在matlab中不使用for循环计算矩阵的直方图

Question

i have an image which i have read in and its pixel values are stored in a matrix. I am trying to get a frequency table for the matrix of which i intend to plot a histogram. I am trying to do this using only matrix expressions(ie no for loops/ imhist function). I looked at a function called histc() that can count the values in a matrix, but i don't know how to use it. I would really appreciate it if someone could point me in the right direction. Thanks

Answer 1

try:

hist(image(:),min(image(:)):max(image(:)));

this will plot a histogram of the pixel values including the entire range of values that the image has.

Answer 2

Though this is an old post, we can also use accumarray :

h = accumarray(double(im(:))+1, 1, [double(intmax(class(im)))+1 1]);

h will contain a histogram / frequency count of how many pixels are encountered per intensity level. We take all of the values in im and offset by 1 as MATLAB indexes arrays starting from 1 instead of 0 . The intensities for an image will start at 0 . We also want to specify the size of the histogram to go from 0 to as many intensities that we have that is supported by this class. We can figure out what the greatest intensity that is supported for an image by doing intmax(class(im)) . As such, the total number of possible intensities that we can support is intmax(class(im)) + 1 . The third parameter of accumarray specifies the output size of our histogram.

Take note that you need to cast the intmax call as well as the image im with double because what is returned is the maximum integer of that type and it is of that type . Therefore, adding 1 will simply saturate the value and not register. Therefore, you need to cast as double so that the addition can properly register.