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So I have this .gif file that has values from 0-255 but not entirely grayscale. I tried the r.report and it does not work. I also tried the raster calculator like: "myraster@1"= 1 (because I want to count how many pixels have the value 1) then I get a completely wrong output with nonsense values. Is it that the r.report and the raster calculator can't handle .gif files or am I doing something wrong? The white pixels have a value of 1 and I want to count these pixels. Here is the .gif file. enter image description here

If it helps I am adding the colortable of the .gif: enter image description here

Like I said, I need the number of white pixels that have the value 1.

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  • is there a color map or is ita muliband raster ?
    – radouxju
    Sep 17, 2015 at 19:40
  • @radouxju no it only has 1 band.
    – Stelios M
    Sep 17, 2015 at 19:46

2 Answers 2

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6

Try out my code (as script) at the Python Console of QGIS:

from os import path
import struct
from osgeo import gdal

def countRasterValue(val):

    layer = iface.activeLayer()
    provider = layer.dataProvider()

    fmttypes = {'Byte':'B', 'UInt16':'H', 'Int16':'h', 'UInt32':'I', 'Int32':'i', 'Float32':'f', 'Float64':'d'}

    my_path = provider.dataSourceUri()

    (root, filename) = path.split(my_path)

    dataset = gdal.Open(my_path)

    band = dataset.GetRasterBand(1)

    print "rows = %d columns = %d" % (band.YSize, band.XSize)

    BandType = gdal.GetDataTypeName(band.DataType)

    print "Data type = ", BandType

    print "Executing for %s" % filename
    print "in %s" % root

    count_value = 0

    for y in range(band.YSize):

        scanline = band.ReadRaster(0, y, band.XSize, 1, band.XSize, 1, band.DataType)
        values = struct.unpack(fmttypes[BandType] * band.XSize, scanline)

        for value in values:
            if value == val:
                count_value += 1

    print "Raster count = %d of %d" % (count_value, val)

    dataset = None

countRasterValue(1)

I tested my code with a gif (Graphics Interchange Format) raster whose values were between 1 and 3 (see next image).

enter image description here

It worked with a count of 205 for 1 values (red color).

enter image description here

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2
  • You are a wizard. It worked like a charm. So I guess the lesson here is that the processing tools of QGIS can't handle .gif files properly?
    – Stelios M
    Sep 18, 2015 at 7:18
  • 1
    You're welcome. As answer to your question, I don't know. By the way, you have a recipe that it could be adapted for many purposes (stats determination, raster characterization, raster classification, etc). I used it for creating some useful plugins.
    – xunilk
    Sep 18, 2015 at 9:45
4

Simpler and faster with gdalnumeric (gdal+numpy):

import gdalnumeric

raster_file = gdalnumeric.LoadFile("/path/to/file.tif")
pixel_count = (raster_file == 1).sum()  # for pixel value = 1
print(pixel_count)

Inside Qgis (for the active layer selected):

import gdalnumeric
layer = iface.activeLayer()
provider = layer.dataProvider()

raster_file = gdalnumeric.LoadFile(provider.dataSourceUri())
pixel_count = (raster_file == 1).sum()  # for pixel value = 1
print(pixel_count)
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