QGIS Raster Calculator using Python to combine two layers

Question

I am a QGIS beginner and I want to combine two layers using the raster calculator via Python. For the beginning, I simply want to add the two layers but currently, I don't get an output file.

Code is the following:

lyr1 = QgsRasterLayer(r'C:\layer1.tif')
lyr2 = QgsRasterLayer(r'C:\layer2.tif')
output = r'C:\output.tif'
entries = []

ras = QgsRasterCalculatorEntry()
ras.ref = 'ras@1' 
ras.raster = lyr1
ras.bandNumber = 1
entries.append(ras)


ras = QgsRasterCalculatorEntry()
ras.ref = 'ras@2'
ras.raster = lyr2
ras.bandNumber = 1
entries.append(ras)

calc = QgsRasterCalculator('ras@1 + ras@2', output, 'GTiff', \
lyr1.extent(), lyr1.width(), lyr1.height(), entries)
calc.processCalculation()

Answer 1

For visualizing an output file in QGIS Map Canvas, you need additional bottom lines in following code for retrieving new computed layer.

lyr1 = QgsRasterLayer(r'C:\layer1.tif')
lyr2 = QgsRasterLayer(r'C:\layer2.tif')
output = r'C:\output.tif'
entries = []

ras = QgsRasterCalculatorEntry()
ras.ref = 'ras@1' 
ras.raster = lyr1
ras.bandNumber = 1
entries.append(ras)


ras = QgsRasterCalculatorEntry()
ras.ref = 'ras@2'
ras.raster = lyr2
ras.bandNumber = 1
entries.append(ras)

calc = QgsRasterCalculator('ras@1 + ras@2', output, 'GTiff', \
lyr1.extent(), lyr1.width(), lyr1.height(), entries)
calc.processCalculation()

layer_sum = QgsRasterLayer(output,
                           'output')

QgsProject.instance().addMapLayer(layer_sum)