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I have geometry that have attached some "ID" data. I have generated stencil buffer, that holds these IDs. Now, in second pass, I render different geometry, but this time I need to mask it with already existing stencil. Each part of geometry can have different "ID". Is there a way, how to control stencil buffer test value from shader?

I give an example:

  1. render geometry with ID = 1, writes 1 to stencil
  2. render geometry with ID = 2 if ID in stencil != 1, writes 2 to stencil
  3. do some other rendering with stencil turn off
  4. render bunch of particles at one go, but some of them have ID 1, some of them ID 2 -> I want to mask them, so 1 is rendered only if 1 is in stencil and 2 only if 2 is in stencil

I need this for OpenGL ES 2.0

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  • \$\begingroup\$ Wouldn't you have to pass the ID through to a pixel shader, then discard the pixel depending on the ID value? You can bind a pixel shader to depth pass, it just needs to return null if I remember (This is in DirectX 11, so it obviously maybe different but the concept might be feasible).. \$\endgroup\$ Commented Feb 18, 2018 at 20:47

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I don't know a way to do it with pure OpenGL ES 2.0, but it would be possible with the ARM_shader_framebuffer_fetch_depth_stencil extension.

You can not control the stencil test value from the shader, but it allows you to read the current value from the stencil using gl_LastFragStencilARM and then you can compare and discard as you wish.

There are also ways to attach stencil as a texture, so you can do something similar, but as far as I can find all need at least OpenGL ES 3.0.

If a majority of your target devices support this, you can use it and implement a fallback where the particles are split into groups by ID and rendered separately.

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