Get a relationship between position vector return by getposition method in matlab - matlab

Question

I wrote a code for image cropping. I used imrect to draw a rectangle on the image and then get the position of it by using the method getposition. I wrote a function which uses image pixel coordinates for cropping operation. How can I create a relationship between values return by getposition method and image pixel coordinates.My code for cropping is as follows,
[rnum cnum dim]=size(img);
for h=1:dim
for i=1:width
for j=1:height
negative(i,j,h)=img(xmin+i,ymin+j,h);
end
end
end
width,height,xmin,ymin have to found from getposition method

Answer 1

Like you said, imrect's getPosition method will return:
[xmin ymin width height] = getPosition( h );
The first two values are the top-left corner of the rectangle, and the next two values are the length of the sides of the rectangle. These should all be in pixel coordinates if you are using imrect.
To crop an image based on these position values, you will start at the top-left corner of (xmin, ymin) and go to the bottom-right corner at (xmin+width-1, ymin+height-1).
You should not use for loops to get the pixel data, you can take advantage of MATLAB's vectorization characteristics and do the following:
CroppedImageMatrix = OriginalImageMatrix( [ymin : 1 : ymin+height-1],
[xmin : 1 : xmin+width-1],
: );
This will immediately "crop" the image and place the cropped data into the new matrix. You can do this because you are using a rectangular crop and all of the indices correspond to create a rectangular lattice of points. It would be "trickier" if this was not a rectangular crop.
This also will work the same for color or grayscale images because you do not need to index the channel dimension, you just take the values from every available channel.
P.S. - Documentation page for imrect: http://www.mathworks.com/help/images/ref/imrect.html