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

Question

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 1

On later version of QGIS is easier to perform the requested task.

Just open Layer Properties, Style Tab, and make sure Single Symbol is selected. Click the box next to the "Fill" color and choose "Edit.

You will be editing the base color of your polygon bases on RGB colors present on the data table, with a constant border color (you can apply the same criteria if needed).

After hitting "Edit" just put the 3 RGB columns names of your table using the following expression:

color_rgb("R-column","G-column","B-column")

Ok, Apply and your done.

If you need to label and characterize the data in the same manner (for example, to build a legend), just use the Categorized option and apply the same expression to the Symbol and to the Column, identified in the image bellow.

Answer 2

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) :

  <qgis>
  <transparencyLevelInt>255</transparencyLevelInt>
   <classificationattribute>MAPCODE</classificationattribute>
   <uniquevalue>
      <classificationfield>MAPCODE</classificationfield>
         <symbol>
             <lowervalue>Oc</lowervalue>
             <uppervalue>Oc</uppervalue>
             <outlinecolor blue="143" green="255" red="255" />
             <outlinestyle>SolidLine</outlinestyle>
             <outlinewidth>0.26</outlinewidth>
             <fillcolor blue="143" green="255" red="255"/>
             <fillpattern>SolidPattern</fillpattern>
          </symbol>
          <symbol>
             <lowervalue>WAT</lowervalue>
             <uppervalue>WAT</uppervalue>
             <outlinecolor blue="255" green="255" red="255" />
             <outlinestyle>SolidLine</outlinestyle>
             <outlinewidth>0.26</outlinewidth>
             <fillcolor blue="255" green="255" red="255" /> 
             <fillpattern>SolidPattern</fillpattern>
          </symbol>
              and so...
   </uniquevalue>
</qgis>

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...

Answer 3

If you want to classify some vector data in a repeatable way then here is what you can do:

• Load your vector into QGIS.
• Right-click on the layer in the "Layers" pane on the left side of the screen.
• Click on "Properties" in the menu that appears.
• Click on the "Style" tab in the window that appears.
• There should be a drop-down list item on the screen that says "Single Symbol". Click on it, and change its value to "Categorized".
• The interface layout will change, giving you a new option labeled "Column". Click on it and select the "map_symb" field.
• Click the "Classify" button below the large empty field. The empty field will be populated with the contents of the "map_symb" column in your dataset.
• Double-click on the symbols you want to change. In the window that appears, click the button labeled "Change".
• This allows you to change the color for the given attribute value.
• Keep doing this until you have changed colors as required.
• When you're done, you may want to save the style by clicking the "Save Style ..." button. This will allow you to apply the same style to any other dataset, merely by clicking "Load Style ..." when in this same window. This may be what you were trying to achieve by linking the text file with the dataset, which isn't possible AFAIK. Once you have saved the style, though, you will be able to use it on any following datasets (assuming they have the same attribute values). If you open the style's save file, you'll see that it's just plain text, which you can edit if you want. It is of course possible to just write this file yourself in any text editor, but it's faster and easier doing it via GUI.
• Click "Apply" to see your changes and "OK" to close the Layer Properties window.