[Bf-committers] Proposition: Multilayer EXR support

[Jeroen Bakker]

  * Problem statement
Multilayer EXR files are used in 2 places.

1. to temporarily store the render buffers to reduce the memory 
footprint between the render pass and the composition pass.
2. to store the final output image as a multilayer exr.

the problem is that multilayer exr's are stored upside down. For 1 it is 
not a problem as the file is temporarily and used by blender renderer. 
for 2: the output image makes the file's not easilly usable. You first 
have to flip the image, before you can use it.

 * Expectation
the temporarily file must still be stored in the current form (upside 
down) for performance reasons.
the output file must not be stored upside down, but still have all 
render layers available. (open file with exrdisplay -C 
RenderLayer.Combined.R filename.exr to see problem)

Identifying the problem.
both accesses the same function in openexr_api.cpp and both are 
triggered from the pipeline.c (blender renderer), but both have 
different requirements:

 * Proposition/alternatives
there are several options how we can solve this:
1. leave all the same, but just before writing the multilayer exr output 
image, flip all render buffers. The implementation of the multilayer 
does not use the imbuf API, so will not effects other file formats and 
is easilly doable
2. update the imbuf api, to support multilayer images. more work, but 
more futureproof to possible other (professional, non existing) output 
formats
3. use the exr tiles to save the image upside down (is something in 
between solution 1 and 2)

I would have chosen for option 1 as it is not that much work, and I 
don't know what the requirements are for other professional output 
formats. Solution 3

Other issues in this area: the multilayer has is no default image/layer; 
all images are default black in other applications like nuke or exrdisplay.

What do you think what is the wise thing to do?
@Brecht, Campbell, is it OKE to patch pipeline as you are restructuring 
the renderer?

Greetings,
Jeroen Bakker.