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To: DIRK LOEDDING Number: 4581
From: GENE KWIECINSKI Refer #: None (Echo)
Date: 01-23-01 23:19 Recvd: No
Subj: Re: bulk zip batch file? Conf: 63 I_DOS.Tips
BBS: The Safe House -------------------------------------------------------------------------------

I couldn't tell you exactly how it's formatted, but I'd imagine you
need a bit for each of the 3 colours that define a pixel, times
whatever your bit depth is, and that can be a lot of bits per pixel.

So how is it that a .GIF or .JPG file can be so small compared to the DL>.BMP it was converted from, without any apparent loss of picture DL>quality/information?

BMPs (BitMaP) are strictly uncompressed image data, as if you had 2
nested loops and just grabbed an entire image pixel-by-pixel. Eg, a 640x480x256 image would be 640x480B large (256 colors == 1 byte/pixel)
or around 300kB (add some overhead for headers, end-of-row, color map,
etc.). A 1280x1024x256 would be over 1.2MB. Make that 24-bit color,
and that balloons up to over 3.6MB.

A GIF compresses data linearly, ie, it takes raw data and compresses it,
so that a 128 and 129 won't compress at all (lossless, after all). A
JPEG/JFIF compresses based on image data, so that 128 is close enough to
129 for it to figure, "Hey, close enough", and compresses 'em as 2
same-color pixels, plus it knows enough about hues, saturation, and
other color factors, not just the blind r/g/b conversion, so that it'll compress more intelligently, if the colors are *VISUALLY* similar
enough.

Eg, if you have a 640x480x256 image of all blue with one aqua pixel in
the middle somewhere, a GIF will compress and reconstitute it to the
blue field with one aqua pixel, whereas a JPEG will compress it to one
big blue field, tossing the "anomaly" as useless.
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