light gray image background from v4l cam after dark subtraction

What steps will reproduce the problem?
1.  Pick any 8bit v4l camera as input (lifecam cinema hd in my case)
2.  Choose say 1sec exposure and shoot darks
3.  uncover lens and observe preview frames

What is the expected output? What do you see instead?
Expected to see dark background, instead it's a light gray

What version of the product are you using? On what operating system?
Latest svn checkout, ubuntu 10.10 I think

Please provide any additional information below.
 I traced the culprit, and it's in image_math.cpp in the dark subtract method.  For some reason, the threshold for dark subtraction is 100, which also gets added into the pixel value when subtracting.  I changed it to zero and everything is now fine.  I'm thinking this was done so only really bright pixels are subtracted rather than also noise, but if a camera has a high bias (as in my case) it just elevates the bias signal by 100, which makes the background light gray. Once I use straight subtraction (took out the +100) it works fine.  Perhaps this is worth asking Craig or investigating further?

Thanks.

Daniel

Original issue reported on code.google.com by ciob...@gmail.com on 14 Jun 2012 at 7:02

olegkutkov / open-phd-guiding

light gray image background from v4l cam after dark subtraction #30