QGIS » QGIS Application

This issue seems related to #20586.

In QGis 2.18 we were using "gdalogr:warpreproject" passing '-srcnodata {some_value} -dstalpha' as 'extra' param. The processing toolbox allowed setting it using the item 'Additional creation parameters'.
In our case, this 'extra' param was needed to correctly get the transparency on a given raster.

In QGis 3.4 and 3.5(master), this 'extra' argument is not available any more: nor in the dialog opened from the processing toolbox, nor in the arguments shown by processing.algorithmHelp("gdal:warpreproject")

So, with QGis 3 we have lost the availability to pass gdal the extra params -srcnodata and -dstalpha.

Dirty workaround¶

By the way, we have found and we are using a quite dirty trick to have gdal receive this -srcnodata and -dstalpha parameters. We pass 'options' set with a value such as this:

'COMPRESS=JPEG|JPEG_QUALITY=75|-dummyworkaround -srcnodata {} -dstalpha'

That, just by chance, seems that makes gdal receive a "-co -dummyworkaround" that is ignored, and then the -srcnodata and -dstalpha that we need...

04-12-2018 16:39:19 - gdalwarp -s_srs EPSG:32635 -t_srs EPSG:32635 -r near -ot Byte -of GTiff -co COMPRESS=JPEG -co JPEG_QUALITY=75 -co -dummyworkaround -srcnodata -9999.0 -dstalpha D:\users\jmperez\temp\newTpzLayer7offnjok\tmp_merged_via_virtual_raster.tif D:\users\jmperez\temp\newTpzFilemfoh41a0.tif
04-12-2018 16:39:19 - Warning 6: creation option '-dummyworkaround' is not formatted with the key=value format
04-12-2018 16:39:19 - Creating output file that is 269P x 177L.
04-12-2018 16:39:19 - Processing D:\users\jmperez\temp\newTpzLayer7offnjok\tmp_merged_via_virtual_raster.tif [1/1] : 0...10...20...30...40...50...60...70...80...90...Warning 1: TIFFReadDirectory:Sum of Photometric type-related color channels and ExtraSamples doesn't match SamplesPerPixel. Defining non-color channels as ExtraSamples.
04-12-2018 16:39:19 - 100 - done.