I left a very simple HDR rendering thing I made last year in a half done state, and finally got around to finishing it today.
See here for the finer details : http://www.cis.rit.edu/people/faculty/ferwerda/publications/sig02_paper.pdf, speciafically equations 1,2 and 3 or 4 (you can comment one or the other out in the shader)
The up and down arrow keys control the brightness of the background. Shift and the same keys control smallest luminance that will be mapped to pure white (in the case of equation 4 only)
get the example here https://github.com/welford/opengl_samples it’s in sample 20