Get the position of pixels with specifed color

Hi! I have a problem.I have a picture with two circle in it.A red and a green.I want to get the postion of their centers and connecting them and how can I get the position of all red pixel in that picture.Please help me and thanks a lot

Answer 1

[Walter Roberson]

I answered about finding the positions a moment ago in response to your earlier post.

For the matter of finding all the red pixel in an image, first you have to define what "red" means. If a pixel has red turned on as high as it can go also happens to have blue turned on to (say) 1/10th of the maximum, then is that pixel still "red" ? If so, then how far could the intensity of the red component be reduced and you would still deem the pixel to be "red" instead of "purple" ?

[nghia nguyen]

I'm sorry but I really don't understand.I'm just a beginner in IP.Can you tell me more clearly.Thanks so much.

[Walter Roberson]

Start with something that is pure red. Mix more and more white in by increase the G and B. With G and B at their lowest non-zero settings, pretty much everyone would agree that the color was still "red". Okay, now bump up the G and B to make it a lighter red. At some point it starts to look pink-ish but you might still identify it as red. Keep pushing up the G and B until you get something that you say is no longer "red" but is instead "pink". How where _exactly_ is that boundary between "red" and "pink" ? And how would the boundary change if the original red was not 100% red but was (say) only 95% of the maximum strength of red? If it was 75%? 50%? 25% red to start with?

Red is not an absolute class: it is a fuzzy concept. You have to tell us exactly what *you* mean by red before we can tell you how to detect the "red" pixels apart from the ones that are (say) merely a mauvey shade of pinky-russet.

[nghia nguyen]

Thanks so much.You helped me a lot.