Due to space constraints, the entire fragment shader code isn't shown here. The full code is available from the GitHub repository. The fragment shader is implemented with five methods, one for each pass. The first pass renders the scene normally to the HDR texture. During this pass, the active framebuffer object is the one associated with the texture corresponding to HdrTex, so the output is sent directly to that texture.

The second pass reads from HdrTex, and writes out only pixels that have a luminance above the threshold value LumThresh. The value is (0,0,0,0) for pixels that have a brightness (luma) value below LumThresh. The output goes to the second framebuffer, which contains a much smaller texture (one-eighth the size of the original).

The third and fourth passes apply the basic Gaussian blur operation (refer to the Applying a Gaussian blur filter recipe in this chapter). In these passes, we ping-pong between BlurTex1 and BlurTex2, so we must be careful to swap the appropriate texture into the framebuffer.

In the fifth pass, we switch back to the default framebuffer, and read from HdrTex and BlurTex1. BlurTex1 contains the final blurred result from step four, and HdrTex contains the original render. We apply tone mapping to the results of HdrTex and add to BlurTex1. When pulling from BlurTex1, we are applying a linear filter, gaining additional blurring.