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December 1994
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1995.01.06 04:55 "Re: xtiff", by Ed Millard
>
> Dear Sir(s),
>
> We are using xtiff program (from contrib of Sam Leffler's software on
> TIFF v3.3 ) to view TIFF images.For some images it is displaying the contents
> without any problems (They happen to be of Bits/Sample = 1) but for the other
> we are not able to view the image (They are of Bits/Sample = 8) instead we are
> getting the following error.
>
> xtiff: 24-bit DirectColor visual not available
>
> We would be grateful to you if you can help us out of this problem.
>
> Thanking you, and awaiting for your reply
>
What kind of graphics hardware do you have. 8 bits/sample with 3 samples
per pixel(i.e. RGB) will need 24 bitplanes to display. xtiff is requesting
a 24 bit Truecolor X visual from your X server and your X server is saying
it doesn't have one.
On SGI hardware if you don't have 24 bitplanes we use dithering to
approximate a 24 bit Truecolor visual so xtiff will work on all our systems
though the image will not be of the highest quality on 8 bit machines.
If your on an 8 bit system(not an SGI) without dithering capability 24 bit
Truecolor is not be an option. Systems like this usually only offer
Psuedocolor visuals which use a color map (256 entries on an 8 bit system).
Some viewers can attempt to map the colors in the Truecolor image to the
available 256 colormap entries but this usually results in a serious
degradation in the image.
You can run xdpyinfo on your machine and if you don't see any Truecolor
visuals then the problem is your lacking the graphics hardware or X
server software for this capibility. This is a typical 24 bit Truecolor
visual in xdpyinfo:
visual:
visual id: 0x2e
class: TrueColor
depth: 24 planes
available colormap entries: 256 per subfield
red, green, blue masks: 0xff, 0xff00, 0xff0000
significant bits in color specification: 8 bits
This is a typical Psuedocolor entry:
visual:
visual id: 0x25
class: PseudoColor
depth: 8 planes
available colormap entries: 256
red, green, blue masks: 0x0, 0x0, 0x0
significant bits in color specification: 8 bits
=============================================================================
Ed Millard
Silicon Graphics Inc.
Developer's Support Group
millard@sgi.com
=============================================================================