[Qgis-developer] Pseudo-color raster drawing

Barry Rowlingson b.rowlingson at lancaster.ac.uk
Tue Apr 24 13:04:01 EDT 2007

At last I've found something I can't do with a python plugin[1]...

I've got an ESRI grid file with integer values 1, 2, and 3. I want to 
map this with specific colours for values 1, 2 and 3 (red, amber, green, 
for example).

BUT I can't set a colour table for a QgsRasterLayer (the system gets 
colour tables from the source, and this source doesn't have a colour 
table palette)

AND since sometimes my grid might only have one or two of the values in 
it, the pseudo colour drawing routine won't be consistent in its mapping 
of values to colours

BUT I discovered I could use the grayscale mapping, and use 
set{Min,Max}GrayDouble() to clamp the mapping of values to colours. I 
could draw value 1 as white, value 2 as gray, value 3 as black.

BUT I don't want to use gray scales! Red is danger!

So I patched QgsRasterLayer.cpp so that if userDefinedGrayMinMax is set 
then it maps the values to the pseudocolor palette using the set values. 
  Its about five lines.

Would this little patch be worth putting into Qgis? Or shall we wait 
until someone has completely rewritten the raster table pseudo colour 
mapping to be a bit more general and doesn't have 'FREAK_OUT' hard coded 
into the C++...


[1] I tried making a new class that inherited from QgsRasterLayer, but 
because the gdalDataset member is private I can't get to it from the 
over-ridden 'draw' method!

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