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fix TIFF encoder regarding rgbA/RGBA

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Encoder:
We were always using ExtraSamples=1, which means associated-alpha.
But we don't need the (lossy) excursion to rgbA format. We can save
the samples as RGBA directly, by changing ExtraSamples to '2'.
The TIFF encoder now checks the colorspace properly, to handle
premultiplied format as well as non-premultiplied.

Decoder:
The result of TIFFReadRGBAImageOriented() is always pre-multiply.
So, in case an alpha channel is present, we need to unmultiply it before
calling WebPPictureImportRGBA().

See:
https://www.itu.int/itudoc/itu-t/com16/tiff-fx/docs/tiff6.pdf (page 31)
and also http://www.asmail.be/msg0055469184.html

Change-Id: I3258bfdb0eb2e1a53d6c04414f55edb2926c938c
This commit is contained in:
Pascal Massimino
2017-02-25 09:10:38 -08:00
parent 17e3c11f99
commit 9273e441da
3 changed files with 62 additions and 7 deletions
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5
examples/dwebp.c
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@ -332,9 +332,8 @@ int main(int argc, const char *argv[]) {
case BMP:
output_buffer->colorspace = bitstream->has_alpha ? MODE_BGRA : MODE_BGR;
break;
case TIFF: // note: force pre-multiplied alpha
output_buffer->colorspace =
bitstream->has_alpha ? MODE_rgbA : MODE_RGB;
case TIFF:
output_buffer->colorspace = bitstream->has_alpha ? MODE_RGBA : MODE_RGB;
break;
case PGM:
case RAW_YUV: