Auto Exposure bug

Hi, well I don’t find Auto Exposure to be a bad feature as a concept, so turning it off isn’t really a solution for me. However, it is bugged in my case.

There are multiple lights in this scene. From this spot, where the lights could be seen, the Auto Exposure actually further brightened the screen.

From this spot, where I am further away from the lights, the Auto Exposure actually darkened the scene.

Needless to say, this is of course opposite to what you’d expect. Our eyes adjust to see dark areas as brighter and bright areas as darker.

And here’s a even clearer picture of what went wrong behind the scene.

As you can see, the dark areas are red. However, in all the official tutorials I have seen, red areas represent bright areas, which means the Auto Exposure is actually detecting it completely wrong. That also explains why it turns dark areas even darker.

Not sure which tutorials you were watching, but this is incorrect. As the documentation notes:

Red pixels represent anything below the auto exposure adaptation ranges set by Histogram Log Min (or Histogram Min EV100). Blue pixels represent anything above the auto exposure adaptation ranges set by Histogram Log Max (or Histogram Max EV100). These pixel ranges ensures that the values set for Low Percent and High Percent are removing these unwanted pixels from being calculated.

Generally speaking when the exposure darkens unexpectedly in low light it is usually because the Histogram Min EV100 is set too high. Not entirely sure why, but it seems like the exposure target seems to fall off the histogram range or something and results in a higher than expected target. If you set your SpeedUp/SpeedDown to a high value it actually seems to result in bouncing exposure.

Frustratingly I find this is pretty much always the case with the default setting of -10 and I typically lower it to -16.


I was watching this Learn About Post-Process Effects in Unreal Engine | Webinar - YouTube
at 7:45. But I remember hearing that on another official tutorial on post processing too.

Anyway, well, maybe I was mistaken since the documentation says so. Thank you for giving me an explanation and a solution.

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Seems he’s referring to the border around the histogram, which is interesting, I never noticed the colors were reversed compared to the pixel clipping debug.

Seems like something they should maybe change to be consistent.