[GRASS5] GRASS very slow with raster maps > 8000 colors

[Eric G. Miller]

On Thu, Mar 08, 2001 at 08:25:51PM +0000, Markus Neteler wrote:
> Hi again,
> 
> some of you already know: GRASS is very slow displaying raster 
> maps > 8000 colors. Just curious (as I don't know too much about this
> problem): is there are chance to speed up GRASS' color system?
> I don't understand the reason for this slowness.

8000 is a *big* colormap.  So each cell's color has to be looked up in
a 8000 element long structure of colors.

> To give you an idea: a 250x250 pixel color infrared image takes
> 4sec to display (3.5sec black screen, then rather fast display)
> on a 1Ghz Athlon. How do "xv", "gimp" etc. manage this color
> troubles? 250x250 pixel are not really much.

You're comparing pure image formats (all they have is color) and cells
that have some information with an associated color map.  In the first,
there's a direct correspondance while the second requires a palette/rule
lookup (rules will be faster than large pallette I think).  Things like
GIF/TIFF with palette max out at 256 entries -- otherwise you have to
go to 24bit RGB pixels.

It might be useful to have a pure "image" type in GRASS.  Then the cell
value is its color.  But maybe that's too cumbersome in practice?  It's
be nice to be able to use arbitrary images in GRASS displays for
background (or other) purposes.  Classic example I use at work all the
time is scanned DRG images stored on CD-ROMs.  It's worth thinking
about...

-- 
Eric G. Miller <egm2 at jps.net>

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