[gdal-dev] gtiff with internal mask

[Even Rouault]

Le mercredi 17 juillet 2013 14:23:29, Duarte Carreira a écrit :
> Hi Even.
> 
> Thanks so much for your tip! It works. I did have to specify I did not want
> an alpha band when cutting with the shapefile:
> 
> gdalwarp -multi -wm 480 --config GDAL_CACHEMAX 256 -co photometric=ycbcr
> -co compress=jpeg -co alpha=no --config GDAL_TIFF_INTERNAL_MASK YES
> -cutline shapes\index_diss.shp vrtini.vrt testmask.tif

Hum, what I don't understand is how the above will produce a mask band in the 
output file...

Does gdalinfo on testmask.tif show something like :

Band 1 Block=.... Type=Byte, ColorInterp=Red
  Mask Flags: PER_DATASET 
Band 2 Block=.... Type=Byte, ColorInterp=Green
  Mask Flags: PER_DATASET 
Band 3 Block=... Type=Byte, ColorInterp=Blue
  Mask Flags: PER_DATASET 

I think you would need to do the above in 2 steps:
1) gdalwarp to an uncompressed RGBA TIFF
2) gdal_translate to turn the RGBA into a YCbCr JPEG-compressed GTiff + mask 
band

> 
> Seems arcgis recognizes the masked tiff, but is much much slower displaying
> than using a "normal" alpha band.

Perhaps adding -co TILED=YES will help ? (random guess)

> QGIS does need the vrt trick but then is
> as fast as usual.
> 
> Also, the size is must smaller when using an internal mask: from a 219MB
> rgba,band interleaved,jpegd image to a 27MB ycbcr,pixel interleaved,jpegd
> masked tiff (8x smaller). Unless I missed something...

Not completely surprising although more important than I would have expected. 
YCbCr compression usually gives a 2x to 3x compression bonus, and due to the 
usual nature of mask bands, the 1bit deflate compression of the mask band will 
give better results (both visually and in size) that the 8bit JPEG compression 
of the alpha band.

> 
> Thanks again!
> Duarte
> 
> -----Mensagem original-----
> De: Even Rouault [mailto:even.rouault at mines-paris.org]
> Enviada: terça-feira, 16 de Julho de 2013 18:58
> Para: gdal-dev at lists.osgeo.org
> Cc: Duarte Carreira
> Assunto: Re: [gdal-dev] gtiff with internal mask
> 
> Le mardi 16 juillet 2013 12:29:37, Duarte Carreira a écrit :
> > I'm trying to create gtiffs with transparent areas where there are
> > voids between my original images.
> > 
> > I have successfully done it by using an alpha band but this process
> > created 3x larger images. I think partly because of an additional 4th
> > band and partly because I cannot use ycbcr to compress with jpeg. So I
> > thought of using internal masks expecting to get smaller images but
> > cannot get it to work.
> > 
> > When converting from the rgba image using -mask 4 I cannot "see" the
> > transparency and get black areas:
> > 
> > gdal_translate -b 1 -b 2 -b 3 -ma sk 4 --config
> > GDAL_TIFF_INTERNAL_MASK YES test_alpha.tif test_internalmask.tif
> > 
> > I tried looking at the images using qgis and paint.net.
> 
> I know QGIS doesn't use the GDALGetMaskBand() API, so no real surprise
> (that would likely be a possible QGIS enhancement). And paint.net probably
> doesn't support this feature of the TIFF format either.
> 
> MapServer does support mask bands however.
> 
> For QGIS, you could workaround that however. Imagine that you have a YCbCr
> TIFF + internal mask in.tif.
> 
> You can do : "gdal_translate -of VRT -b 1 -b 2 -b 3 -b mask in.tif out.vrt"
> 
> And QGIS should handle out.vrt as a regular RGBA dataset.
> 
> > Any hints or help appreciated.
> > 
> > Thanks,
> > Duarte
> > 
> > 
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> > Duarte Carreira
> > Diretor | Dep. Informa??o Geogr?fica e Cartografia
> > 
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