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Gamma error in scaling images

sendhil76

Active Member
Hello!

I would like to share this interesting link that i came across while researching and working with linear gamma images.

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The results are quite intriguing...and after trying out the tests myself...i am reading the article again...very slowly this time :) to figure out what is really going on.

Best regards.
Sendhil.
 

kakou

Active Member
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Don't waste your time with that.

That's nothing more than a specially constructed image that is intentionally designed to go to grey when reduced.

Zoom in and take a look for yourself. It's made up of alternating green and magenta lines of varying intensities to fake out the full spectrum. It's not a real photo. It doesn't even look that good before reducing it.

Guess what happens when you combine green and magenta (complementary colors)?
 

Arvo

Well-Known Member
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Absolutely correct. Why then it does produce quite wrong shade of gray (or even color, depend on source image) when reduced (in most editors)?

For real images these problems are substantially less visible, but they exist.

I think I could find example. I can't upload original (too big), only crop from it:

SDIM1132-bw-crop.jpg

Left is incorrectly resized 20%, right correctly:

SDIM1132-bw-bicubic-wrong.jpg SDIM1132-bw-bicubic-correct.jpg

(Crop was from bush at right central part.)
If you can't notice difference, save both images and compare (left one is much darker). If you need original (not crop), just ask by PM.

It is possible that difference is caused by different bicubic algorithms in different programs; I'm thinking that different gamma handling is the real cause however.
 
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