Hi,
My viewer (mrViewer) uses part of the code used in playexr to create the
lut table for CTL transforms. This lut is created in log space, like (
as the playexr explains ) :
//
// We build a 3D color lookup table by running a set of color
// samples through a series of CTL transforms.
//
// The 3D lookup table covers a range from lutMin to lutMax or
// NUM_STOPS f-stops above and below 0.18 or MIDDLE_GRAY. The
// size of the table is lutN by lutN by lutN samples.
//
// In order make the distribution of the samples in the table
// approximately perceptually uniform, the Cg shaders that use
// the table perform lookups in "log space":
// In a Cg shader, the lookup table is represented as a 3D texture.
// In order to apply the table to a pixel value, the Cg shader takes
// the logarithm of the pixel value and scales and offsets the result
// so that lutMin and lutMax map to 0 and 1 respectively. The scaled
// value is used to perform a texture lookup and the shader computes
// e raised to the power of the result of the texture lookup.
//
Recently, a user, trying to match nuke and with my suggestion, used an
identity transform as RRT and a linear_to_sRGB function (from the CTL
utilities) to create an sRGB transform. To make it clear:
/// Utility -- convert linear value to sRGB
float linear_to_sRGB (float x)
{
if (x < 0.0)
return 0.0;
if (x <= 0.0031308)
return (12.92 * x);
else
return (1.055 * pow (x, 1.0/2.4) - 0.055);
}
The problem the user and I are facing is that the LUT table clamps (and
raises) the black levels to 0.0182 and makes it not match the sRGB curve
(black levels) as seen in nuke.
I am wondering if anyone has seen something similar with playexr (or
your own code) and if someone knows how to address this. Any code or
pointers would be highly appreciated.
--
Gonzalo Garramuño
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