20160721 - Why Motion Blur Trivially Breaks Tone-Mapping - Part 1

Some food for visual thought ...

You have a scene with a bunch of High Dynamic Range (HDR) light sources, or bright secondary reflections. Average Picture Level (APL) of the scene as displayed is relatively low, which is standard practice and expected behavior for displayed HDR scenes. Because the display cannot output the full dynamic range of the scene, the scene is tone-mapped before display, and thus the groups of pixels representing highlights appear not as bright as they should.

Now the camera moves, and the scene gets motion blur applied in engine prior to tone-mapping. All the sudden the scene gets brighter. The APL increases. In fact the larger the motion, the brighter the scene gets.

You, yes you the reader, have seen this before in many games. And technically speaking the game engine isn't doing anything wrong.

What is happening is those small bright areas of the scene, get blurred, distributing their energy over more pixels. Each of these blurred pixels have less intensity than the original source. As the intensity lowers, it falls more away from the aggressive areas of highlight compression, approaching tonality which can be reproduced by the display. So the APL increases, because less of the scene's output energy is getting limited by the tone-mapper.

The irony in this situation is that as motion and motion blur increases, the scene is actually getting closer to it's correct energy conserving visual representation as displayed.

Note the same effect applies as more of the scene gets out of focus during application of Depth of Field.