Comparing images for brightness programmatically

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I have a two bmp files of the same scene and I would like determine if one is more bright than the other. Similarly I have a set of bmps with different contrasts and another set of bmps with different saturation. How do I compare these images for brightness,contrast and saturation ? These test images are saved by a tool provided by the sensor manufacturer. I am using gcc 4.5.

Posted 27-Jun-12 23:33pm

johnny alpaca

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Sergey Alexandrovich Kryukov · Answer 1 · 2012-06-28T08:28:00

The term "brightness" is a very unlucky term as it is often used incorrectly, especially in computer-related color space issues, mostly due to poor understanding of physics by the people who introduced the terms. Please see:
http://en.wikipedia.org/wiki/Brightness[^].

I would prefer to call it "value", as in the name of the color space "HSV". Please see:
http://en.wikipedia.org/wiki/HSV_color_space[^].
This article explains some controversies in "brightness" or "lightness".

For more general understanding of color spaces, please see:
http://en.wikipedia.org/wiki/Color_space[^].

[EDIT]

Please also see my notes to Solution 1.

—SA

nv3 · Answer 2 · 2012-06-28T01:17:00

Your question is interesting, but hard to answer in short. First of all I would recommend you get some image library on board that allows you to access each pixel of your images without writing tons of code to read the bmp files.

Then you want to take a close look at your images. Are they precisely registered, i.e. taken from the exact same point with the exact same camera viewing direction and the same focal length? In that case you can compare pixels from one image directly with pixels from the other image. If not you will first have to align the images.

You might want to think about what exactly "brighter" means. Brighter in parts of the image, brighter in average, brighter in a particular area of the image?

In the simplest case, you just calculate the average pixel brightness of image 1 and compare that with the average pixel brightness in image 2. If the brightness difference resulted from different lighting or different exposure settings, that is probably a valid method.

Similar things apply to contrast and saturation. For contrast you might want to calculate the standard deviation of pixel brightness values. For saturation you might go by the average saturation of all image pixels.

Hope that gives you a rough road map for your task.