Rendering color for specific value using PyQGIS

Question

I'm a QGIS beginner and I want to do a specific operation with Python scripts.

I have a GeoTIFF file corresponding to a raster layer with 6 bands, and each of these bands are 2 values (like 0 and 1, or 0 and 255, or 0 and 100 for example). What I need to do is to open each of this band separately.

rlayer = QgsRasterLayer("maps.geotiff", "viewed")

fcn = QgsColorRampShader()
fcn.setColorRampType(QgsColorRampShader.Interpolated)
lst = [ QgsColorRampShader.ColorRampItem(0, QColor(0,0,0)), QgsColorRampShader.ColorRampItem(100, QColor(255,0,0))]
fcn.setColorRampItemList(lst)
shader = QgsRasterShader()
shader.setRasterShaderFunction(fcn)
renderer = QgsSingleBandPseudoColorRenderer(rlayer.dataProvider(), 1, shader)
rlayer.setRenderer(renderer)

QgsProject.instance().addMapLayer(rlayer)

I write this according to the docs finding here : https://github.com/qgis/QGIS-Documentation/blob/master/docs/pyqgis_developer_cookbook/raster.rst#single-band-rasters

This works but not the optimal way but I have only two values and shader goes from 0 to 255 like we can see here:

What I want to do is to specify color only for specific value like proposed in "render type > unique value" with python scripts etc.

Answer 1

You need to convert the raster color table to paletted renderer class data. For that, use QgsPalettedRasterRenderer class.

rlayer = QgsRasterLayer("maps.geotiff", "viewed")
provider = rlayer.dataProvider()

lst = [ QgsColorRampShader.ColorRampItem(1, QColor(0,0,0)),
        QgsColorRampShader.ColorRampItem(100, QColor(255,0,0))]
        
class_data = QgsPalettedRasterRenderer.colorTableToClassData(lst) # <-

band = 1
renderer = QgsPalettedRasterRenderer(provider, band, class_data)  # <-

rlayer.setRenderer(renderer)
rlayer.triggerRepaint()