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coherence of the standard deviation value displayed
Posted: Wed Aug 28, 2024 7:23 am
by Jdef
Hi,
I have remarked that the standard deviation value displayed in the histogram window is usualy not coherent with the true SD of the image, the value displayed is about 1.7 (pi/2?) the true value.
Here an example:
- Capture d'écran 2024-08-20 225844.png (492.4 KiB) Viewed 1914 times
- Capture d'écran 2024-08-28 090234.png (234.58 KiB) Viewed 1914 times
- Capture d'écran 2024-08-28 092042.png (268.21 KiB) Viewed 1914 times
Best regards
jacques
Re: coherence of the standard deviation value displayed
Posted: Wed Aug 28, 2024 2:46 pm
by admin
Hi Jacques,
yes, something not quite right here - I tested and didn't get quite the same factor but still not the right result. The calculation is a bit odd since it tries to work from the (already calculated) histogram rather than the raw image data to avoid the performance hit of scanning the whole image twice, but it still should be possible to get an accurate value. I will check the code.
cheers,
Robin
Re: coherence of the standard deviation value displayed
Posted: Wed Aug 28, 2024 7:32 pm
by Jean-Francois
Hello Jacques,
To be sure ... do you check that the selection red rectangle is over the full image ?
You need to select the button left of the reticules ... it shows the red rectangle.
If it is not over the full image, then the statistic in SharpCap will be different as the statistic over the full image in a different software.
Jean-Francois
Re: coherence of the standard deviation value displayed
Posted: Thu Aug 29, 2024 12:59 pm
by admin
Hi,
I tracked this down to some of the processing done on the histogram data when the camera is in a high bit depth mode (12,14,16). The algorithm was designed to make the histogram look more like a true histogram of all the levels, even though the number of points in the graph is reduced for performance reasons. However, the data was changed in a way that tended to increase the variance calculated
For now, I've gone back to not trying to manipulate how the histogram looks in this situation, which fixes the variance - it remains to be seen if the histogram becomes slightly less useful...
cheers,
Robin