Is gamma-correct resizing really an issue, and is the article's test valid?

Question

I found an article claiming that many image editors resize images incorrectly because they average gamma-encoded RGB values instead of resizing in linear light. The article suggests testing this with patterns like a black-and-white checkerboard and says software that is not gamma-aware will produce the wrong brightness when downscaling.

Is this technically correct? Is using that kind of test a valid way to identify software that resizes incorrectly? I'm trying to understand whether this is a real issue in image processing or mostly a theoretical edge case.

user1024 · Accepted Answer

Before even looking at the article, I say that there certainly is some amount of skew in levels when resizing a JPEG image. This is due to the fact that the RGB values are obtained from the RAW after a gamma (non-linear exponential) correction was applied to the RAW pixel values. Then, while scaling, values from adjacent pixels are averaged through some method (linear, cubic, lanczos) in a manner that is different from the specific gamma correction. This averaging is a linear operation by its nature, and applying it over the non-linear gamma correction introduces the skews.

That said, I think if you scale the RAW itself, assuming it's done properly before the gamma correction, you should be fine. I am not sure how the various software do it, though.

BTW, the RGB planes themselves are obtained by interpolation of near pixels on the RAW matrix - this is called the Bayer pattern demosaicing.

That said, I think if you scale the RAW itself, assuming it's done properly before the gamma correction, you should be fine. I am not sure how the various software do it, though.

BTW, the RGB planes themselves are obtained by interpolation of near pixels on the RAW matrix - this is called the Bayer pattern demosaicing.

Is gamma-correct resizing really an issue, and is the article's test valid?

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