src/core/SkCubicMap.cpp

/*
 * Copyright 2018 Google LLC
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "include/core/SkCubicMap.h"

#include "include/private/SkTPin.h"
#include "src/core/SkVx.h"

#include <algorithm>
#include <cmath>

static float eval_poly(float t, float b) { return b; }

template <typename... Rest>
static float eval_poly(float t, float m, float b, Rest... rest) {
    return eval_poly(t, std::fma(m, t, b), rest...);
}

static float cubic_solver(float A, float B, float C, float D) {
#ifdef SK_DEBUG
    auto valid = [](float t) { return t >= 0 && t <= 1; };
#endif

    auto guess_nice_cubic_root = [](float a, float b, float c, float d) { return -d; };
    float t = guess_nice_cubic_root(A, B, C, D);

    int iters = 0;
    const int MAX_ITERS = 8;
    for (; iters < MAX_ITERS; ++iters) {
        SkASSERT(valid(t));
        float f = eval_poly(t, A, B, C, D);        // f   = At^3 + Bt^2 + Ct + D
        if (std::fabs(f) <= 0.00005f) {
            break;
        }
        float fp = eval_poly(t, 3*A, 2*B, C);      // f'  = 3At^2 + 2Bt + C
        float fpp = eval_poly(t, 3*A + 3*A, 2*B);  // f'' = 6At + 2B

        float numer = 2 * fp * f;
        float denom = std::fma(2 * fp, fp, -(f * fpp));

        t -= numer / denom;
    }

    SkASSERT(valid(t));
    return t;
}

static inline bool nearly_zero(SkScalar x) {
    SkASSERT(x >= 0);
    return x <= 0.0000000001f;
}

static float compute_t_from_x(float A, float B, float C, float x) {
    return cubic_solver(A, B, C, -x);
}

float SkCubicMap::computeYFromX(float x) const {
    x = SkTPin(x, 0.0f, 1.0f);

    if (nearly_zero(x) || nearly_zero(1 - x)) {
        return x;
    }
    if (fType == kLine_Type) {
        return x;
    }
    float t;
    if (fType == kCubeRoot_Type) {
        t = std::pow(x / fCoeff[0].fX, 1.0f / 3);
    } else {
        t = compute_t_from_x(fCoeff[0].fX, fCoeff[1].fX, fCoeff[2].fX, x);
    }
    float a = fCoeff[0].fY;
    float b = fCoeff[1].fY;
    float c = fCoeff[2].fY;
    float y = ((a * t + b) * t + c) * t;

    return y;
}

static inline bool coeff_nearly_zero(float delta) {
    return std::fabs(delta) <= 0.0000001f;
}

SkCubicMap::SkCubicMap(SkPoint p1, SkPoint p2) {
    // Clamp X values only (we allow Ys outside [0..1]).
    p1.fX = std::min(std::max(p1.fX, 0.0f), 1.0f);
    p2.fX = std::min(std::max(p2.fX, 0.0f), 1.0f);

    auto s1 = skvx::float2::Load(&p1) * 3;
    auto s2 = skvx::float2::Load(&p2) * 3;

    (1 + s1 - s2).store(&fCoeff[0]);
    (s2 - s1 - s1).store(&fCoeff[1]);
    s1.store(&fCoeff[2]);

    fType = kSolver_Type;
    if (SkScalarNearlyEqual(p1.fX, p1.fY) && SkScalarNearlyEqual(p2.fX, p2.fY)) {
        fType = kLine_Type;
    } else if (coeff_nearly_zero(fCoeff[1].fX) && coeff_nearly_zero(fCoeff[2].fX)) {
        fType = kCubeRoot_Type;
    }
}

SkPoint SkCubicMap::computeFromT(float t) const {
    auto a = skvx::float2::Load(&fCoeff[0]);
    auto b = skvx::float2::Load(&fCoeff[1]);
    auto c = skvx::float2::Load(&fCoeff[2]);

    SkPoint result;
    (((a * t + b) * t + c) * t).store(&result);
    return result;
}