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22nd March 2002, 13:19 | #2 | Link |
Kilted Yaksman
Join Date: Oct 2001
Location: South Carolina
Posts: 1,303
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Re: What is a quantizer?
Quick answer:
You have "information" which tells you about an image - not pixel data, but frequencies which *describe* pixels. Now that information can be quite complex, you could have values like: 112, -300, 1, 0, -1003, 24, 0, 0 Just as an example I pulled out of thin air. Now instead of storing these weird numbers (which would take up quite a bit of space), I propose that we divide them all by 100: 1, -3, 0, 0, -10, 0, 0, 0 Those numbers are a lot simpler. That is the idea behind quantization - get the "information" to describe pixel values (this is what the DCT does - transforms pixel data into frequency values), then divide the DCT result by a value to make them smaller, thus consuming less space. The value we divide them by is the quantizer. That's not a 100% correct explanation, as there are different quantization models, different pre-dividing steps, etc., but that's the crux of quantization - throw away information through division. -h |
9th April 2002, 12:44 | #6 | Link |
Kilted Yaksman
Join Date: Oct 2001
Location: South Carolina
Posts: 1,303
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higher quantizers mean more compression,
The size of a frame is always inversely proportional to its quantizer, all other things equal. So yes that's true. but more compression doesn't always mean lower quality... am i right? It is possible to quantize a frame/area heavily, without influencing perceived (or even measured) quality. This can be messy. -h |
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