[gdal-dev] What happens by default when writing RGBA into jpeg?

[Jukka Rahkonen]

Even Rouault <even.rouault <at> mines-paris.org> writes:

> 
> Selon Jukka Rahkonen <jukka.rahkonen <at> mmmtike.fi>:
> 
> > Hi,
> >
> > I would like to know what happens if I convert an original RGBA tiff image
> > into jpeg as
> >
> > gdal_translate -of jpeg rgba_in.tif jpeg_out.jpg
> >
> > I know that for getting a good result I can add "-b 1 -b 2 -b 3" but I am
> > curious to know what happens and how GDAL computes sort of a transparent
> > sepia coloured mask when run without selecting bands.
> 
> It will result in a TIFF with separate bands (PLANARCONFIG_SEPARATE :
>
http://www.awaresystems.be/imaging/tiff/tifftags/planarconfiguration.html), each
> of them JPEG compressed. So for a transparency mask with "instantenous"
changes
> between zones of fully transparence and fully opacity, you'll have JPEG
> artifacts.
> I'm wondering if the "sepia coloured mask" is not caused by a viewer that will
> be confused by this TIFF configuration and would assume a CMYK color space
> instead of RGB.

Hi Even,

Sorry, I must have been unclear. The question was about writing out a real
JPEG file, not JPEG compressed TIFF
The original is of this type:
Band 1 Block=5000x1 Type=Byte, ColorInterp=Red
Band 2 Block=5000x1 Type=Byte, ColorInterp=Green
Band 3 Block=5000x1 Type=Byte, ColorInterp=Blue
Band 4 Block=5000x1 Type=Byte, ColorInterp=Undefined

The command gdal_translate -of jpeg rgba_in.tif jpeg_out.jpg creates a jpeg
file with this gdalinfo

C:\Jukka>gdalinfo jpeg_out.jpg
Driver: JPEG/JPEG JFIF
Files: jpeg_out.jpg
       jpeg_out.jpg.aux.xml
Size is 5000, 5000
Coordinate System is:
PROJCS["NAD83 / Indiana West (ftUS)",
    GEOGCS["NAD83",
        DATUM["North_American_Datum_1983",
            SPHEROID["GRS 1980",6378137,298.2572221010002,
                AUTHORITY["EPSG","7019"]],
            AUTHORITY["EPSG","6269"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4269"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",37.5],
    PARAMETER["central_meridian",-87.08333333333333],
    PARAMETER["scale_factor",0.999966667],
    PARAMETER["false_easting",2952750],
    PARAMETER["false_northing",820208.333],
    UNIT["US survey foot",0.3048006096012192,
        AUTHORITY["EPSG","9003"]],
    AUTHORITY["EPSG","2966"]]
Origin = (2947500.000000000000000,2265000.000000000000000)
Pixel Size = (0.500000000000000,-0.500000000000000)
Metadata:
  AREA_OR_POINT=Area
  TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
  TIFFTAG_XRESOLUTION=1
  TIFFTAG_YRESOLUTION=1
Image Structure Metadata:
  COMPRESSION=JPEG
  INTERLEAVE=PIXEL
  SOURCE_COLOR_SPACE=CMYK
Corner Coordinates:
Upper Left  ( 2947500.000, 2265000.000) ( 87d 6' 8.96"W, 41d27'59.66"N)
Lower Left  ( 2947500.000, 2262500.000) ( 87d 6' 8.95"W, 41d27'34.96"N)
Upper Right ( 2950000.000, 2265000.000) ( 87d 5'36.12"W, 41d27'59.66"N)
Lower Right ( 2950000.000, 2262500.000) ( 87d 5'36.12"W, 41d27'34.96"N)
Center      ( 2948750.000, 2263750.000) ( 87d 5'52.54"W, 41d27'47.31"N)
Band 1 Block=5000x1 Type=Byte, ColorInterp=Red
  Overviews: 2500x2500, 1250x1250, 625x625
  Image Structure Metadata:
    COMPRESSION=JPEG
Band 2 Block=5000x1 Type=Byte, ColorInterp=Green
  Overviews: 2500x2500, 1250x1250, 625x625
  Image Structure Metadata:
    COMPRESSION=JPEG
Band 3 Block=5000x1 Type=Byte, ColorInterp=Blue
  Overviews: 2500x2500, 1250x1250, 625x625
  Image Structure Metadata:
    COMPRESSION=JPEG

The mask I told can be seen in this stackexchange question on the left side
of the image. I do not believe that a viewer plays any role here.

http://gis.stackexchange.com/questions/101393/gdal-tif-to-jpg-creates-dark-image

-Jukka-