Extract ROI from image by hand into Matrix Form

I have a 3D Image (each pixel contains information for various wavelengths). I draw the image for one wavelength by using

Image[img[[All, All, 100]]] // ImageAdjust


whereas img is my Image. Now I want to extract the Data in a certain ROI. I do so by using the mask tool, but when I copy the mask, I get a fullsize image where the selected mask is white (I selected the following three squares):

How to extract the information from img (fully including the wavelength-dimension) for these regions? I.e. discarding the black areas completely. ImageMultiply doesnt help me, as this would just make those areas black again and not really discarding them.

Thanks

• I guess that's "region of interest"? (I had at first thought "return on investment" but clearly that does not go with the question at hand.) – Daniel Lichtblau Dec 1 '14 at 15:41
• yes region of interest ;) – tim Dec 2 '14 at 7:56

mask = (* As seen above, copied from masking tool as a "Image *)