Efficient downsampling for post-processing effects in opengl 3.3

Question

I understand the idea behind the bloom/glow effect: we downsample the texture to keep our convolution kernels small. Now that I am trying to implement it, I am not quite sure which road I should take.

My first idea was to use glGenerateMipMap to do the downsampling. However, I cannot tell it to stop after, say, 4 steps. It's a bit of a black box for me, and for all I know, it may generate 10 images to sample my screen from 1024*768 down to 1*1. Maybe these last steps are cheap because everything is so small already, but maybe they are not.

I googled around and found that many people were relying on FBOs rather than glGenerateMipMap. I am familiar with FBOs since I use deferred lighting. My second idea is to simply render a 'quad' with a linear sampler into a smaller texture. I would do that four times in a row, halving width and height each time. However, I found that some people preferred using their own fragment shader for downsampling rather than relying on GL_LINEAR and I wonder why; maybe it is faster?

What would be a way to quickly downsample my full-screen texture 4 times in a row, keeping each version? I have no need for fancy edge-preserving sampling algorithms as I am going to blur everything anyway.

Answer 1

we downsample the texture to keep our convolution kernels small.

Or you simply render the bloom/glow layer at a smaller resolution in the first place. This saves both fillrate and you don't have to minify afterwards.

My second idea is to simply render a 'quad' with a linear sampler into a smaller texture.

This is no downsampling it all. It's linear interpolation between sampling points and may create artifacts.