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/*
* Copyright 2018 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#include "../skcms.h"
#include "GaussNewton.h"
#include "PortableMath.h"
#include <limits.h>
// Evaluating skcms_TF13{A,B} at x:
// f(x) = Ax^3 + Bx^2 + (1-A-B)x
//
// ∂f/∂A = x^3 - x
// ∂f/∂B = x^2 - x
static float eval_13(float x, const void* ctx, const float P[4]) {
(void)ctx;
return P[0]*x*x*x
+ P[1]*x*x
+ (1 - P[0] - P[1])*x;
}
static void grad_13(float x, const void* ctx, const float P[4], float dfdP[4]) {
(void)ctx;
(void)P;
dfdP[0] = x*x*x - x;
dfdP[1] = x*x - x;
}
bool skcms_ApproximateCurve13(const skcms_Curve* curve, skcms_TF13* approx, float* max_error) {
// Start a guess at skcms_TF13{0,1}, i.e. f(x) = x^2, i.e. gamma = 2.
// TODO: guess better somehow, like we do in skcms_ApproximateCurve()?
float P[4] = { 0,1, 0,0 };
if (curve->table_entries > (uint32_t)INT_MAX) {
// That's just crazy.
return false;
}
const int N = curve->table_entries == 0 ? 257 /*TODO: tune?*/
: (int)curve->table_entries;
for (int i = 0; i < 3/*TODO: Tune???*/; i++) {
if (!skcms_gauss_newton_step(skcms_eval_curve, curve,
eval_13, NULL,
grad_13, NULL,
P,
0,1,N)) {
return false;
}
}
*max_error = 0;
for (int i = 0; i < N; i++) {
float x = i * (1.0f / (N-1));
float err = fabsf_( skcms_eval_curve(x, curve) - eval_13(x,NULL,P) );
if (err > *max_error) {
*max_error = err;
}
}
approx->A = P[0];
approx->B = P[1];
return true;
}
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