``, ``yMin``, ``xMax``, ``yMax``       |
+--------------------+----------------------------------------------------------------+
| ``box2f``          | Four ``float``\ 's: ``xMin``, ``yMin``, ``xMax``, ``yMax``     |
+--------------------+----------------------------------------------------------------+
| ``chlist``         | A sequence of channels followed by a null byte (``0x00``).     |
|                    | Channel layout:                                                |
|                    +----------------+-----------------------------------------------+
|                    | name           | zero-terminated string, from 1 to 255 bytes   |
|                    |                | long                                          |
|                    +----------------+-----------------------------------------------+
|                    | ``pixel type`` | ``int``, possible values are:                 |
|                    |                |                                               |
|                    |                | * ``UINT`` = 0                                |
|                    |                | * ``HALF`` = 1                                |
|                    |                | * ``FLOAT`` = 2                               |
|                    |                |                                               |
|                    +----------------+-----------------------------------------------+
|                    | ``pLinear``    | ``usigned char``, possible values are 0 and 1 |
|                    +----------------+-----------------------------------------------+
|                    | ``reserved``   | three ``char``, should be zero                |
|                    +----------------+-----------------------------------------------+
|                    | ``xSampling``  | ``int``                                       |
|                    +----------------+-----------------------------------------------+
|                    | ``ySampling``  | ``int``                                       |
+--------------------+----------------+-----------------------------------------------+
| ``chromaticities`` | Eight ``float``: ``redX``, ``redY``, ``greenX``,               |
|                    | ``greenY``, ``blueX``, ``blueY``, ``whiteX``, ``whiteY``       |
+--------------------+----------------------------------------------------------------+
| ``compression``    | ``unsigned char``, possible values are:                        |
|                    |                                                                |
|                    | * ``NO_COMPRESSION`` = 0                                       |
|                    | * ``RLE_COMPRESSION`` = 1                                      |
|                    | * ``ZIPS_COMPRESSION`` = 2                                     |
|                    | * ``ZIP_COMPRESSION`` = 3                                      |
|                    | * ``PIZ_COMPRESSION`` = 4                                      |
|                    | * ``PXR24_COMPRESSION`` = 5                                    |
|                    | * ``B44_COMPRESSION`` = 6                                      |
|                    | * ``B44A_COMPRESSION`` = 7                                     |
|                    |                                                                |
+--------------------+----------------------------------------------------------------+
| ``double``         | ``double``                                                     |
+--------------------+----------------------------------------------------------------+
| ``envmap``         | ``unsigned char``, possible values are:                        |
|                    | * ``ENVMAP_LATLONG`` = 0                                       |
|                    | * ``ENVMAP_CUBE`` = 1                                          |
+--------------------+----------------------------------------------------------------+
| ``float``          | ``float``                                                      |
+--------------------+----------------------------------------------------------------+
| ``int``            | ``int``                                                        |
+--------------------+----------------------------------------------------------------+
| ``keycode``        | Seven ``int``\ 's: ``filmMfcCode``, ``filmType``, ``prefix``,  |
|                    | ``count``,``perfOffset``, ``perfsPerFrame``, ``perfsPerCount`` |
+--------------------+----------------------------------------------------------------+
| ``lineOrder``      | ``unsigned char``, possible values are:                        |
|                    | * ``INCREASING_Y`` = 0                                         |
|                    | * ``DECREASING_Y`` = 1                                         |
|                    | * ``RANDOM_Y`` = 2                                             |
|                    |                                                                |
+--------------------+----------------------------------------------------------------+
| ``m33f``           | 9 ``float``\ 's                                                |
+--------------------+----------------------------------------------------------------+
| ``m44f``           | 16 ``float``\ 's                                               |
+--------------------+----------------------------------------------------------------+
| ``preview``        | Two ``unsigned int``\ 's, width and height, followed by        |
|                    | 4×width×height ``unsigned char``\ 's of pixel data.            |
|                    | Scan lines are stored top to bottom; within a scan line        |
|                    | pixels are stored from left to right. A pixel consists of      |
|                    | four ``unsigned char``\ 's, ``R``, ``G``, ``B``, ``A``.        |
+--------------------+----------------------------------------------------------------+
| ``rational``       | An ``int``, followed by an ``unsigned int``.                   |
+--------------------+----------------------------------------------------------------+
| ``string``         | String length, of type ``int``, followed by a sequence of      |
|                    | ``char``\ 's.                                                  |
+--------------------+----------------------------------------------------------------+
| ``stringvector``   | A sequence of zero or more text strings. Each string is        | 
|                    | represented as a string length, of type ``int``, followed by a |
|                    | sequence of ``chars``. The number of strings can be inferred   |
|                    | from the total attribute size                                  |
|                    | (see the `Attribute Layout`_ section.                          |
+--------------------+----------------------------------------------------------------+
| ``tiledesc``       | Two ``unsigned int``\ 's: ``xSize``, ``ySize``, followed       |
|                    | by ``mode``, of type ``unsigned char``, where                  |
|                    |                                                                |
|                    |     mode = levelMode + roundingMode×16                         |
|                    |                                                                |
|                    | Possible values for ``levelMode``:                             |
|                    | * ``ONE_LEVEL`` = 0                                            |
|                    | * ``MIPMAP_LEVELS`` = 1                                        |
|                    | * ``RIPMAP_LEVELS`` = 2                                        |
|                    |                                                                |
|                    | Possible values for ``roundingMode``:                          |
|                    | * ``ROUND_DOWN`` = 0                                           |
|                    | * ``ROUND_UP`` = 1                                             |
|                    |                                                                |
+--------------------+----------------------------------------------------------------+
| ``timecode``       | Two ``unsigned int``\ 's: ``timeAndFlags``, ``userData``.      |
+--------------------+----------------------------------------------------------------+
| ``v2i``            | Two ``int``\ 's                                                |
+--------------------+----------------------------------------------------------------+
| ``v2f``            | Two ``float``\ 's                                              |
+--------------------+----------------------------------------------------------------+
| ``v3i``            | Three ``int``\ 's.                                             |
+--------------------+----------------------------------------------------------------+
| ``v3f``            | Three ``float``\ 's.                                           |
+--------------------+----------------------------------------------------------------+


