Question

For a thumbnail-engine I would like to develop an algorithm that takes x random thumbnails (crop, no resize) from an image, analyzes them for contrast and chooses the one with the highest contrast. I’m working with PHP and Imagick but I would be glad for some general tips about how to compute contrast of imagery.

It seems that many things are easier than computing contrast, for example counting colors, computing luminosity,etc.

What are your experiences with the analysis of picture material?

Answer 1

I’d do it that way (pseudocode):

L[256] = {0,0,0...}

loop over each pixel:
    luminance = avg(R,G,B)
    increment L[luminance] by 1

for i = 0 to 255:
    if L[i] < C: L[i] = 0       // C = threshold of your chose

find index of first and last non-zero value of L[]
contrast = last - first