how to get the values of blue channel minus green channel of an rgb image?

that is m=G-B this is done because soil and vegetation both have high hue as well, but these objects tend to have relative high value of green and red, respectively. So m is employed to distinguish between these objects and real shadows areas. i can get the blue channels and green channels separately.

[Guillaume]

Tagging your question as urgent tends to antagonise contributors and thus makes it less likely you'll get an answer.

http://www.mathworks.com/matlabcentral/answers/29922-why-your-question-is-not-urgent-or-an-emergency

[Guillaume]

Moreover, since you say you can get the value of blue and green channels separately, what stopping you to take their difference?

Answer 1

[Image Analyst]

" soil and vegetation both have high hue" <== Not true. A high hue value is purple. I've not seen purple soil and only occasionally purple vegetation. Vegetation will have high saturation (chroma), and soil may (red, brown) or may not (black dirt) have high saturation.

I rarely do color segmentation in RGB color space. I have several color segmentation demos in my File Exchange: http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862 Using a different color space may make it easier.

Try the Color Thresholder App on the Apps tab of the tool ribbon.

To answer your question

m = double(greenChannel) - double(blueChannel);

[Walter Roberson]

By the way, purple potatoes are yummy. :-) So is purple cabbage; and purple corn makes a nice change. But Eggplant -- well, keep in mind that Eggplant is a member of the deadly Nightshade family ;-(