Cryptomatte is a standard to efficiently create mattes for compositing. The
renderer outputs the required render passes, which can then be used in the
compositor to create masks for specified objects. Unlike the Material and Object
Index passes, the objects to isolate are selected in compositing, and mattes
will be anti-aliased.
Cryptomatte was already available in Cycles this patch adds it to the EEVEE
render engine. Original specification can be found at
https://raw.githubusercontent.com/Psyop/Cryptomatte/master/specification/IDmattes_poster.pdf
Accurate mode
Following Cycles, there are two accuracy modes. The difference between the two
modes is the number of render samples they take into account to create the
render passes. When accurate mode is off the number of levels is used. When
accuracy mode is active, the number of render samples is used.
Deviation from standard
Cryptomatte specification is based on a path trace approach where samples and
coverage are calculated at the same time. In EEVEE a sample is an exact match on
top of a prepared depth buffer. Coverage is at that moment always 1. By sampling
multiple times the number of surface hits decides the actual surface coverage
for a matte per pixel.
Implementation Overview
When drawing to the cryptomatte GPU buffer the depth of the fragment is matched
to the active depth buffer. The hashes of each cryptomatte layer is written in
the GPU buffer. The exact layout depends on the active cryptomatte layers. The
GPU buffer is downloaded and integrated into an accumulation buffer (stored in
CPU RAM).
The accumulation buffer stores the hashes + weights for a number of levels,
layers per pixel. When a hash already exists the weight will be increased. When
the hash doesn't exists it will be added to the buffer.
After all the samples have been calculated the accumulation buffer is processed.
During this phase the total pixel weights of each layer is mapped to be in a
range between 0 and 1. The hashes are also sorted (highest weight first).
Blender Kernel now has a BKE_cryptomatte header that access to common
functions for cryptomatte. This will in the future be used by the API.
Known Limitations
- Alpha blended materials aren't supported. Alpha blended materials support in render passes needs research how to implement it in a maintainable way for any render pass.
Future work
This is a list of tasks that needs to be done for the same release that this
patch lands on (Blender 2.92)
- T82571 Add render tests.
- T82572 Documentation.
- T82573 Store hashes + Object names in the render result header.
- T82574 Use threading to increase performance in accumulation and post processing.
- T82575 Merge the cycles and EEVEE settings as they are identical.
- T82576 Add RNA to extract the cryptomatte hashes to use in python scripts.
