Problem with exposure filter

[Michael Koch]

Make a 10-bit grayscale test image:

ffmpeg -f lavfi -i color=black:s=16x64,format=gray10le -lavfi geq=lum='X+16*Y',format=gray10le,datascope=s=640x768:mode=color2:format=hex -frames 1 -y test.png

Apply the exposure filter, followed by a limiter:

ffmpeg -i test.png -vf format=rgb48,exposure=exposure=1:black=0.4,limiter -y out.png

exposure=1 should be equivalent to a factor 2, but the amplification in the output image is much stronger. It becomes even worse if black=0.5 is used, then the output image contains only black or white pixels, that's infinite amplification.
Either something is wrong, or I did misunderstand what the filter is supposed to do.

[Michael Koch]

P.S. If black is set bigger than 0.5, then the amplification seems to become negative. Black and white are swapped.

[Elon Musk]

It is using base-e exponential function, not what you want/imagined.

[Michael Koch]

Why does the polarity change at black=0.5? Does that make any sense at all?

[Michael Koch]

The subtraction of s->black in line 71 doesn't look correct.

[Elon Musk]

It is 1000000% correct. Check your factchecker.

[Elon Musk]

The invert of pixels bug have been fixed.

[Michael Koch]

Replying to Elon Musk:

The invert of pixels bug have been fixed.

The subtraction of s->black (now in line 70) doesn't make any sense in my opinion.
And it would be better if the unit of "exposure" is photographic EV values, I mean exposure=1 should be a factor of 2 and exposure=-1 should be a factor of 0.5.

I suggest to replace lines 70-73 by

s->scale = exp(s->exposure * log(2.0))

It's always positive.