[FFmpeg-user] exponential decay, is it possible?

[Michael Koch]

Am 05.08.2018 um 15:42 schrieb Gyan Doshi:
>
>
> On 05-08-2018 06:41 PM, Michael Koch wrote:
>> I'd like to ask if it's possible to make an exponential decay effect 
>> with FFmpeg.
>> Let's assume a pixel is fashing to luminance = 1 in just one frame. 
>> Then in the output video I'd like to have the pixel flashing to 1 
>> immediately, followed by an exponential decay with a time constant of 
>> a few seconds.
>>
>> The mathematics would be something like this:
>>
>> luminance = 0.95 * luminance_in_last_frame;     // exponential decay
>> if (luminance_in_new_frame > luminance)
>>    luminance = luminance_in_new_frame;
>
> So,
>
> frame 0    --> luminance 0
> frame 1    --> luminance 1
> frame 2    --> as per exprs above
> ...
> frame 2+T  -->       "
> frame NEXT  --> source luminance
> ...
> frame LAST -->       "
>
> ?
>
> Or would the exponential decay restart each time a pixel of lum 1 is 
> encountered?
>

Every time when the input becomes brighter than the last output frame, 
the output is set to the input value. If input is below the last output 
value, then exponential decay.
I have a timelapse video of the night sky with meteors. Each meteor is 
visible only in one frame. I'd like to make them visible a little 
longer, decaying over several frames.

Example:
Input   Output
0     0
1     1            // set to input value
0     0.95      // exponential decay begins
0     0.90
0.50     0.86
0.50     0.81     // until here
0.90     0.90     // now set again to input value, because input is 
brighter
0.90     0.90     // keep at the same level
0.60     0.86     // exponential decay begins
0.60     0.81
0.60     0.77

Michael

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   ASTRO ELECTRONIC   Dipl.-Ing. Michael Koch
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