Hi Richard,
Back from holidays. I realize this message is quite old, so I apologize...
Ok, There was a reason to limit the tone curve plug-in and disallow the
file IO. It happens that the file format is defined by the ICC (and only
by the ICC!), so any addition made by anyone else would turn the profile
in not compliant, and I'm afraid many CCM will just discard it. So the
plug-in feature allows you to create new tone curves and play with those
tone curves in pipelines, evaluators, etc. but keeping in mind this is
only for data handling and not to be saved as a profile. An example
would be to create a special tone curve for a profiler to be used to
compute LUT points, the points then are meant be saved as a CLUT in the
final profile but the math curve is not.
But anyway, I think there is a clean way to do what you wish, and this
needs only a few math.
It seems to me the issue comes because parametric 5 is in the form
(a*x)^g
and you need
a*(x)^g
But nothing prevents you to compute a new a' instead
a'=a^(1/g)
See below the parameters disposition
params[0] = 0.45; /* y */
params[1] = pow(1.099, 1.0 / 0.45); /* a' */
params[2] = 0.0; /* b */
params[3] = 4.5; /* c */
params[4] = 0.018; /* d */
params[5] = -0.099; /* e */
params[6] = 0.0; /* f */
I've checked this against the "normal" Rec709, and it works at 16 bits
precision. See the testing code below. Sorry again for the delay
Regards
Marti
-----
static
double Rec709(double L)
{
if (L <0.018) return 4.5*L;
else
{
double a = 1.099* pow(L, 0.45);
a = a - 0.099;
return a;
}
}
static
cmsInt32Number CheckParametricRec709(void)
{
cmsFloat64Number params[7];
cmsToneCurve* t;
int i;
params[0] = 0.45; /* y */
params[1] = pow(1.099, 1.0 / 0.45); /* a */
params[2] = 0.0; /* b */
params[3] = 4.5; /* c */
params[4] = 0.018; /* d */
params[5] = -0.099; /* e */
params[6] = 0.0; /* f */
t = cmsBuildParametricToneCurve (NULL, 5, params);
for (i=0; i < 256; i++)
{
cmsUInt16Number f1 = floor(255.0*cmsEvalToneCurveFloat(t, i /
255.0) + 0.5);
cmsUInt16Number f2 = floor(255.0*Rec709((double) i / 255.0) + 0.5);
if (f1 != f2)
{
return 0;
}
}
return 1;
}
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