Read depth values while stencil testing(same texture)

Question

I know that it is a very bad idea to read/write from/to the same texture/location, because this would result in undefined behaviour. But in my case, if depth testing is disabled and I read the depth values in a shader, is it ok to do the stencil testing at the same time as reading the depth values within the same texture?

In my opinion there should not be any problems, because i'm not reading the stencil buffer values in the shader. Or, could there by any hardware related problems when the texture is bound for reading in a shader and OpenGL uses it to do the stencil testing?

This texture is filled with depth/stencil values. I wan't to avoid some heavy BRDF lighting(Directional light) calculations on specific pixels(the sky).

Example code:

//Contains the depth/stencil texture(deferredDepthStencilTextureID).
//Two FBO's are sharing the same depth/stencil texture.
lightAccumulationFBO->bind();

glEnable(GL_STENCIL_TEST);
glStencilFunc(GL_EQUAL, 1, 0xFF);
//Disable writing to the stencil buffer, i.e. all the bits is write-protected.
glStencilMask(0x00);

glDisable(GL_DEPTH_TEST);
glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE); //Additive blending. Light accumulation.

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, deferredDepthStencilTextureID); //GL_DEPTH24_STENCIL8
//Bind other textures here...

shader.bind();
//Uniforms here...
postProcessQuad->renderQuad();
shader.unbind();

//Unbind all the textures here.
...

glDisable(GL_BLEND);
glEnable(GL_DEPTH_TEST);
glDisable(GL_STENCIL_TEST);

lightAccumulationFBO->unbind();

Do you only do stencil testing in this rendering pass, or do you use a stencil operation that modifies the stencil values as well? — Reto Koradi
I have updated my post with example code. I have not included the stencil operation in this rendering pass. I have also set the stencil mask like this: glStencilMask(0x00); — karl88

Nicol Bolas Nicol Bolas · Accepted Answer · 2016-04-22T04:49:07

But in my case, if depth testing is disabled and I read the depth values in a shader, is it ok to do the stencil testing at the same time as reading the depth values within the same texture?

No. Whether an operation is defined or not is based on the images attached to the FBO and read from. Not the components of said images. And no, write masking will not save you from undefined behavior.

So unless your stencil texture is separate from your depth texture, that's not going to work. And good luck finding hardware that will allow you to separate depth/stencil images.

Even with GL 4.5/NV/ARB_texture_barrier, the answer is still no. That functionality makes certain exceptions to the above rule, but only for operations that are due to fragment shader outputs. Stencil test operations are not fragment shader outputs, so they don't apply.

Read depth values while stencil testing(same texture)

1 Answers