Re: 256 web page

[P3D Greg Marshall <greg.marshall@xxxxxxxxxxxxxxx>]

Jonathan Gross  wrote a very informative and useful post on this topic,
but
I am compelled to correct two errors (that is, as I understood his words
they
are erroneous):

"High color" mode is not limited to 256 colors per screen.  This mode is
a
(more or less) uniform distribution of colors like "true color" mode,
except
that the accuracy of each color component (R,G, and B) is limited to 5
or
6 bits for a total of 32K or 65K colors.  For many images this is nearly
indistinguishable from true color.  You run into contouring problems
when
the image contains large areas of gradual shading.

JPEG does affect spatial resolution.  I always encourage people to use
the 
highest quality level that will produce an acceptable file size.  What
happens 
as you decrease the quality level is that more and more high-frequency
information is discarded, which results both in blurring and (usually
more
apparent) disparities between adjacent blocks (JPEG processes image
data in blocks of 8 by 8 pixels).  With more severe compression blurring
becomes very bad and you begin to see contouring as large sections of
adjacent pixels become exactly the same color.

GIF is great for drawings and icons and such.  JPEG or some other
compressed true color format should generally be used for photos.

Greg Marshall


------------------------------