In step 1, we specify 625 shader invocations per work group, 25 in each dimension. Depending on the system on which the code is running, this could be changed to better match the hardware available.
The uniform image2D variables (step 2) are the input and output images. Note the binding locations indicated in the layout qualifier. These correspond to the image units specified in the glBindImageTexture call within the main OpenGL program. The input image should contain the rendered scene, and corresponds to the image texture bound to the FBO. The output image will receive the result of the filter. Also note the use of rgb8 as the format. This must be the same as the format used when creating the image using glTexStorage2D
