First my problem: I’m trying to render to multiple buffers in an FBO. I set multiple buffers using glDrawBuffers, and rendering to them using the appropriate gl_FragData. All good and well, but in my situation one of the buffers should be downsampled, by a quarter to be exact (w/2, h/2).
Of course, I can do this by blitting those specific buffers afterwards or I can simply do the downsampling on the CPU (current solution). But then I read about viewport arrays and found this quote in the ARB specification, which seems to be exactly what I want, without any extra conversions.
Additionally, when combined with multiple framebuffer attachments, it
allows a different viewport rectangle to be selected for each.
Of course, the specification never mentions afterwards how to do this or what they actually mean, multiple framebuffer attachments is quite generic. I only noticed I can set a specific viewport as an output of the geometry shader (output gl_ViewportIndex). So I could call the geometry twice for each viewport in the array. But as far as I understand, this will simply call the fragment shader with another viewport transformation applied, not one per target buffer. This of course makes not much sense for my usecase, also I can’t see how this could ever help to select a viewport per framebuffer attachment
For my situation it does not make much sense to add a geometry shader. And as viewport transform is only applied after the fragment shader, it does make sense to have a viewport per render target, which the previous quote seems to confirm. Is this actually possible, and if so, how would I accomplish this?
Oh, and I’ve tried the obvious already: resizing the renderbuffer of that target (let’s say I use GL_COLOR_ATTACHMENT1) to the downsampled version and setting index 1 of the viewport array to the according size. I ended up with a picture of the lower left quadrant of the image, essentially telling me the viewport was unchanged.
Viewport arrays can only be used with geometry shaders; without them, array index 0 will be used for all rendering.
Remember: the viewport transform happens before rasterization. Thus, if you want to transform a triangle by multiple viewports, you’re effectively asking the system to render that triangle multiple times. And the only way to do that is with a geometry shader that outputs the primitive multiple times.