gdal python background not really transparent

Question

When I merge my geotiffs using buildVRT and translate from gdal the transparity is showed as transparity blocks instead of the content of layer behind it.

see image:

used code for merging geotiff:

demList = glob.glob(os.path.dirname(os.path.abspath(__file__)) + "//..//icons//*.tif")
print(demList)

VRToptions = gdal.BuildVRTOptions(srcNodata="255 255 255")
vrt = gdal.BuildVRT(os.path.dirname(os.path.abspath(__file__)) + "//merged.vrt", demList, options=VRToptions)
translateOptions = gdal.TranslateOptions(creationOptions=["TILED=YES"])
gdal.Translate(os.path.dirname(os.path.abspath(__file__)) + "//mergedDEM.tif", vrt, options=translateOptions)
vrt = None

my question is how i can use the previous layer as background instead of the transparity blocks.

Compleet code with problem: https://github.com/NickSmeding/MergeGeotiff

Please provide a reproducible example or some more information about your dataset(s) — Val, Dec 08 '20 at 13:26
Do you have an option to use a .png file instead? I would try that. — mgmussi, Dec 12 '20 at 00:55
y done that i merge the images before making it a geotiff, was just wondering if it was possible. — N. Smeding, Dec 13 '20 at 18:45

tgrandje · Accepted Answer · 2020-12-15T09:21:13.333

What about this (using rasterio and it's extension rioxarray) and using both your tifs :

import rioxarray
import rioxarray.merge

icon = "./tiffs/horse2.tif"
background = "./tiffs/4c5ae720f274505972ecedf4066bb32dfb28b63e5785030a7eb55b4e9978602c.tif"

# open the rasters
with rioxarray.open_rasterio('./tiffs/horse2.tif') as rds1:
    with rioxarray.open_rasterio(background) as rds2:

        # ensure the rasters have the same projection/transform/shape
        rds3 = rds1.rio.reproject_match(rds2)
        ar = rds3.values
        
        #look for pixels in icon having alpha = 255
        ix = (ar[3] == 255)
        
        #update background pixels with icon
        for k in range(4):
            rds2.values[k][ix]=ar[k][ix]
        
        #save updated raster
        rds2.rio.to_raster("test.tif")

The icon is a bit pixelised, but it's only because the resolution of you background is less than the one in your icon/tif. Maybe you could reproject both the arrays to the maximum resolution of both layers, but IMHO that would be another subject.

(Note that you icon TIF is ok ; I tried to open it in QGIS and you can superpose the layers alright. I also tried to edit it to re-declare a nodata value, but I still got the same result as you. Maybe if your background and icon tif shared the same nodata value there could be a way to compute it without accessing the pixels as I did,...)

thx man, I think this is the best possible optie if you have only access to the geotiff. — N. Smeding, Dec 15 '20 at 09:51
You're welcome. I just realise I've forgotten to explain why the "ar[3]" accessor : this is about rasterio / gdal which is converting the tuples of the image's pixels to independant layers/bands. So when you access an Image's data (with PIL for example) you will call ar[:,:,3] to get the alpha channel though with rasterio it will be ar[3] — tgrandje, Dec 15 '20 at 09:58

gdal python background not really transparent

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