I was always thinking that JPEG doesn't support transparency so that it would not be possible to convert a 4 bands GeoTIFF using JPEG compression (without using internal binary mask).
However, I just did a gdal_translate -ot byte -scale -CO COMPRESS=JPEG 4bands.tif rgba_jpeg.tif and it "worked".
The gdalinfo output on the new file reports 4 bands dataset with Gray,Green,Blue,Alpha colors.
Does the TIFF specification allows that?
I still have to read it again to check if/how alpha and JPEG compression are related, but I'm wondering if you have any feedback on this topic in order to understand if a "standard" GeoTIFF reader should be able to deal with that or not and how to deal with the Alpha band (which at this point would be a Lossy band, containing artifacts).
Feedbacks are welcome.
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> Dear all,
> I was always thinking that JPEG doesn't support transparency so that it
> would not be possible to convert a 4 bands GeoTIFF using JPEG compression
> (without using internal binary mask).
> However, I just did a gdal_translate -ot byte -scale -CO COMPRESS=JPEG
> 4bands.tif rgba_jpeg.tif and it "worked".
> The gdalinfo output on the new file reports 4 bands dataset with
> Gray,Green,Blue,Alpha colors.
I guess you have a .aux.xml file with color interpreation in them. To get a proper RGBA file, add -co PHOTOMETRIC=RGB
> Does the TIFF specification allows that?
I didn't find anything in the TechNote 2, which defines JPEG in TIFF compression, that would restrict using RGBA
Using imagemagick or gimp seems to show they can decode JPEG-compressed RGBA files correctly, but I've no idea how interaporable it is outside of libtiff-based decoders (which they are)
> I still have to read it again to check if/how alpha and JPEG compression
> are related, but I'm wondering if you have any feedback on this topic in
> order to understand if a "standard" GeoTIFF reader should be able to deal
> with that or not and how to deal with the Alpha band (which at this point
> would be a Lossy band, containing artifacts).
Yes, the alpha band will be lossily compressed. I guess it would be possible to tweak the encoder to use a very conservative quantization table for the alpha component so as to limit the loss on this channel (not implemented right now in GDAL)
Basically, you are limited by the combined limitations of TIFF and JPEG spec
So for a pixel-interleaved TIFF (the default with GDAL), up to 4 components (since the JPEG spec limits to 4 components per JPEG "scan", and libtiff only knows to drive libjpeg to generate a single scan), which could be RGBA or CMYK depending on the PhotometricInterpreation and ExtraSamples tag values.
For a band-interleave TIFF ( -co INTERLEAVE=BAND), you can have an arbitrary number of bands.
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