[gdal-dev] gdal2tiles.py of transparent background vrt/png gives black background tiles

[Grégory Bataille]

Hey John,

Is your TIF transparent background using properly the NODATA special value?
If so you can tell gdal2tiles how to interpret and translate the nodata in
RGBA using the -a option

gdal2tiles.py SOURCE DEST -a 0,0,0,0

I think that should produce the desired effect

Let me know


---
Gregory Bataille

On Sun, Apr 23, 2017 at 7:55 AM, John W. Glendening <glendening at drjack.net>
wrote:

> I have been creating a tileset from a .vrt based upon .tif files using
> gdal2tiles.py.  The .tif files consist of lines overlaid upon a
> background map but I now want to eliminate the background map,
> i.e. make that area transparent, so the tiles will have only the lines
> on a transparent background, so they can be used as an overlay.
>
> I successfully produced the desired .tif files with transparent
> background and created a .vrt.  But using gdal2tiles.py gives a black
> background (RGBA=0,0,0,255) where a transparent background is
> expected. (The expected lines _are_ present, on top of the black
> background.)
>
> Thinking that there might be a problem with the gdal2tiles.py
> interpretation of the .vrt, I used gdal_translate to convert it to a
> (large!) .png file and used that as the gdal2tiles.py argument
> instead.  But the background remains black.  As a check of the png
> file, I used gdal_translate to crop out and view a section and its
> background was indeed transparent (RGBA=0,0,0,0) in the expected
> places.
>
> I also tried two other variants of the gdal2tiles.py file that I found
> on the web, one from osgeo (with a 2013 copyright) and one from a
> geophython site - with identical results.
>
> I'm hoping for some guidance here.
>
> FYI an extract of gdalinfo for the .vrt file is:
>  Driver: VRT/Virtual Raster
>  Files: 24ksections.vrt
>         /home/glendeni/HIKES/TRAILMAP/TILES/BigSurTrailmap-
> sectionA1-map.tif
>         ...
>  Size is 37840, 48993
>  Coordinate System is:
>  PROJCS["unnamed",
>      GEOGCS["unnamedellipse",
>          DATUM["unknown",
>              SPHEROID["unnamed",6378137,0]],
>          PRIMEM["Greenwich",0],
>          UNIT["degree",0.0174532925199433]],
>      PROJECTION["Mercator_1SP"],
>      PARAMETER["central_meridian",0],
>      PARAMETER["scale_factor",1],
>      PARAMETER["false_easting",0],
>      PARAMETER["false_northing",0],
>      UNIT["metre",1,
>          AUTHORITY["EPSG","9001"]]]
>  Origin = (-13565727.275918338447809,4356632.541813874617219)
>  Pixel Size = (1.836533043499999,-1.836542168400001)
>  Corner Coordinates:
>  Upper Left  (-13565727.276, 4356632.542) (121d51'46.81"W,
> 36d24'21.64"N)
>  Lower Left  (-13565727.276, 4266654.831) (121d51'46.81"W, 35d45'
> 9.94"N)
>  Upper Right (-13496232.866, 4356632.542) (121d14'19.40"W,
> 36d24'21.64"N)
>  Lower Right (-13496232.866, 4266654.831) (121d14'19.40"W, 35d45'
> 9.94"N)
>  Center      (-13530980.071, 4311643.687) (121d33' 3.10"W, 36d
> 4'48.23"N)
>  Band 1 Block=128x128 Type=UInt16, ColorInterp=Red
>  Band 2 Block=128x128 Type=UInt16, ColorInterp=Green
>  Band 3 Block=128x128 Type=UInt16, ColorInterp=Blue
>  Band 4 Block=128x128 Type=UInt16, ColorInterp=Alpha
>
> The gdalinfo for the .png created by gdal_translate from the .vrt is:
>  Driver: PNG/Portable Network Graphics
>  Files: 24ksections.png
>         24ksections.png.aux.xml
>  Size is 37840, 48993
>  Coordinate System is:
>  PROJCS["unnamed",
>      GEOGCS["unnamedellipse",
>          DATUM["unknown",
>              SPHEROID["unnamed",6378137,0]],
>          PRIMEM["Greenwich",0],
>          UNIT["degree",0.0174532925199433]],
>      PROJECTION["Mercator_1SP"],
>      PARAMETER["central_meridian",0],
>      PARAMETER["scale_factor",1],
>      PARAMETER["false_easting",0],
>      PARAMETER["false_northing",0],
>      UNIT["metre",1,
>          AUTHORITY["EPSG","9001"]]]
>  Origin = (-13565727.275918338447809,4356632.541813874617219)
>  Pixel Size = (1.836533043499999,-1.836542168400001)
>  Image Structure Metadata:
>    INTERLEAVE=PIXEL
>  Corner Coordinates:
>  Upper Left  (-13565727.276, 4356632.542) (121d51'46.81"W,
> 36d24'21.64"N)
>  Lower Left  (-13565727.276, 4266654.831) (121d51'46.81"W, 35d45'
> 9.94"N)
>  Upper Right (-13496232.866, 4356632.542) (121d14'19.40"W,
> 36d24'21.64"N)
>  Lower Right (-13496232.866, 4266654.831) (121d14'19.40"W, 35d45'
> 9.94"N)
>  Center      (-13530980.071, 4311643.687) (121d33' 3.10"W, 36d
> 4'48.23"N)
>  Band 1 Block=37840x1 Type=UInt16, ColorInterp=Red
>  Band 2 Block=37840x1 Type=UInt16, ColorInterp=Green
>  Band 3 Block=37840x1 Type=UInt16, ColorInterp=Blue
>  Band 4 Block=37840x1 Type=UInt16, ColorInterp=Alpha
>
>
> --
> John W. (Jack) Glendening
>
> _______________________________________________
> gdal-dev mailing list
> gdal-dev at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/gdal-dev
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