MinusculeRender/src/rasterizer.cc

// Copyright 2024 Bytedance Inc. All Rights Reserved.
// Author: tianlei.richard@qq.com (tianlei.richard)

#include "rasterizer.h"
#include <limits>

Rasterizer::Rasterizer(const int width, const int height)
    : width_(width), height_(height),
      pixels_(std::vector<std::vector<PixelProperty>>(
          height,
          std::vector<PixelProperty>(
              width, PixelProperty{0., 0., 0.,
                                   std::numeric_limits<double>::infinity()}))) {
}

void Rasterizer::rasterize(const Triangle &t) {
  const auto &aabb = t.axis_align_bbox();
  const int x_min = aabb.x;
  const int x_max = aabb.x + aabb.width;
  const int y_min = aabb.y;
  const int y_max = aabb.y + aabb.height;

  for (int i = x_min; i < x_max; ++i) {
    for (int j = y_min; j < y_max; ++j) {
      auto &property = pixels_[j][i];
      const auto &[is_inside, z_screen] = inside(Point2d{i + 0.5, j + 0.5}, t);
      if (is_inside && z_screen > property[3]) {
        property[0] = 1;
        property[1] = 0;
        property[2] = 0;
        property[3] = z_screen;
      }
    }
  }
}

std::pair<bool, double> Rasterizer::inside(const Point2d &p_screen,
                                           const Triangle &t) {
  const auto points = t.get_points<3>();

  const auto plane_normal = t.normal_vector();
  const auto plane_point = points[0];
  const auto z_screen =
      plane_point.z() - (plane_normal.x() * (p_screen.x() - plane_point.x()) +
                         plane_normal.y() * (p_screen.y() - plane_point.y())) /
                            plane_normal.z();
  const Point3d p{p_screen.x(), p_screen.y(), z_screen};

  const auto v1 = points[1] - points[0];
  const auto v2 = points[2] - points[1];
  const auto v3 = points[0] - points[2];

  const auto c1 = v1.cross(p - points[0]).normalized();
  const auto c2 = v2.cross(p - points[1]).normalized();
  const auto c3 = v3.cross(p - points[2]).normalized();

  const auto r1 = c1.dot(c2);
  const auto r2 = c1.dot(c3);
  const auto r3 = c2.dot(c3);

  return {r1 > 0 && r2 > 0 && r3 > 0, z_screen};
}