View Full Version : What is a quantizer?

22nd March 2002, 12:26
I know it affect's the quality and size of a movie, but what are this things and how do they work?

22nd March 2002, 13:19
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.


9th April 2002, 11:52
so higher quantizers means lower quality?

9th April 2002, 12:21
Simple answer, yes. Correct answer, not always :)


9th April 2002, 12:34
higher quantizers mean more compression, but more compression doesn't always mean lower quality... am i right?

9th April 2002, 12:44
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.