Using field-to-RGB mapping for symbology in QGIS?

Using QGIS version 1.7.

I have a plain text file that lists a set of rgb values against a code. I want to use this colour table to colour a polygon layer by mapping one of its attribute fields ('map_symb') to a code in the text file.

the colour table is very long, and looks like this:

$ head gsv1Msymbology.txt
MAPCODE RED GREEN   BLUE
Oc  143 255 255

WAT 255 255 255
Qa  244 250 202
Qdl 195 239 218
Na  248 255 238
Qd2 227 255 190
Qxw 248 255 238
Qns 255 148 83
Qn  255 202 190
.... 

I want to match my 'map_symb' attribute to a value in MAPCODE, and use the corresponding RGB values to colour the polygons.

Is there a gui way to do this?

Answer

You can use Python with ElementTree module :

from string import *
from xml.etree import cElementTree as ET

class symbol:
    def __init__(self,b=[]):
            self.typec= typec

            self.b = b
            self.key = ['MAPCODE','R','G','B']
            self.data = dict(zip(self.key,self.b))
            self.symb = ET.SubElement(typec,"symbol")
            self.lower = ET.SubElement(self.symb, "lowervalue")
            self.upper = ET.SubElement(self.symb, "uppervalue")
            self.outline = ET.SubElement(self.symb,"outlinecolor")
            self.outsty = ET.SubElement(self.symb, "outlinestyle")
            self.outtail = ET.SubElement(self.symb, "outlinewidth")
            self.fillc = ET.SubElement(self.symb,"fillcolor")

            self.fillp = ET.SubElement(self.symb,"fillpattern")

    def creation(self):
            self.lower.text = self.data['MAPCODE']
            self.upper.text = self.data['MAPCODE']
            self.outsty.text="SolidLine"
            self.outtail.text="0.26"
            self.outline.set("red",str(self.data['R']))
            self.outline.set("green",str(self.data['G']))
            self.outline.set("blue",str(self.data['B']))

            self.fillc.set("red",str(self.data['R']))
            self.fillc.set("green",str(self.data['G']))
            self.fillc.set("blue",str(self.data['B']))
            self.fillp.text = "SolidPattern"

# QML file creation
intro = ET.Element("qgis")
transp = ET.SubElement(intro,"transparencyLevelInt")
transp.text = '255'
classatr = ET.SubElement(intro, "classificationattribute")

classatr.text= "MAPCODE"
typec = ET.SubElement(intro,"uniquevalue")
classif = ET.SubElement(typec,"classificationfield")
classif.text="MAPCODE"

# RGB file processing              
def main():
    file = "RGB.txt"
    f= open(file,"r")
    while 1 :

        line = f.readline()
        if not line :
            break
        elem = split(line,',') #or tab, or space, or
        symboltag = symbol(elem)
        symboltag.creation()
     result = ET.ElementTree(intro)
     result.write("RGB.qml")

if __name__ == '__main__':

    main()

The style file generated by this script is (and it works) :

  
  255
   MAPCODE
   
      MAPCODE
         

             Oc
             Oc
             
             SolidLine
             0.26
             
             SolidPattern
          
          
             WAT

             WAT
             
             SolidLine
             0.26
              
             SolidPattern
          
              and so...
   

You can also use the shapefile module ([shapefile])1 for shapefiles with RGB columns

import shapefile ....
[....]
noduplicates = []

def main():
sf = shapefile.Reader("RGBshape")
for rec in enumerate(sf.records()):
    if rec[1][0] not in noduplicates:

        noduplicates.append(rec[1][0])
        symboltag = symbol(rec[1])
        symboltag.creation()      
    else:
        continue

and so...