What is cGBufferOffsets?

Hello! I’ve been toying around with shaders and I’m trying to understand these two functions:

vec4 GetScreenPos(vec4 clipPos)

{
    return vec4(
        clipPos.x * cGBufferOffsets.z + cGBufferOffsets.x * clipPos.w,
        clipPos.y * cGBufferOffsets.w + cGBufferOffsets.y * clipPos.w,
        0.0,
        clipPos.w);
}

vec2 GetScreenPosPreDiv(vec4 clipPos)
{
    return vec2(
        clipPos.x / clipPos.w * cGBufferOffsets.z + cGBufferOffsets.x,
        clipPos.y / clipPos.w * cGBufferOffsets.w + cGBufferOffsets.y);
}

Can somebody tell me what does cGBufferOffsets stands for? Also, any help on this “wierd” looking multiplactions and divisions by its components in both of the upper functions would be more than welcome :slight_smile:

Disclaimer: I’m not an expert and this is just from reading the shaders + renderpaths xmls. I might be wrong so I recommend waiting for someone other than me (^:

The link above talks a bit about these functions. Tl;dr they’re to access G-Buffers correctly.


A G-Buffer is used when deferred lighting is specified by the render path. cGBufferOffset a shader parameter used to know where to fetch data from the G-Buffer. It’s usually used via a call to these two functions above, which tell where on the rendertarget the shader is at and from where to sample data from the G-Buffers. See DeferredLight.glsl for an example usage.

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In this particular case, cGBufferOffsets is an (xy_offset, xy_scale) that transforms from viewport space [-1,1]x[-1,1] to UV space [0,1]x[0,1] (or arbitrary subregion of UV space). Used to sample all display textures, including but not limited to GBuffers.

Also, the following vectors in homogeneous coordinates are equivalent: (x, y, 0, w) ~ (x/w, y/w, 0, 1). First function makes first vector, second one makes second.

Thanks! Now it’s a bit clearer now.