Cryptomattes - understand how they work!

[m_hinks]

Hi. I wanted to understand how Vay assigns ids to make cryptomattes.

We've had instances where we render a model and do post work using the masks, then render the model again (with a model change/material addition, not sure exactly on what was done) and all the masks have been re-id'd.

I guess Im trying to understand what we can change and masks not be affected, and what will cause Vray to reorder and id?

Thanks

[vladimir_krastev]

HI, thanks for posting.

then render the model again (with a model change/material addition, not sure exactly on what was done) and all the masks have been re-id'd

This is how the cryptomatte works in general. We have no control over this. Please use the MultiMatteElement if you want to keep your masks consistent but it is more time consuming to set up a scene with multimatte.

[m_hinks]

Hi Vladimir, So to confirm, been a long week - If any new objects or materials are added to a scene, then the ids assigned to the objects/materials depending on the crypto mask method chosen, will likely change all the masks?

[m_hinks]

Further to that, just thinking more. If in Nuke you select via obj name, would that get round it Or as you have to select the obj via a colour picker, the colour could change and therefore break the mask as well? So many questions!

[vladimir_krastev]

Hi, actually it keeps the same colours for the previously existing objects/material as long as you don't change material/object names. It assigns colours based on the categories on the dropdown:

Objects need proper names. For instance if you add a new box and leave it with it's default name the cryptomatte won't catch it.
I personally don't know of other way for masking with cryptomatte in Nuke other than using the colour picker.

[m_hinks]

Ok interesting so if you have it set to by node name, then you add some new geo in the scene, the old geo will keep the ids/mask colour, and the new geo will pick up new ids, so the masks shouldnt be damaged by adding new geo to a seen/render? Same if you use materials, once a material is made it gets an id and that doesnt change in new renders, even if a new material is made?

[vladimir_krastev]

Hi, this is true. As long as you use the corresponding category from the dropdown and don't change names midway you should be fine.
For example if you use layer name all object that are in one layer will be one color but if you change the name of the layer the color will change.

[m_hinks]

Bit of a mine field isnt it!!
So it sounds like the id's/colours are being assigned by names, either geo/layer/material. And once assigned they should stay locked unless you change the name? So any new materials/layers/geo will get new id's and not break what has been made before?

[vladimir_krastev]

Hi, the colors/masks are assigned based on the main categories in the dropdown (post #5). You can combine a few cryptomatte render elements and also combine with the multimatte (post #2).