MS RFC 21: MapServer Raster Color Correction

[Frank Warmerdam]

Steve Lime wrote:
> I like the idea. 
> 
> One comment I have is related to an idea that has been floating around
> for overall map output, especially with tiled output. When Assefa and
> Paul where playing around with 24-bit to 8-bit quantitization (sp?) they
> found that the GD functionality was not consistent- a big problem
> between tiles for something like ka-map. An idea was to allow the
> definition of a pallete that could easily be pre-computed with a tool
> like ImageMagick. You'd render a representative image (big enough to
> capture occurances of all features) in a 24-bit image and reduce to
> 8-bit and save the palette. The palette would be read as the output
> image is created. In the 8-bit context (e.g. 8-bit image in memory) the
> palette would simply serve as a lookup table, in the 24-bit context you
> would pre-load the output image (8-bit) with the palette and us
> gdImageColorClosest to convert individual pixels.
> 
> I was thinking of a MAP level parameter called PALETTE that would take a
> filename. Could the same parameter be used at the LAYER level with
> handling as Frank describes?
> 
> My thought was not to allow definition within the mapfile, rather as an
> external file with just a straight list of RGB values in the output
> file:
> 
> 0,0,0
> 255,0,0
> ...
> 
> So 0 would map to 0,0,0 and so on. The support at the MAP level could be
> just a naive implementation 

Steve,

I think a PALETTE keyword that works as you suggest would be a good idea
for the map object.  In fact, it might also be nice to be able to apply
it to a raster layer as a short cut rather than creating many classes for
some raster images.

However, such an activity is essentially orthagonal to the color correction
proposal which applies individual 8bit lookup tables to each band prior to
other processing.

PS. The proposed map level palette would fix the ka-map tile variation
problem.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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