[GRASS-user] error message coming with r.colors

[Nikos Alexandris]

On Wed, 2008-08-06 at 01:14 +0100, Glynn Clements wrote:
> Nikos Alexandris wrote:
> 
> > > The -g and -e switches make grey.log and grey.eq redundant
> > 
> > By the way, something I don't understand (copy-paste from the manual):
> > 
> > The -e flag equalizes the original raster's color table. It can preclude
> > the need for grey.eq rule, when used as -e color=grey. Note however,
> > that this will not yield a color table identical to color=grey.eq,
> > because grey.eq scales the fraction by 256 to get a grey level, while -e
> > uses it to interpolate the original colour table.
> 
> It then goes on to say:
> 
> 	If the original colour table is a 0-255 grey scale, -e is
> 	effectively scaling the fraction by 255. Different algorithms
> 	are used. -e is designed to work with any color table, both
> 	the floating point and the integer raster maps.
> 
> Essentially the only difference is in how a floating-point value
> between 0 and 1 is converted to an integer between 0 and 255. In
> practice, the difference will be invisible to the eye.
> 
> Here's a concrete example:
> 
> 	$ r.colors elevation.dem -e color=grey
> 	Reading elevation.dem ...
> 	Color table for <elevation.dem> set to grey
> 	$ r.mapcalc 'tmp1 = r#elevation.dem'
> 	$ r.colors elevation.dem color=grey.eq
> 	Reading elevation.dem ...
> 	Color table for <elevation.dem> set to grey.eq
> 	$ r.mapcalc 'tmp2 = r#elevation.dem'
> 	$ r.mapcalc 'diff = tmp1 - tmp2'
> 	$ r.stats -c diff
> 	-1 155417
> 	0 147001
> 	r.stats complete.
> 
> So, roughly half the cells are identical between the two, while half
> differ by one intensity level.

Thank you for your time and the concrete example. Nikos