The RGB vectors for white and red color are same for this image (Matlab)

Question

I was working with this picture in Matlab to detect the color of the circles. This is a 512 by 512 jpeg image.

I am finding the centers of circles using imfindcircles, then I am taking the R, G, B components of some points near the center of each circle to detect color.

But, I am confused because, for both the red and white circles, I find that the R, G, B components are same [239 227 175].

I am new to image processing, so can anyone explain what's actually happening here.

You have to show us some actual code....I'm guessing that you're selecting the beige background pixels (because of incorrect indexing) rather than the actual circles — Suever
You are sampling outside. That is the color of the background — Ander Biguri
Don't put your code on pastebin, edit your question and add it here (formatted by indenting it)! — Wolfie

Suever Suever · Accepted Answer · 2017-04-04T21:25:37

The centers output of imfindcircles gives the coordinates of the centers in x/y coordinates and you need to index into your image using row/column coordinates so you need to be sure to reverse the two columns when indexing into the image

centers = imfindcircles(IM);

center1 = IM(centers(1,2), centers(1,1),:);
center2 = IM(centers(2,2), centers(2,1),:);

Presumably you are not doing this because you are instead sampling pixels from the background which obviously result in the same RGB values for the centroids.

Update

It appears that actual issue is that you are casting the location of the centroids to a uint8 to make it an integer value so you can then use it as an index. The maximum integer representable by uint8 is 255 and the number of rows and columns in your image is large than 255 (and so are the centroids) so they will get truncated to 255 resulting in the wrong pixel being sampled.

Rather than using uint8, just use round to round the centroids to their nearest integers

cX = round(centers(n_c,1));
cY = round(centers(n_c,2));

The RGB vectors for white and red color are same for this image (Matlab)

1 Answers