Tim,
Reviewing the warping logic, I see we have support for a *input* mask,
but not creating/updating an *output* mask. AFAICS, there isn't any
particular strong reason not to have support for that. Likely just a few
hours of coding away. Your best workaround is to generate an output with
an alpha band (possibly as a VRT) and use gdal.Translate(dst, src,
options="-b 1 -b 2 -b 3 -mask 4") for example to transform a RGBA into a
RGB+mask product
Even
Le 12/02/2025 à 23:47, Tim Harris via gdal-dev a écrit :
Not sure if this is a bug, or expected behavior, or user error. If I
warp one TIF into another, and both have nodata masks, it seems that
gdalwarp isn't updating the destination TIF's nodata mask to unmask
the new pixels.
Here's a short script to generate two example TIFs. One has a red box
in its upper left corner, the other has a green box in its upper right
corner. Both have nodata masks so that only the colored boxes are visible.
---
import numpy as np
from osgeo import gdal
gdal.UseExceptions()
gdal.SetConfigOption("GDAL_TIFF_INTERNAL_MASK", "YES")
drv = gdal.GetDriverByName("GTiff")
zeros = np.zeros((3, 1024, 1024))
red = np.copy(zeros)
red[0, 100:200, 100:200] = 255
green = np.copy(zeros)
green[1, 100:200, 800:900] = 255
ds = drv.Create("red.tif", 1024, 1024, 3, gdal.GDT_Byte)
ds.SetGeoTransform((0, 1, 0, 0, 0, -1))
ds.WriteArray(red)
ds.CreateMaskBand(gdal.GMF_PER_DATASET)
ds.GetRasterBand(1).GetMaskBand().WriteArray(red[0, :, :])
ds = None
ds = drv.Create("green.tif", 1024, 1024, 3, gdal.GDT_Byte)
ds.SetGeoTransform((0, 1, 0, 0, 0, -1))
ds.WriteArray(green)
ds.CreateMaskBand(gdal.GMF_PER_DATASET)
ds.GetRasterBand(1).GetMaskBand().WriteArray(green[1, :, :])
ds = None
---
Then if you copy red.tif to warp.tif, then warp green.tif into warp.tif:
cp red.tif warp.tif
gdalwarp green.tif warp.tif
I would expect the result to be a combined image with both the red and
green boxes. But when I load this result into QGIS, I only see the red
box. If I change the layer in QGIS to ignore the mask band, I do see
both boxes (and the remaining image is black, as expected). So the
pixel data is there, but the mask didn't get updated.
Is there a way to get gdalwarp to update the nodata mask
automatically? Maybe some hidden setting I'm missing? I can work
around this by separately warping the masks, then copying the result
into the final TIF's mask band. But it would be nice if the warp took
care of this for me.
Thanks
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