MATLAB: Image processing

image analysis

What is the relationship between gray level of an image to histogram. I know about histogram that implies the number of pixels in particular gray level.But is it possible to calculate gray percentage using histogram value?…..

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Histogram is a statistic of the image that gives information about the number of pixels of a particular intensity. It is irreversible, in the sense that with a histogram, you cannot recreate the image, and hence cannot predict the pixel value at a particular point. Having said this, it is possible to calculate the percentage of a particular gray level. This is equivalent to just normalizing the histogram on a unit scale. For example,

I = imread('rice.png');

hist = imhist(I);

%percentage of pixels that have the value 105, is

hist(105)/sum(hist) %This is the fraction.

and as a check, sum(hist) should always be equal to the number of pixels in the image.

Hope this helps!

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MATLAB: How the Peak value of the image histogram can be obtaind

[pixelCounts, grayLevels] = imhist(grayImage);
[maxCount, indexOfMax] = max(pixelCounts);
grayLevelAtMax = grayLevels(indexOfMax);

MATLAB: Image Analysis

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)

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MATLAB: How the Peak value of the image histogram can be obtaind

MATLAB: Image Analysis

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