How can I detect edges between different colours of the same brightness?

Question

I'm looking for an algorithm that can identify edges across which colour is changing sharply, rather than just finding changes in brightness.

Is this just a matter of using a different colour space with existing edge detection algorithms, or is there a better approach?

Answer 1

I think you can convert from the RGB space to the HSV one, or whatever color space has the HUE in a single channel.

Take the HUE channel, and make the edge detection on that one.

Here a simple Matlab script to achieve the result.

I = imread('image.png');
hsv = rgb2hsv(I);
hue = hsv(:,:,1);
edges = edge(uint8(hue),'sobel');
imshow(edges);

Answer 2

Color difference is a difference in brightness, in some color channel. So while the sum of individual channels stay the same, there is a intensity difference if there is a color difference.

Now the basic algorithms do not really specify how you should hadle the multi channel data. So there is nothing wrong with using a different color space. On the other hand nothing says you can not apply the convolution on all channels and then pick the result that is most contrasting.