[~, lowMap] = histeq(gra​yImage(bin​aryImageLo​w));

what does the above code do ? and what is ~ ? can i've a simple example

Answer 1

[Walter Roberson]

In that context, the ~ means to throw away the result that would normally go into that position. It is almost identical to saying

[TEMP17393, lowMap] = histeq(grayImage(binaryImageLow)); clear TEMP17393

except with no possibility of clashing variable names.

Answer 2

[Image Analyst]

grayImage(binaryImageLow) takes all the pixels that are "true" (white) in the binary image (mask) and strings them together into a 1D array - essentially a list of all the gray levels in the masked area.

histeq() then does histogram equalization on that 1D array. In my experience, histogram equalization is rarely helpful or even needed at all. It returns the lookup table that tells what input gray level should be mapped to what output gray level.