ladybird/Userland/Libraries/LibSoftGPU/Clipper.cpp

/*
 * Copyright (c) 2021, Jesse Buhagiar <jooster669@gmail.com>
 * Copyright (c) 2021, Stephan Unverwerth <s.unverwerth@serenityos.org>
 * Copyright (c) 2022, the SerenityOS developers.
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Vector.h>
#include <LibGPU/Vertex.h>
#include <LibGfx/Vector4.h>
#include <LibSoftGPU/Clipper.h>

namespace SoftGPU {

template<Clipper::ClipPlane plane>
static constexpr bool point_within_clip_plane(FloatVector4 const& vertex)
{
    if constexpr (plane == Clipper::ClipPlane::LEFT)
        return vertex.x() >= -vertex.w();
    else if constexpr (plane == Clipper::ClipPlane::RIGHT)
        return vertex.x() <= vertex.w();
    else if constexpr (plane == Clipper::ClipPlane::TOP)
        return vertex.y() <= vertex.w();
    else if constexpr (plane == Clipper::ClipPlane::BOTTOM)
        return vertex.y() >= -vertex.w();
    else if constexpr (plane == Clipper::ClipPlane::NEAR)
        return vertex.z() >= -vertex.w();
    else if constexpr (plane == Clipper::ClipPlane::FAR)
        return vertex.z() <= vertex.w();
    return false;
}

template<Clipper::ClipPlane plane>
static constexpr GPU::Vertex clip_intersection_point(GPU::Vertex const& p1, GPU::Vertex const& p2)
{
    constexpr FloatVector4 clip_plane_normals[] = {
        { 1, 0, 0, 0 },  // Left Plane
        { -1, 0, 0, 0 }, // Right Plane
        { 0, -1, 0, 0 }, // Top Plane
        { 0, 1, 0, 0 },  // Bottom plane
        { 0, 0, 1, 0 },  // Near Plane
        { 0, 0, -1, 0 }  // Far Plane
    };
    constexpr auto clip_plane_normal = clip_plane_normals[to_underlying(plane)];

    // See https://www.microsoft.com/en-us/research/wp-content/uploads/1978/01/p245-blinn.pdf
    // "Clipping Using Homogeneous Coordinates" Blinn/Newell, 1978

    float const w1 = p1.clip_coordinates.w();
    float const w2 = p2.clip_coordinates.w();
    float const x1 = clip_plane_normal.dot(p1.clip_coordinates);
    float const x2 = clip_plane_normal.dot(p2.clip_coordinates);
    float const a = (w1 + x1) / ((w1 + x1) - (w2 + x2));

    GPU::Vertex out;
    out.position = mix(p1.position, p2.position, a);
    out.eye_coordinates = mix(p1.eye_coordinates, p2.eye_coordinates, a);
    out.clip_coordinates = mix(p1.clip_coordinates, p2.clip_coordinates, a);
    out.color = mix(p1.color, p2.color, a);
    for (size_t i = 0; i < GPU::NUM_SAMPLERS; ++i)
        out.tex_coords[i] = mix(p1.tex_coords[i], p2.tex_coords[i], a);
    out.normal = mix(p1.normal, p2.normal, a);
    return out;
}

template<Clipper::ClipPlane plane>
FLATTEN static constexpr void clip_plane(Vector<GPU::Vertex>& read_list, Vector<GPU::Vertex>& write_list)
{
    auto read_from = &read_list;
    auto write_to = &write_list;

    write_to->clear_with_capacity();
    for (size_t i = 0; i < read_from->size(); i++) {
        auto const& curr_vec = read_from->at((i + 1) % read_from->size());
        auto const& prev_vec = read_from->at(i);

        bool const is_curr_point_within_clip_plane = point_within_clip_plane<plane>(curr_vec.clip_coordinates);
        bool const is_prev_point_within_clip_plane = point_within_clip_plane<plane>(prev_vec.clip_coordinates);
        if (is_curr_point_within_clip_plane != is_prev_point_within_clip_plane) {
            auto const intersect = clip_intersection_point<plane>(prev_vec, curr_vec);
            write_to->append(intersect);
        }

        if (is_curr_point_within_clip_plane)
            write_to->append(curr_vec);
    }
    swap(write_list, read_list);
}

void Clipper::clip_triangle_against_frustum(Vector<GPU::Vertex>& input_verts)
{
    list_a = input_verts;
    list_b.clear_with_capacity();

    // FIXME C++23. Static reflection will provide looping over all enum values.
    clip_plane<ClipPlane::LEFT>(list_a, list_b);
    clip_plane<ClipPlane::RIGHT>(list_a, list_b);
    clip_plane<ClipPlane::TOP>(list_a, list_b);
    clip_plane<ClipPlane::BOTTOM>(list_a, list_b);
    clip_plane<ClipPlane::NEAR>(list_a, list_b);
    clip_plane<ClipPlane::FAR>(list_a, list_b);

    input_verts = list_a;
}

}