Sample File
===========

The following is an annotated byte-by-byte listing of a complete OpenEXR
file. The file contains a scan-line based image with four by three
pixels. The image has two channels: G, of type ``HALF``, and Z, of type
``FLOAT``. The pixel data are not compressed. The entire file is 415 bytes
long.

The first line of text in each of the gray boxes below lists up to 16
bytes of the file in hexadecimal notation. The second line in each box
shows how the bytes are grouped into integers, floating-point numbers
and text strings. The third and fourth lines indicate how those basic
objects form compound objects such as attributes or the line offset
table.

.. code-block::
       
     76   2f   31   01   02   00   00   00   63   68   61   6e   6e   65   6c   73
          20000630     |         2         |  c    h    a    n    n    e    l    s
        magic number   |  version, flags   | attribute name
                       |                   | start of header
.. code-block::
    
     00   63   68   6c   69   73   74   00   25   00   00   00   47   00   01   00
     \0 |  c    h    l    i    s    t   \0 |        37         |  G   \0 | HALF
        | attribute type                   | attribute size    | attribute value
    
.. code-block::
    
     00   00   00   00   00   00   01   00   00   00   01   00   00   00   5a   00
             |  0 |      0       |         1         |         1         |  Z   \0 |
    
    
.. code-block::
    
     02   00   00   00   00   00   00   00   01   00   00   00   01   00   00   00
     FLOAT             |  0 |      0       |         1         |         1         |
                                                                                   |
.. code-block::
    
     00   63   6f   6d   70   72   65   73   73   69   6f   6e   00   63   6f   6d
     \0 |  c    o    m    p    r    e    s    s    i    o    n   \0 |  c    o    m
        | attribute name                                            | attribute type
    
.. code-block::
    
     70   72   65   73   73   69   6f   6e   00   01   00   00   00   00   64   61
      p    r    e    s    s    i    o    n   \0 |         1         | NONE| d    a
                                                | attribute size    |value|
    
.. code-block::
    
     74   61   57   69   6e   64   6f   77   00   62   6f   78   32   69   00   10
      t    a    W    i    n    d    o    w   \0 |  b    o    x    2    i   \0 |
     attribute name                             | attribute type              |
    
.. code-block::
    
     00   00   00   00   00   00   00   00   00   00   00   03   00   00   00   02
        16        |         0         |         0         |         3         |
    attribute size| attribute value
    
.. code-block::
    
