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Comment by kazinator | Hacker News Reader

A pixel is not the sample(s) that is value came from. Given a pixel (of image data) you don't know what samples are behind it. It could have been point-sampled with some optical sensor far smaller than the pixel (but not infinitely small, obviously). Or it could have been sampled with a gaussian bell shaped filter a bit wider than the pixel.

A 100x100 thumbnail that was reduced from a 1000x1000 image might have pixels which are derived from 100 samples of the original image (e.g. a simple average of a 10x10 pixel block). Or other possibilities.

As an abstraction, a pixel definitely doesn't represent a point sample, let alone an infinitely small one. (There could be some special context in which it does but not as a generality.)

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> A 100x100 thumbnail that was reduced from a 1000x1000 image might have pixels which are derived from 100 samples of the original image (e.g. a simple average of a 10x10 pixel block). Or other possibilities.

And if a downsampling algorithm tries to approximate a point sample, it'll give you a massively increased chance of ugly moire patterns.

The audio equivalent is that you drop 3/4 of your samples and it reflects the higher frequencies down into the lower ones and hurts the quality. You need to do a low-pass filter first. And "point samples from a source where no frequencies exist above X, also you need to change X before doing certain operations" is very different and significantly more complicated than "point samples". Point samples are one leaky abstraction among many leaky abstractions, not the truth. Especially when an image has a hard edge with frequencies approaching infinity.

But from the pixels alone, you don't know whether the moire is an artifact of sampling of something that was free of moire, or whether an existing image of moire was sampled and reproduced.

I have the source image. I know the downscaled version looks awful and wrong. I know the naive point algorithm was the wrong one to use.

Someone with just the small version wouldn't know if it's supposed to look like that, but we're not asking them.

Unless they can infer it's a picture of normal objects, in which case they can declare it's moire and incorrect.

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