[gdal-dev] NITF YCbCr To RGB?

[M Lewis]

We have a software component where users can view/zoom/pan different map
types.
We use GDAL to read the specific part of the image using RasterIO as raw
bytes and then convert to a bitmap for display.
We are now incorporating viewing NITF files and am a little confused in how
the NITF driver handles YCbCr colour space (which some of the NITFs have).
I have found these little snippets from Frank that I found in another post
from 2005:

/'....Othewise YCbCr images are automatically converted to RGB internally.
However, even in the (default) case where YCbCr images are converted to RGB
internally I *think* that the NITFDataset will still return the YCbCr band
types. This is a bug, and if you want it fixed please file a bug'/

Also in the NITFDataset code there is a comment:

/ /* FIXME? Does it make sense if the JPEG/JPEG2000 driver decodes */
                /* YCbCr data as RGB. We probably don't want to set */
                /* the color interpretation as Y, Cb, Cr *//

Here is a typical gdalinfo dump of a NITF file that I have to deal with:

Band 1 Block=640x1 Type=Byte, ColorInterp=YCbCr_Y
  Overviews: 320x240, 160x120
  Image Structure Metadata:
    COMPRESSION=JPEG
Band 2 Block=640x1 Type=Byte, ColorInterp=YCbCr_Cb
  Overviews: 320x240, 160x120
  Image Structure Metadata:
    COMPRESSION=JPEG
Band 3 Block=640x1 Type=Byte, ColorInterp=YCbCr_Cr
  Overviews: 320x240, 160x120
  Image Structure Metadata:
    COMPRESSION=JPEG

The question is can it be assumed (based on Franks comments) that in the
decoding by the JPEG driver that Band 1 has been converted to Red even
though it states above that the band is YCbCr?

Also if this is not the case and the bands are YCbCr, can somebody give any
clues of converting a YCbCr image in raw data bytes to an RGB bitmap?

Thanks

Martin



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