docs: clarify exr2aces output format (AcademySoftwareFoundation#1963)

* docs: clarify exr2aces output format

Inspired by discussion in AcademySoftwareFoundation#1962.

Adds additional background to the ACES Image Container format produced by exr2aces.

Signed-off-by: Patrick Hulce <[email protected]>

* docs: update exr2aces.rst with recommended edits

Signed-off-by: Patrick Hulce <[email protected]>

* docs: add justified indentation

Signed-off-by: Patrick Hulce <[email protected]>

* Use inline link instead.

Co-authored-by: Cary Phillips <[email protected]>
Signed-off-by: Patrick Hulce <[email protected]>

* Remove link reference

Signed-off-by: Patrick Hulce <[email protected]>

---------

Signed-off-by: Patrick Hulce <[email protected]>
Co-authored-by: Cary Phillips <[email protected]>

Author: patrickhulce

website/bin/exr2aces.rst

@@ -15,26 +15,46 @@ Description
 Read an OpenEXR file from infile and save the contents
 in ACES image file outfile.
 
-The ACES image file format is a subset of the OpenEXR file
-format.  ACES image files are restricted as follows:
-
-- Images are stored as scanlines; tiles are not allowed.
-
-- Images contain three color channels, either R, G, B (red, green,
-  blue) or Y, RY, BY (luminance, sub-sampled chroma)
-
-- Images may optionally contain an alpha channel.
-
-- Only three compression types are allowed:
-
-  * NO_COMPRESSION (file is not compressed)
-
-  * PIZ_COMPRESSION (lossless)
-
-  * B44A_COMPRESSION (lossy)
-
-- The "chromaticities" header attribute must specify
-  the ACES RGB primaries and white point.
+The ACES Image Container File Format specified in SMPTE ST 2065-4 
+is a subset of the OpenEXR file format defined by the implementation 
+of the time (roughly OpenEXR 1.6). 
+
+This imposes considerable restrictions on what image data and metadata 
+can be transported from a modern OpenEXR file into an ACES Image 
+Container File, including (but not limited to) the following:
+
+- The version field must be either be 2 or 1026; a value of 2 implies 
+  attribute names cannot be longer than 31 characters in length, 
+  as was the case prior to OpenEXR 1.7.
+
+- Images must be stored as scanlines; tiles are not allowed.
+
+- Images must contain RGB and possibly an A channel if they are monoscopic; 
+  images representing stereo pairs would add another three (or four if A is 
+  present) channels. Other channels are not permitted, including channels 
+  that otherwise might represent a combination of a luminance channel and 
+  chromaticity channels.
+
+- The image must not be compressed.
+
+- The `chromaticities` attribute must specify the ACES RGB primaries and 
+  the ACES neutral as specified in SMPTE ST 2065-1.
+
+- The `acesImageContainerFlag` flat must be present and have the value 1.
+
+For the full set of restrictions, see `SMTPE ST 2065-4  <https://doi.org/10.5594/SMPTE.ST2065-4.2013>`_ (or any 
+superseding later version of that standard).
+
+In practice, facilities and productions often use the term "ACES file" 
+to mean OpenEXR files containing linear scene data expressed as 
+combinations of R, G and B whose chromaticities match those found in 
+ST 2065-1, where equal amounts of those primaries produce a color the 
+chromaticity of which matches that of the ACES neutral. The image data 
+might be compressed using one of the OpenEXR's library's built-in compression 
+functions, even though strict compliance with ST 2065-4 would forbid such 
+compression; the `acesImageContainerFile` flag might be missing; and the 
+`chromaticities` attribute might contain chromaticities that do not actually 
+match those found in ST 2065-1.
 
 Options:
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