What do you want to do exactly ?
Check if a value is between 150 and 200 ?
What kind of value ?

thank you for the reply!

oh yes..i want to detect blobs..not too dark and in the meanwhile not too light. In my case, I want to videoproject in the same position of the blobs..and the blobs are made by objects where the light is the only condition to detect them.

I usually use some spot lamps in order to have some tiny light on the object..but when I videoproject, i want the detection not so sensable to the highlight...

i've seen there are many kinds of threshold in opencv library..but dunno exactly if there is a way which works in my case.

Is it possible just programming..or should i try something different?

hope it is clear the problem..

well..actually it was easy! i'll never end to thank Cousot and Stanley..

what i wanted is:

THRESH_BINARY_INV : dst(x,y) = src(x,y) > value ? 0 : max


threshold(value, max, type);


http://ubaa.net/shared/processing/opencv/opencv_threshold.html