I am working on QGIS 3.20 on a raster layer symbology using PyQGIS singlebandpseudocolor and min/max values from the target raster layer.

My own color ramp (from blue to red) does not spread homogeneously the raster values, since it uses 0 to 255 range, instead of the min/max values. e.g. for a raster layer with min/max [150-300], the layer will be mostly blueish, whereas a raster layer with min/max [0.1-0.15] will be mostly reddish.

enter image description here

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I would like the min value to be always blue and the max value to be red.

I am missing something I guess in the following code (e.g. for raster_interpol_ibco_clip and repeated for raster_interpol_shannon_clip):

# ibco:
rlayer = QgsProject.instance().mapLayersByName('raster_interpol_ibco_clip')[0]
extent = rlayer.extent()
provider = rlayer.dataProvider()
myColorRamp = QgsColorRampShader()
stats = provider.bandStatistics(1, QgsRasterBandStats.All, extent, 0)
if (stats.minimumValue < 0):
   min = 0  
   min = stats.minimumValue
max = stats.maximumValue
etape = (stats.maximumValue - stats.minimumValue) //4
interval_orange = min + (etape*1)
interval_jaune = min + (etape*2)
interval_vert = min + (etape*3)
lst2 = [ QgsColorRampShader.ColorRampItem(min, QColor(colDic['1']), str(min)),
        QgsColorRampShader.ColorRampItem(interval_orange, QColor(colDic['2']), str(interval_orange)),
        QgsColorRampShader.ColorRampItem(interval_jaune, QColor(colDic['3']), str(interval_jaune)),
        QgsColorRampShader.ColorRampItem(interval_vert, QColor(colDic['4']), str(interval_vert)),
        QgsColorRampShader.ColorRampItem(max, QColor(colDic['5']), str(round(max)))]
myRasterShader = QgsRasterShader()
myPseudoRenderer = QgsSingleBandPseudoColorRenderer(rlayer.dataProvider(), 1, myRasterShader)

1 Answer 1


I found the issue. The code is correct, but I did not know how Python was dealing with floating values! My calculation for the color ramp was rounded and thus wrong.

The solution:

from decimal import Decimal

if (stats.minimumValue < 0):
   min = Decimal('0')
   min = Decimal(stats.minimumValue)

max = Decimal(stats.maximumValue)

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