It allows to implement tricks based on a knowledge whether the path
ever cam through a portal or not, and even something more advanced
based on the number of portals.
The main current objective is for strokes shading: stroke shader
uses Ray Portal BSDF to place ray to the center of the stroke and
point it in the direction of the surface it is generated for. This
gives stroke a single color which matches shading of the original
object. For this usecase to work the ray bounced from the original
surface should ignore the strokes, which is now possible by using
Portal Depth input and mixing with the Transparent BSDF. It also
helps to make shading look better when there are multiple stroke
layers.
A solution of using portal depth is chosen over a single flag due
to various factors:
- Last time we've looked into it it was a bit tricky to implement
as a flag due to us running out of bits.
- It feels to be more flexible solution, even though it is a bit
hard to come up with 100% compelling setup for it.
- It needs to be slightly different from the current "Is Foo"
flags, and be more "Is Portal Descendant" or something.
An extra uint16 is added to the state to count the portal depth,
but it is only allocated for scenes that use Ray Portal BSDF.
Portal BSDF still increments Transparent bounce, as it is required
to have some "limiting" factor so that ray does not get infinitely
move to different place of the scene.
Ref #125213
Pull Request: https://projects.blender.org/blender/blender/pulls/143107
dcae48d1d3