1995.01.05 11:54 "", by Anuraag Agrawal

1995.01.06 04:55 "Re: xtiff", by Ed Millard

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.

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 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 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