Get indices from pixels with a certain value in PyQGIS

Question

I'm working in the Python-Console of QGIS and I have an object form the class QgsRasterLayer. I want to get the indices of those pixels that have certain pixel value (e.g. I need the indices of all the pixels with a pixel value = 50), but I don't know how.

Answer 1

This script should work:

# lyr = iface.mapCanvas().currentLayer() # You don't need this if you already have a QgsRasterLayer
prov = lyr.dataProvider()

extent = prov.extent()
width = prov.xSize()
height = prov.ySize()
block = prov.block(1, extent, width, height)

for x in range(width):
    for y in range(height):
        val = block.value(y, x)
        if val == 50:
            print x, y

Note that this gives you the indices of the pixels, and not their geographic coordinates.