MATLAB: Cropping greyscale image based on pixel

Question

Hi all,

I intend to crop images based on its pixel value or intensity. Here are my images below, labeled as image1, image2… image 7.

All these images are from the taken from the object but replaced with different optical density filters. I performed a simple imcrop() rectangle and subplotted to obtain the following.

For example if I take image3, I am mostly interested in the center and would like to crop out that weird eclipse area.

I would like to somehow be able to define a range to crop and keep the cropped area. For each image, I will need to define a new range, depending on the section I want to keep or discard. Can I do something like, if the pixelvalue < 50, then make it transparent?

Later on, I would like to paste these cropped areas on top of each other.

Thanks, Victor

Answer 1

The quick-n-dirty way that comes to mind is to just use a bitmask: 0 to toss a pixel's value and 1 to keep it. It's a little memory intensive but it won't care about the shape of the ROI.

For a given filtered image,

    %Returns a logical matrix equal in size to your image, doing a per-pixel comparison.
    Mask1 = (Im1 > 50);

Then to combine, just add everyone together with the bitmasks, performing a bitwise mult to only keep the pixels you want. The other pixels will be zero. So not exactly transparent, but since you're working in grayscale I suspect you don't have (or need) an alpha channel.

ImAll = Im1.*Mask1 + Im2.*Mask2 + ...

Obviously you could use N-D arrays instead of discrete variables and matrices and a half dozen other tricks if you wanted, depending on your end application. Is that sufficient? Or too coarse a solution?