Cryptomatte & Render Layers: is it possible to recover Cryptomatte from a .PNG sequence?

Question

I will try to explain my noob-problem a little deeper.

I have a 3-days-render just done as a PNG sequence. I enabled Cryptomatte pass before rendering, but I eventually realized that the png sequence doesn't have Cryptomatte pass.

In other words, I need the Cryptomatte pass, but I can't find a way to not to re-render the whole scene, possibly in another more compositing-oriented format like Open Exr Multilayer, making it possible to load the actual render in the compositor.

Another thing: I know that my render is stored somewhere in cache, where Render Layers load the actual render into the compositor. The problem is that this 3-day-render I've just done, has crashed 2 or 3 times during the whole process, so the Render Layer in my Blender project seems to be empty.

Is there a way to recover my Cryptomatte pass, without re-rendering again the whole scene?

Why is it, every time I close Blender, my Render Layer in the compositor seems empty, losing the Cryptomatte pass?

Thanks all Guglielmo Ruben

Answer 1

Unfortunately there is no way to recover the Cryptomatte render passes, since .png isn't capable of storing them. For an explanation of the Cryptomatte format you can take a look at this answer. You will have to render it again and store the result in a Multi-layer OpenEXR.

Why is it, every time I close Blender, my Render Layer in the compositor seems empty, losing the Cryptomatte pass?

Blender caches the last rendered frame and its render passes for compositing, they're not stored in the .blend file though and therefore not persistently written to your hard drive. Once you close Blender that data is gone.