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52 lines
1.9 KiB
HTML
52 lines
1.9 KiB
HTML
<html><head><link rel="stylesheet" href="style.css"></head><body><div class="page">
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<h1>20160721 - Why Motion Blur Trivially Breaks Tone-Mapping - Part 1</h1>
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<i>Some food for visual thought ...</i>
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You have a scene with a bunch of High Dynamic Range (HDR) light sources, or bright secondary reflections.
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Average Picture Level (APL) of the scene as displayed is relatively low, which is standard practice and expected behavior for displayed HDR scenes.
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Because the display cannot output the full dynamic range of the scene, the scene is tone-mapped before display,
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and thus the groups of pixels representing highlights appear not as bright as they should.
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Now the camera moves, and the scene gets motion blur applied in engine prior to tone-mapping.
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All the sudden the scene gets brighter.
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The APL increases.
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In fact the larger the motion, the brighter the scene gets.
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<i>You, yes you the reader, have seen this before in many games. And technically speaking the game engine isn't doing anything wrong.</i>
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What is happening is those small bright areas of the scene, get blurred,
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distributing their energy over more pixels.
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Each of these blurred pixels have less intensity than the original source.
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As the intensity lowers, it falls more away from the aggressive areas of highlight compression,
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approaching tonality which can be reproduced by the display.
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So the APL increases, because less of the scene's output energy is getting limited by the tone-mapper.
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<i>The irony in this situation is that as motion and motion blur increases,
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the scene is actually getting closer to it's correct energy conserving visual representation as displayed.</i>
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Note the same effect applies as more of the scene gets out of focus during application of Depth of Field.
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</div></body></html>
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