Hello, I have version 3.6.2 of GDAL and I need to crop an image using a contour
from a GML file, so far there is no problem.
The problem begins when the original image is a GEOTIFF with transparency and
it generates a cropped image, the information of both images is a little lower.
The color table changes from having 2 values to having 256, this causes it to
lose transparency although it has the label "Mask Flags: PER_DATASET ALPHA"
which indicates that it has an alpha channel.
My question is, why does the color table change?, and is there any parameter
that can be used with gdalwarp or gdal_translate to avoid this and keep the
image transparent?
I use this command:
gdalwarp.exe -cutline fichero.gml -crop_to_cutline -dstalpha ficheroIN
ficheroOUT --config GDAL_DATA rutaGDAL
ORIGINAL IMAGE
Driver: GTiff/GeoTIFF
Size is 5616, 3655
…
Metadata:
TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
TIFFTAG_XRESOLUTION=300
TIFFTAG_YRESOLUTION=300
Image Structure Metadata:
INTERLEAVE=BAND
Corner Coordinates:
…
Band 1 Block=5616x1493 Type=Byte, ColorInterp=Palette
Image Structure Metadata:
NBITS=1
Color Table (RGB with 2 entries)
0: 0,0,0,255
1: 255,255,255,255
CROPPED IMAGE
Driver: GTiff/GeoTIFF
Size is 5605, 2689
….
Metadata:
TIFFTAG_DATETIME=2024:02:09 08:49:12
TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
TIFFTAG_SOFTWARE=Microsoft Windows Photo Viewer 10.0.19041.1
TIFFTAG_XRESOLUTION=300
TIFFTAG_YRESOLUTION=300
Image Structure Metadata:
COMPRESSION=LZW
INTERLEAVE=PIXEL
Corner Coordinates:
….
Band 1 Block=5605x1 Type=Byte, ColorInterp=Palette
Mask Flags: PER_DATASET ALPHA
Color Table (RGB with 256 entries)
0: 0,0,0,255
1: 255,255,255,255
2: 0,0,0,255
3: 0,0,0,255
….
252: 0,0,0,255
253: 0,0,0,255
254: 0,0,0,255
255: 0,0,0,255
I await your response, thank you and regards.
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