Image Analysis

How to calculate gray percentage of an gray scale image ? Like 50 % gray Level, 32% gray level....Please post your answers....Thanks in advance........

Answer 1

[Image Analyst]

This is ambigous so I guess I'll have to solve both scenarios. If you want to know what "percent gray" an image is, like you want to know if it really is "18% gray" then you'd do this:

maxGrayValue = intmax(class(grayImage))
meanGray = mean2(grayImage) 
percentGray = meanGray / single(maxGrayValue)

This can of course be combined into one line if you want. You may have heard about the "18% gray" assumption in photography: http://en.wikipedia.org/wiki/Gray_card , or http://en.wikipedia.org/wiki/Middle_gray

On the other hand, if you're wanting to find out what gray level has 32% of the pixels darker than that, you can use the CDF:

% Get the histogram.
[pixelCounts GLs] = imhist(grayImage);
% Get the cumulative distribution function.
cdf = cumsum(pixelCounts) / sum(pixelCounts);

% For example, find the gray level where 32% of the pixels are darker.
indexGray32 = find(cdf<=0.32, 1, 'last');
percentGray32 = GLs(indexGray32)

Answer 2

[Amith Kamath]

see http://www.mathworks.com/matlabcentral/answers/19673-image-processing

but if you want to measure gray level in the way you have described above, you would want to replace the hist(<pixel value>) with hist(<pixel value range>) in the explanation in the link above. For example,

sum(hist(120:145)./sum(hist)) to check for fraction of pixels that are between 120 and 145 in value.

Thanks!