Re: DCT coefficient quantization factors


"Thomas Richter" <thor@xxxxxxxxxxxxxxxxx> wrote in message
news:4v8g02F1ajbk3U1@xxxxxxxxxxxxxxxx
fulltime wrote:
In JPEG, after passing the pixel values thru DCT computation, we
quantatize it b4 encoding the code. I understand tat the quantization
matrix given can be scaled by a factor.

This is one way to setup a quantization matrix for an arbitrary quality.
This is definitely not the only way to do it, but only a *recommendation*.


yes, for example, my implementation makes a pass over all the output DCT
blocks, calculates the general energy distributions, and uses this, along
with the quality factor, to attempt to come up with a "good" set of
quantization values.


DCT quantized = ROUND (DCTcoefficient/(Q*Scale Factor))

where Q is a value of the quantization matrix and
scale factor =
i) 50/Quality Factor for QF <50
ii) 2- QF/50 for QF>=50

My qn is to get the best quality factor, we set QF = 100, therefore
scale factor = 2-100/5 = 0
that means the DCT quantized value will be 0 as well???

No, it means that the formula above is no longer well-defined and you
cannot compress to 100% quality. In fact, the DCT mode of JPEG-1 is
*never* lossless, and the scaling of the quantizer matrix for 100% quality
returns nonsense.

JPEG-1 does have a lossless mode, but that works completely different,
and not by setting quality to 100.


usually though, since 0 makes no sense in a quantizer table, one clamps the
range to 1..255.

closest one can get to 100% quality is a quantizer table filled with all
1's...


but, really, it is not that majorly important really. just try for something
sane that gives good-looking results and decent filesizes...

So long,
Thomas


.