Batch NDVI calculation with PyQGIS

Question

I'm trying to run an NDVI calculation on a set of raster layers. The approach is to adapt this QGIS 2.x code to run in GQIS 3.x. A far as I'm aware it's the "legendInterface" that's changed, but I'm getting lost trying to suss out how it changed, and what to use as an alternative.

lddLrs = qgis.utils.iface.legendInterface().layers()
path = "/QGIS/Project/"
for lyr in lddLrs:
    entries = []
    #Define red band#
    ras1 = QgsRasterCalculatorEntry()
    ras1.ref = 'layer.name() +@121'
    ras1.raster = lyr
    ras1.bandNumber = 121
    entries.append(ras1)
    #Define nir band#
    ras2 = QgsRasterCalculatorEntry()
    ras2.ref = 'layer.name() +@192'
    ras2.raster = lyr
    ras2.bandNumber = 192
    entries.append( ras2 )
    calc = QgsRasterCalculator( '("layer.name() +@192" -  "layer.name() 
   +@121") / ("layer.name() +@192" + "layer.name() +@121")', path + lyr.name() 
   + "_NDVI.tif", 'GTiff', lyr.extent(), lyr.width(), lyr.height(), 
   entries )
   calc.processCalculation()

Answer 1

Firstly, to update the first line of your code, I would suggest something like this to collect all raster layer objects in the current project:

lddLrs = [l for l in QgsProject().instance().mapLayers().values() if isinstance(l, QgsRasterLayer)]

Secondly, unless I am missing something, it looks like you are referencing a variable "layer" e.g. layer.name() which is undefined. You define a variable lyr when you construct your for loop- this is what will store the layer object on each iteration of the loop.

Therefore, I think you should change layer.name() to lyr.name() throughout your code.

Also, creating a string like: ras1.ref = 'layer.name() +@121' will not work. You should use string formatting such as:

ras1.ref = '{}@121'.format(lyr.name())

I have not tested your script, but it looks like it should work with those changes.

Answer 2

Okay, sussed it thanks to Ben and others on this thread: QGIS python script loop for raster calculator

It looks like the references to the layers in the calculation weren't needed, and my pathway wasn't quite right.

Anyone confused and looking to batch calculate NDVIs in QGIS 3x, this worked for me:

from qgis.analysis import QgsRasterCalculatorEntry, QgsRasterCalculator
from qgis.core import QgsProject

lddLrs = lddLrs = [l for l in QgsProject().instance().mapLayers().values() if isinstance(l, QgsRasterLayer)]
path = "/desired/output/location"
for lyr in lddLrs:
    entries = []
    r = QgsRasterCalculatorEntry()
    ir = QgsRasterCalculatorEntry()
    r.ref = 'r@121'
    ir.ref = 'ir@192'
    r.raster = lyr
    ir.raster = lyr
    r.bandNumber = 121
    ir.bandnumber = 192
    entries.append( r )
    entries.append( ir )
    calc = QgsRasterCalculator( '(ir@192 - r@121) / (ir@192 + r@121)', path + lyr.name() + "_c3_ndvi.tif", 'GTiff', lyr.extent(), lyr.width(), lyr.height(), entries )
    calc.processCalculation()