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convert -average darky1.jpg darky2.jpg lighty1.jpg lighty2.jpg l.ppm
convert -average lighty1.jpg lighty2.jpg lighty1.jpg lighty2.jpg d.ppm

l.ppm is much lighter than d.ppm. The order of images matters (but should not).

The same for using evaluate-sequence mean.

How to do it properly?

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Belongs in ? – akseli Dec 24 '10 at 3:36
May be I ask other, more general question there, but here is "how to use this program". – Vi. Dec 24 '10 at 13:14
up vote 1 down vote accepted

Just use GraphicsMagick: gm convert -average *.JPG q.ppm

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Big disadvantage: it tries to load all input images into memory. /* Alt+SysRq+F */ – Vi. Mar 12 '11 at 23:39

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