Checking for clipped pixels?

[dpaul]

I have a cooled CMOS camera (Atik Horizon) and just wanted to check the offset value was set appropriately. I believe there is a way to check for any pixels below zero in Pixel Math (and possibly Statistics?).

I need a very simple explanation, for example I've seen notes on this before but I need to know exactly where to place an expression or formula (Pixel Math) and what does the output mean.  Statistics looks easier to understand if that can do it too?

All help very much appreciated.

Thanks

David

[pfile]

in pixelmath, set up for a single expression, and write "$T==0" in the first field (but remove the quotes). then click "create new image" and apply the process to your frame. you will get a new image with only black and white pixels. every white pixel in the new image corresponds with a black pixel in the original image.

there's no pixels below 0 in any calibrated image generated by PI; 0 is the lowest possible value. if the result of a calibration calculation on any pixel is 0 or < 0, it will be clamped at 0.

rob

[bulrichl]

Alternative: open the frame, call Image/Statistics with 'Unclipped' unchecked and select the frame.

count (%) is the fraction of non-zero pixels, (100 - count) is the fraction of pixels with  zero intensity.

Bernd

[dpaul]

Hi Rob / Bernd

Thanks for the 2 quick replies -

This is what I found after trying a handful of light frames:

Pixel Math:-
On all occasions there were no white pixels evident, all look uniformly black ............so good!

Statistics:-
The same frames gave 99.97%-99.98% count. Ok this is not quite a 100% but I'm assuming this is still good.

One final question - what % would be considered to be good enough, I'm assuming 99.98% is fine?

Many thanks

David