----- Original Message -----
From: "Albert" <olympus@xxxxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Wednesday, June 25, 2003 6:48 AM
Subject: [OM] Photoshop much better than iview, proof!
> http://albert.achtung.com/irfanview/
>
> Just to PROVE what a nerd I am.. I just put up a page comparing the
> images compressed from both..
>
> A picture is worth a thousand words..
Yes there definitely is a difference there. The problem is that the jpeg
compression employs several different steps to compress the data. If memory
serves (I have a book where I could look this up at home but I'm at work
right now), the big 2 steps are Chroma sampling and Quantization. The chroma
sampling step doesn't introduce any artifacts although it can introduce
errors with colours (particularly stripes). The way it works is the colour
values for each pixel are converted to Luminance, Red Chroma, and Blue
Chroma (YCC). Instead of recording 24bits for each pixel (8 bits for each of
the values), the chroma values for the pixels are averaged over more than 1
pixel (usually 1x1 or 2x2 but I think 4x4 gets used in lower quality
settings). By averaging the 2 chroma values for each pixel, you can reduce
the filesize by 1/3. With 2x2 chroma sampling, you can reduce the filesize
by 2.4x (4x8 bits for Luminance, and 1 8 bit value for Chroma instead of
4*24 bits).
The quantization step is what introduces artifacts to the image. I'd suspect
that the 2 programs are using different combinations of quantization and
chroma sampling to achieve aproximately the same filesize in the end.
Judging by the artifacts you point out, I suspect that PS is using 1x1 or
2x2 chroma sampling and less Quantization while iview is not using chroma
sampling and then has to use a higher ammount of Quantization to achieve the
same filesize. I remember discussing a similar issue when comparing PS and
Paintshop Pro's jpeg compression about 2 years ago. I may still have the
information from that kicking around somewhere.
Andrew "frugal" Dacey
frugal@xxxxxxxxxxxxxxx
http://www.tildefrugal.net/
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