P3D Re: JPG and destruction

[Dylan The Hippy Wabbit <spacey@xxxxxxxxxxxxx>]

Hi,

Regarding my words:-

>>Then, based on good
>>psychological principles, the greyscale retains its full resolution but
>>the colour components are reduced by half.

John Roberts asked:-

>Is that based on the idea that spatial resolution of the human visual
>system is much better in green than in blue or red? (I recall being told
>that blue resolution is about half that of green, but I didn't recall a
>specific number for red.)

It's based on acuity for black and white being superior to all colours.
Colour TV uses the same trick (at least it does here, I know nothing about
NTSC) of being a sharp B+W signal overlaid with a softer colour one.

Also:-

>>No, using the minimum compression/maximum quality setting will make no
>>difference.  That affects a later stage of the compression.

>So what does the quality setting affect?

Now here we are getting to where my understanding is a bit ropey, so
forgive any oversimplifications please, but it's basically a rounding
thing.  The bitmap is veiwed as a 2D waveform, and transformed into a set
of cosines which sum into an approximation of the original pattern.  Then
the least significant frequency components are rounded down to zero.  All
those zeros compress well at the next stage.  The quality/compression
setting affects how significant a component has to be to escape being
dumped in that fashion.

I hope that's clear enough.  Damn book made my head spin.  :-D

Dave Spacey

______________________________________________________________________________

Walk tall, walk straight,
Spit the world right in the eye.

-Kevin Godley and Lol Creme


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