Proposed RFC on COLORRAMP Support (Bug 13 05)

[Bill Binko]

On Tue, 27 Sep 2005, Ned Harding wrote:

> Theming demographics is one of the most important things my company does
> with a map, but I'm not sure why the need for RAMPBREAKS.  If you are going
> to calculate your own ranges externally (which we do), it seams like at best
> that is a syntax improvement on the current class system, but it doesn't add
> any new functionality.  We haven't had any problems displaying custom breaks
> in the existing system, since the class let you do almost anything.  
> 
> It seams to me the big advantage of color ramps is continuous coloring
> (which there is no way currently to do), for which the scale would be linear
> or logarithmic.  Not that I have a problem with the RAMPBREAKS idea, but it
> does seams like feature creep.  I'd rather see something get done than wait
> for a better solution for something we can already do well enough.  It would
> be too bad if every RFC that comes up ends up getting expanded to the point
> that people are afraid to propose them because they get too big.
> 

Gentlemen,

I agree that this feels like a bit of feature creep.  I can see why it 
would be useful, but I believe there are two ways to accomplish this with 
only the COLORRAMP changes currently in the RFC.

One approach would simply be to manually create several CLASSes using the
existing EXPRESSION keyword and the new COLORRAMP feature.  So, if you
wanted a stop level at '10', you might have this:

LAYER 
 ....
 CLASSITEM somefield
 CLASS
    EXPRESSION ([somefield] < 10)
    COLORRANGE
        MINVALUE 0
	MAXVALUE 10
	MINCOLOR 255 0 0 #red
	MAXCOLOR 255 255 0 #yellow
	RANGEITEM 'somefield'
	METHOD 'linear'
    END
  END
 CLASS
    EXPRESSION ([somefield] >= 10)
    COLORRANGE
        MINVALUE 10
        MAXVALUE 20
        MINCOLOR 255 255 0 #yellow
	MAXCOLOR 0 255 0 #green
        RANGEITEM 'somefield'
        METHOD 'linear'
    END
  END
...

Another solution would be to use Mapscript to essentially do the same 
thing programatically.  I am sure one of the Mapscript gurus could put 
together a function (buildColorRampClasses for example) that essentially 
would allow this (in PHP):

$theLayer = getTheLayerSomehow();
$stops = array(0 => '255 0 0', 10 => '255 255 0', 20 => '0 255 0');
buildColorRampClasses($theLayer, $stops, 'LINEAR', 'somefield');

I believe the Mapscript class I described in the RFC would make this 
fairly simple.  

A final thought would be to create another METHOD that performed this.  
While I'm not necessarily opposed to that, a part of me worries about any
method that would require additional keywords to do its work.

Stephen, I hope this addresses your concerns.  If not, I'm willing to take 
another stab, or we can try it in a separate RFC.

Thanks for the feedback,
Bill