[FFmpeg-user] Documentation and feature request
astroelectronic at t-online.de
Mon Apr 29 12:15:20 EEST 2019
Am 15.04.2019 um 18:10 schrieb Gyan:
> Hi Michael,
> I'll look at these in detail later on, but just to touch on one point..
> On 15-04-2019 06:15 PM, Michael Koch wrote:
>> "eq" filter:
>> -- It might be helpful to add a note that multiple eq filters can be
>> cascaded, for example if contrast enhancement bigger than 2 is required.
> Contrast can go from -1000 to 1000. The range was expanded a few years
> ago; the doc wasn't updated.
> Do note that what contrast does is scale the distance of a pixel's
> value from the median value i.e. 128 for a 8-bit input. So, if a pixel
> channel has a value of 100, then a contrast of 3 results in a value of
> 128 + 3*(100-128) = 44.
> The order is contrast -> brightness -> gamma.
If we assume the video data is in the [0..1] range, the function (before
applying gamma) is:
out = brightness + 0.5 + contrast * (in - 0.5)
This works fine for images where most pixels are in the center of the
But it fails for images which consist mainly of a dark background, with
only a few bright details. To amplify contrast in such images, both
brightness and contrast must be adjusted.
May I suggest to add a new parameter p (pivot) in the range [0..1] which
is 0.5 by default (so that it's compatible with old command lines).
out = brightness + p + contrast * (in - p)
With the p value properly set, the contrast could be adjusted without
changing the brightness.
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