gaussian.hpp

#pragma once
 
/*
 * Gaussian blur math: sigma<->radius, CDF profile LUT, separable kernel.
 *
 * Based on principles from Skia's SkBlurMask (BSD-3, third_party/skia/LICENSE).
 * Implementations are independent re-expressions; no Skia code is copied.
 */
 
#include <cstdint>
#include <span>
 
#include "qml_material/math/scalar.hpp"
 
namespace qml_material::math
{
 
inline constexpr scalar k_blur_sigma_scale = 0.57735f; // ≈ 1/sqrt(3)
 
[[nodiscard]] inline scalar radius_to_sigma(scalar radius) {
    return radius > 0 ? k_blur_sigma_scale * radius + 0.5f : 0.0f;
}
 
[[nodiscard]] inline scalar sigma_to_radius(scalar sigma) {
    return sigma > 0.5f ? (sigma - 0.5f) / k_blur_sigma_scale : 0.0f;
}
 
[[nodiscard]] float gaussian_cdf(float x);
 
void fill_unit_cdf_profile(std::span<std::uint8_t> out);
 
[[nodiscard]] int gaussian_kernel_radius(scalar sigma);
 
void fill_gaussian_kernel_1d(std::span<float> out, scalar sigma);
 
[[nodiscard]] int rrect_corner_blur_size(scalar sigma, scalar radius);
 
void fill_rrect_corner_blur(std::span<std::uint8_t> out, scalar sigma, scalar radius);
 
} // namespace qml_material::math