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I have a raster layer and a polygon layer. Polygon layer is inside of raster layer and it's extent is smaller than raster layer. Some pixels of raster layer inside the polygon have negative value. I want to change the negative values based on maximum pixel value that is inside the polygon in GDAL or PyQGIS. So In first step I must find the max value based on below code:

r_lyr = QgsRasterLayer('.../aa.tif', 'Raster')
v_lyr= QgsVectorLayer('../bb.shp','bb',"ogr") 
v_ext = v_lyr.extent()
#(xmin, xmax, ymin, ymax) = (v_ext.xMinimum(), v_ext.xMaximum(), v_ext.yMinimum(), v_ext.yMaximum())

provider = r_lyr.dataProvider()
r_ext = provider.extent()


rows = r_lyr.height()
cols = r_lyr.width()

block = provider.block(1, v_ext, cols, rows)
lst = []
for i in range(rows):
    for j in range(cols):
        #print (block.value(i,j)),
        lst.append(block.value(i,j))
max_v = max(lst)

In second step it is important to get pixels inside the polygon and then change them to maximum value.

In @xunilk's answer to Changing pixel values of raster data from QGIS python plugin there is a way to change the pixel value but by this answer it is possible to do for full extent of raster layer.

How can I merge my code with @xunilk's code for doing it?

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    If you wish to also ask about GDAL then please do that in a separate question so that this one can remain focused on PyQGIS.
    – PolyGeo
    Oct 21, 2019 at 21:15

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