Q: the fastest way to perform some image analysis tasks

Dear all,

what is the fastest way to perform the 3 following tasks on 8bpp grey images in GM using VB6?
1) subtract an integer value (e.g. 12) from all grey values in the image (effectively resulting in background subtraction), with resulting negatives being set to 0?
I can do it with low-level data access methods (copyMemory etc) but is there a build-in function that will do this?

2) make a digital mask of a picture where all pixels smaller than an indicated minimum (e.g., <12) is set to 0 and all other pixels are set to 0?

3) set all pixels smaller than a given value to 0, and keep the other pixels unchanged (background suppresion, rather than subtraction)

Thanks, Kees

concerning my question 2:

Oops, that is a typo. I mean:
...is set to 0 and all other pixels are set to 1 (ONE)!!

The idea is to make a digital mask with zero's for low-level pixels, and with 1 for higher levels. That is because I want to perform arithmatic between pixel values from different images, that are taken at extremely low light levels. In these images, a gray level of e.g. 9 is based on collection of only 9 photons for that pixel. The natural variation in this type of low light images is + 2 * sqrt(9) to - 2* sqrt(9), meaning that actual values may change from 9+6 to 9-6, so from 15 to 3. This causes enormous errors in my arithmatic, therefore I wish to exclude these pixels from the calculations using the digital mask.

It seems to be a good solution for problem 2. As for problem 3, the simplest way is to use the mask you get as an alpha channel and blend it with totally black image of the same size as your one. Please let me know if you need a code sample.