In libjpeg-turbo 3.0.x, JSAMPLE is always 8-bit. You have to use
J12SAMPLE for 12-bit and J16SAMPLE for 16-bit. GraphicsMagick still
appears to be using libjpeg. The code below would not work with
libjpeg-turbo 3.0.x.
On 2/6/24 3:47 PM, Bob Friesenhahn wrote:
On Tue, 6 Feb 2024, Ellen Johnson wrote:
Hi Bob and Even,
(I'm copying Even's reply in this thread, and adding Darrell from
libjpeg-turbo after consulting with him).
Thank you for confirming libtiff does not support 16-bit JPEG.
This is very puzzling to me. I think it means that before I built
libtiff v4.6.0 with libjpeg-turbo v3.0.0, we must have had a patch or
modification in our libtiff version or our old libjpeg v6b version
that allowed us to read the 16-bit linearRaw subimage in the TIFF file.
I'll do more investigation on our end.
Something I find interesting is that in GraphicsMagick, the JPEG
reader is not using the libjpeg/libjpeg-turbo dual/triple mode with
its added per-depth APIs (which seemed to originally be added for
libtiff to use). The approach currently used by GM worked with some
other older libjpeg's, including one which supported 16-bit samples.
The approach used is this:
if (jpeg_info.data_precision > 8)
{
unsigned int
scale_short;
scale_short=65535U/MaxValueGivenBits(jpeg_info.data_precision);
for (x=0; x < (long) image->columns; x++)
{
q->red=ScaleShortToQuantum(scale_short*GETJSAMPLE(*p++));
q->green=ScaleShortToQuantum(scale_short*GETJSAMPLE(*p++));
q->blue=ScaleShortToQuantum(scale_short*GETJSAMPLE(*p++));
if (jpeg_info.output_components > 3)
q->opacity=ScaleShortToQuantum(scale_short*GETJSAMPLE(*p++));
else
q->opacity=OpaqueOpacity;
q++;
}
}
This assumes that the size of JSAMPLE will be sufficient for the range
supported by jpeg_info.data_precision. For example, JSAMPLE could be
based on uint16_t, or the macro could be smart enough to form a
uint8_t or uint16_t based on a stream of octets.
I do need to personally re-test with recent libjpeg-turbo to make sure
that the approach works properly, but others have reported success.
Bob
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