[Openexr-devel] Storing multiple layers in OpenEXR files

[Yves Poissant]

This is a sort of follow-up on this thread:
http://lists.gnu.org/archive/html/openexr-devel/2004-08/msg00007.html

At Hash Inc, to store multiple layers for the same render, we have adopted 
the convention:
  "layername.channelname"
Where channelnames are the conventional names like "R", "G", "B", "A", etc. 
This is simple and works well.

However, we also had a need for storing the particular load order for the 
layers/groups for compositing purposes. So we have added some metadata in 
the header. It goes like this:
For each layer, there is a metadata attribute named :
  "LayerO"
where "O" is a non 0-padded load order number starting with 1. This is a 
string attribute with value :
  "layername T R,G,B,..."
where "T" is the enum value of Imf::PixelType
and "R,G,B" is a comma separated list of channel names.

We also considered simply adding the load order number in front of the 
channel name such as "O.layername.channelname"  But the metadata way greatly 
simplifies the channel name searching since we don't have to scan through 
all of then first to parse and regroup them. We simply loop through the 
meatadata and load/regroup the channels as listed in the metadata. 
Furtheremore, it keeps the compatibility of the ad-hoc naming convention 
"layername.channelname" for other applications to load them.

We also use the single buffer names "R", "G", etc to store the fully 
composited final render result. This way, when our EXR files are loaded in 
another application, at least the final render result is displayable.

I'm interested to know if anyone else have developped a way to store layer 
order information in EXR files and if there is an actual need for that kind 
of information.

Yves Poissant
Hash Inc.