Batch processing a folder of pairs of grids in QGIS-python

Question

I am creating a script to run some processing tools on pairs of rasters - in this case VelocityX and VelocityY raster grids. The files are called .Vx and .Vy and are uniquely identified by names in this format:

• StandardBaseName_0001.Vx
• StandardBaseName_0001.Vy
• StandardBaseName_0002.Vx
• StandardBaseName_0002.Vy
• etc etc

This question has some code to loop over a folder of rasters - can this be adapted for identifying each 'pair' of rasters and feeding their names into variables that I can give to the processing algorithms?

def findRasters (path, filter):
    for root, dirs, files in os.walk(path):
        for file in fnmatch.filter(files, filter):
            yield file

for raster in findRasters(INPUT_FOLDER, '*.tif'):

I was thinking that matching the name (without extension) of the .Vx raster to the name of the .Vy raster might work but I'm not sure how to do it.

Answer 1

After much trial and error I've managed to solve this one I think. You can do it either with files in a folder (answer 01 below) or with loaded files in the QGIS TOC (answer 02 below). I suspect these aren't that elegant but they do seem to work; any suggestions on improvements are welcome.

Answer 01 - this takes an input folder and iterates over the rasters in it.

    ##Lisflood - Calculate Velocity Vectors=name
    ##inputfolder=folder
    ##outputfolder=folder

    #import the os and processing modules
    import os
    import processing

    #iterate through all the files in the input folder
    for filename in os.listdir(inputfolder):
        #Compare the file extensions - this is what we're using the select the files
        if filename.endswith(".Vx"):
            #Set the inputvx and inputvy parameters - you need to specify the full filepath
            inputvx = inputfolder + "/" + filename
            inputvy = inputfolder + "/" + filename [:-3] + ".Vy"
            #Specify an output name - this goes into the final processing tool
            OutputName = outputfolder + '/' + filename [:-3] + '_VelocityVectors.shp' 

#Run whatever tools you want - in this case I'm calculating directional vectors from Vx and Vy velocity grids.          outputs_SAGARASTERCALCULATOR_1=processing.runalg('saga:rastercalculator', inputvx,[inputvy],'sqrt(a^2 + b^2)',True,7,None)
            outputs_SAGARASTERCALCULATOR_3=processing.runalg('saga:rastercalculator', inputvx,[inputvy],'(atan2(a,b)) * (180/pi())',True,7,None)
            outputs_SAGAGRIDVALUESTOPOINTS_1=processing.runalg('saga:gridvaluestopoints', [outputs_SAGARASTERCALCULATOR_3['RESULT']],None,True,0,None)
            outputs_SAGAADDGRIDVALUESTOPOINTS_1=processing.runalg('saga:addgridvaluestopoints', outputs_SAGAGRIDVALUESTOPOINTS_1['SHAPES'],[outputs_SAGARASTERCALCULATOR_1['RESULT']],1,OutputName)

Answer 02 - this one is the same but this one uses layers loaded into the QGIS TOC instead of an input folder so you don't need :

##outputfolder=folder
for layer in QgsMapLayerRegistry.instance().mapLayers().values():
    layerfilepath = layer.dataProvider().dataSourceUri()
    NameStub = layer.name()
    if layerfilepath [-3:] == '.Vy':
        layer.setLayerName(NameStub + '.Vy')
    elif layerfilepath [-3:] == '.Vx':
        layer.setLayerName(NameStub + '.Vx')
    layerfilepath = layer.dataProvider().dataSourceUri()
    NameStub = layer.name()
    if NameStub [-3:] == '.Vy':
        inputvy = layerfilepath
        print inputvy
        inputvx = layerfilepath [:-3] + '.Vx'
        print inputvx
        OutputName = outputfolder + '/' + layerfilepath [:-3] + '_VelocityVectors'