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points by solarkraft 11 hours ago | 0 comments
Consider me confused. Isn’t quantizing a value, e.g. reducing the number of bits in an int, a form of discarding information from that value? Isn’t your comparison just about discarding information on another level (discarding entire values)?
[–] edflsafoiewq 9 hours ago
I think the GP's point is, if you don't know what quantization is, the article gives the impression it consists of discarding some of the values, which is where the compression comes from. This is a good "0-th order" intuition, but quantization is a softening of this; instead of fully retaining or fully discarding a value, there is a sliding scale of how much information to discard. If you already know what quantization is, it's easy to see how discarding is a special case, but the article gives the impression discarding = quantization.
[–] dizzant 7 hours ago
Exactly correct. Quantizing is like rounding. This is the same notion as in quantum mechanics: the values aren’t any real number (a huge input space), but rather integer multiples of some constant (a small output space). Quantization is “rounding” to the nearest allowed output.

To build on the example used in the article, you could quantize an image by dropping the lower half of the bits of every pixel value. This discards half the information by mapping the larger input image space (e.g. 8 bits per pixel) to a smaller output image space (e.g. 4 bits per pixel), effectively scaling/rounding the original values.

The fact that higher frequencies can be quantized more intensely with relatively little image quality loss is a completely separate idea from quantization itself. In this context, high frequency masking as described in the article could be accurately described as a spatially dependent quantization to zero-bit output values. But the quantization part is the mapping, not the shape of the mask, and I think the article is misleading about the distinction.

In real codecs, there usually isn’t a hard boundary like a mask, but instead an empirical table of how many bits to keep based on the perceptual value of the information at each frequency.