     00   00   00   64   69   73   70   6c   61   79   57   69   6e   64   6f   77
        2         |  d    i    s    p    l    a    y    W    i    n    d    o    w
                  | attribute name
    
.. code-block::
    
     00   62   6f   78   32   69   00   10   00   00   00   00   00   00   00   00
     \0 |  b    o    x    2    i   \0 |        16         |         0         |
        | attribute type              | attribute size    | attribute value
    
    
.. code-block::
    
     00   00   00   03   00   00   00   02   00   00   00   6c   69   6e   65   4f
        0         |         3         |         2         |  l    i    n    e    O
                                                          | attribute name
    
.. code-block::
    
     72   64   65   72   00   6c   69   6e   65   4f   72   64   65   72   00   01
      r    d    e    r   \0 |  l    i    n    e    O    r    d    e    r   \0 |
                            | attribute type                                  |
    
.. code-block::
    
     00   00   00   00   70   69   78   65   6c   41   73   70   65   63   74   52
        1         |INCY | p    i    x    e    l    A    s    p    e    c    t    R
    attribute size|value| attribute name
    
.. code-block::
    
     61   74   69   6f   00   66   6c   6f   61   74   00   04   00   00   00   00
      a    t    i    o   \0  | f    l    o    a    t   \0 |         4         |
                             | attribute type             | attribute size    |
    
    
.. code-block::
       
     00   80   3f   73   63   72   65   65   6e   57   69   6e   64   6f   77   43
       1.0         | s    c    r    e    e    n    W    i    n    d    o    w    C
    attribute value| attribute name
    
    
.. code-block::
    
     65   6e   74   65   72   00   76   32   66   00   08   00   00   00   00   00
      e    n    t    e    r   \0 |  v    2    f   \0 |         8         |
                                 | attribute type    | attribute size    |
    
    
.. code-block::
       
     00   00   00   00   00   00   73   63   72   65   65   6e   57   69   6e   64
    0.0      |         0.0       |  s    c    r    e    e    n    W    i    n    d
    attribute value              | attribute name
    
    
.. code-block::
       
     6f   77   57   69   64   74   68   00   66   6c   6f   61   74   00   04   00
      o    w    W    i    d    t    h   \0 |  f    l    o    a    t   \0 |
                                           | attribute type              |
    
    
.. code-block::
       
     00   00   00   00   80   3f   00   3f   01   00   00   00   00   00   00   5f
    4        |        1.0       | \0  |                  319                  |
    size     | attribute value  |     |        offset of scan line 0          |
                        end of header | start of scan line offset table
    
.. code-block::
       
     01   00   00   00   00   00   00   7f   01   00   00   00   00   00   00   00
                 351                   |                 383                  |
       offset of scan line 1           |       offset of scan line 2          |
                                                end of scan line offset table |
    
.. code-block::
       
     00   00   00   18   00   00   00   00   00   54   29   d5   35   e8   2d   5c
        0         |         24        |  0.000  |  0.042  |  0.365  |  0.092  |
        y         |  pixel data size  |  pixel data for G channel             |
      scan line 0
    
.. code-block::
       
     28   81   3a   cf   e1   34   3e   8b   0b   bb   3d   89   74   f9   3e   01
    0.000985395   |   0.176643        |   0.0913306       |   0.487217        |
    pixel data for Z channel                                                  |
                                                                              |
.. code-block::
       
     00   00   00   18   00   00   00   37   38   76   33   74   3b   73   38   7f
        1         |         24        |  0.527  |  0.233  |  0.932  |  0.556  |
        y         |  pixel data size  |  pixel data for G channel             |
      scan line 1
    
.. code-block::
       
     ab   e8   3e   8a   cf   54   3f   5b   6c   11   3f   20   35   50   3d   02
    0.454433      |   0.831292        |   0.56806         |   0.0508319       |
    pixel data for Z channel                                                  |
                                                                              |
    
.. code-block::
       
     00   00   00   18   00   00   00   23   3a   0a   34   02   3b   5d   3b   38
        2         |         24        |  0.767  |  0.252  |  0.876  |  0.920  |
        y         |  pixel data size  |  pixel data for G channel             |
      scan line 2
    
.. code-block::
       
     f3   9a   3c   4d   ad   98   3e   1c   14   08   3f   4c   f3   03   3f
    0.0189148     |   0.298197        |   0.531557        |   0.515431       
    pixel data for Z channel                                                 
                                                                  end of file
