In the first step, we render the scene to an HDR texture. In step 2, we compute the log-average luminance by retrieving the pixels from the texture and doing the computation on the CPU (OpenGL side).
In step 3, we render a single screen-filling quad to execute the fragment shader for each screen pixel. In the fragment shader, we retrieve the HDR value from the texture and apply the tone-mapping operator. There are two tunable variables in this calculation. The Exposure variable corresponds to the a term in the tone mapping operator, and the variable White corresponds to Lwhite. For the previous image, we used values of 0.35 and 0.928, respectively.
