MinusculeRender/src/minus_renderer.cc

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

#include <cmath>

#include "spdlog/spdlog.h"
#include <iostream>
#include <opencv2/highgui/highgui.hpp>

#include "minus_renderer.h"
#include "rasterizer.h"

MinusRenderer::MinusRenderer(float near, float far, float fov,
                             float aspect_ratio)
    : near_(near), far_(far), fov_(fov), aspect_ratio_(aspect_ratio) {
  spdlog::info("near: {}, far: {}, fov: {}, aspect ratio: {}", near_, far_,
               fov_, aspect_ratio_);
}

void MinusRenderer::set_meshes(const std::vector<Mesh> &meshes) {
  meshes_ = meshes;
  spdlog::info("Mesh size: {}, the primitives size of the first mesh: {}",
               meshes_.size(), meshes_.front().get_primitives().size());
}

float MinusRenderer::calculate_height(const float fov, const float near) {
  return std::fabs(near) * std::tan(fov * 0.5) * 2;
}

float MinusRenderer::calculate_width(const float height, const float ratio) {
  return height * ratio;
}

std::pair<TransformMatrix, TransformMatrix>
MinusRenderer::construct_transform(const int resolution_width,
                                   const int resolution_height) const {
  // Squish Transform
  const auto &squish_transform =
      TransformMatrix{{near_, 0, 0, 0},
                      {0, near_, 0, 0},
                      {0, 0, near_ + far_, -near_ * far_},
                      {0, 0, 1, 0}};
  const auto &inv_squish_transform =
      TransformMatrix{{1, 0, 0, 0},
                      {0, 1, 0, 0},
                      {0, 0, 0, near_},
                      {0, 0, -1. / far_, (near_ + far_) / far_}};

  // Orthographic transform
  const float height = calculate_height(fov_, near_);
  const float width = calculate_width(height, aspect_ratio_);
  const float right = width * 0.5;
  const float left = -right;
  const float top = height * 0.5;
  const float bottom = -top;

  const auto &ortho_transform =
      TransformMatrix{{2 / (right - left), 0, 0, 0},
                      {0, 2 / (top - bottom), 0, 0},
                      {0, 0, 2 / std::fabs(far_ - near_), 0},
                      {0, 0, 0, 1}} *
      TransformMatrix{{1, 0, 0, -0.5 * (right - left)},
                      {0, 1, 0, -0.5 * (top - bottom)},
                      {0, 0, 1, -0.5 * (far_ - near_)},
                      {0, 0, 0, 1}};
  const auto &inv_ortho_transform = TransformMatrix{
      {(right - left) / 2., 0, 0, 0.5 * (right - left)},
      {0, (top - bottom) / 2., 0, 0.5 * (top - bottom)},
      {0, 0, std::fabs(far_ - near_) / 2., 0.5 * (far_ - near_)},
      {0, 0, 0, 1}};

  // View port transform
  const auto &view_port_transform =
      TransformMatrix{{resolution_width / 2., 0, 0, resolution_width * 0.5},
                      {0, resolution_height / 2., 0, resolution_height * 0.5},
                      {0, 0, 1, 0},
                      {0, 0, 0, 1}};
  const auto &inv_view_port_transform =
      TransformMatrix{{2. / resolution_width, 0, 0, 0},
                      {0, 2. / resolution_height, 0, 0},
                      {0, 0, 1, 0},
                      {0, 0, 0, 1}} *
      TransformMatrix{{1, 0, 0, -resolution_width * 0.5},
                      {0, 1, 0, -resolution_height * 0.5},
                      {0, 0, 1, 0},
                      {0, 0, 0, 1}};

  const auto &ss_transform =
      view_port_transform * ortho_transform * squish_transform;
  const auto &inv_ss_transform =
      inv_squish_transform * inv_ortho_transform * inv_view_port_transform;
  return {ss_transform, inv_ss_transform};
}

std::tuple<double, double, double>
MinusRenderer::calculate_barycentric_coordinate(const Triangle &t,
                                                const Point2d &p) {
  const auto &points = t.get_vertex_position();
  const auto &A = (points[0]).head(2);
  const auto &B = (points[1]).head(2);
  const auto &C = (points[2]).head(2);
  double alpha =
      (-(p.x() - B.x()) * (C.y() - B.y()) + (p.y() - B.y()) * (C.x() - B.x())) /
      (-(A.x() - B.x()) * (C.y() - B.y()) + (A.y() - B.y()) * (C.x() - B.x()));
  double beta =
      (-(p.x() - C.x()) * (A.y() - C.y()) + (p.y() - C.y()) * (A.x() - C.x())) /
      (-(B.x() - C.x()) * (A.y() - C.y()) + (B.y() - C.y()) * (A.x() - C.x()));
  double gamma = 1. - alpha - beta;
  return {alpha, beta, gamma};
}

void MinusRenderer::model_transform(const TransformMatrix &mtx) {
  for (auto &m : meshes_) {
    for (auto &t : m.get_primitives()) {
      const auto &[res, _] = apply_transform(mtx, t.get_vertex_position());
      t.set_points(res);
    }
  }
}

void MinusRenderer::view_transform(const TransformMatrix &mtx) {
  for (auto &m : meshes_) {
    for (auto &t : m.get_primitives()) {
      const auto &[res, _] = apply_transform(mtx, t.get_vertex_position());
      t.set_points(res);
    }
  }
}

cv::Mat MinusRenderer::render(const int resolution_width,
                              const int resolution_height) {
  // 构造 camera space -> screen space 的变换矩阵
  const auto [ss_transform, inv_ss_transform] =
      construct_transform(resolution_width, resolution_height);

  // 主要为了把顶点坐标拷贝一份，否则 camera space 的坐标被变换到 screen space,
  // 下次渲染时拿到的就不是 camera space 的坐标
  std::vector<Mesh> meshes = meshes_;
  Rasterizer rasterizer{resolution_width, resolution_height};
  for (int mesh_index = 0; mesh_index < meshes.size(); ++mesh_index) {
    auto &mesh = meshes[mesh_index];
    auto &primitives = mesh.get_primitives();
    for (int i = 0; i < primitives.size(); ++i) {
      auto &t = primitives[i];
      const auto &triangle_vertices = t.get_vertex_position();
      const auto &[res, _] =
          apply_transform(ss_transform, t.get_vertex_position());
      t.set_points(res);
    }
  }
  const auto &shading_points = rasterizer.rasterize(meshes);
  assert(!shading_points.empty());
  cv::Mat color_image(shading_points.size(), (shading_points[0]).size(),
                      CV_8UC3);
  for (int y = 0; y < shading_points.size(); ++y) {
    for (int x = 0; x < shading_points[y].size(); ++x) {
      auto &pixel_color =
          color_image.at<cv::Vec3b>(shading_points.size() - y - 1, x);
      pixel_color = cv::Vec3b(0, 0, 0);

      const auto &fragment = shading_points[y][x];
      if (fragment.triangle.mesh_index >= 0 &&
          fragment.triangle.mesh_index < meshes_.size()) {
        const auto &mesh = meshes_[fragment.triangle.mesh_index];
        const auto &primitives = mesh.get_primitives();
        if (fragment.triangle.triangle_index >= 0 &&
            fragment.triangle.triangle_index < primitives.size()) {
          const Point3d &position =
              (inv_ss_transform *
               (Point3d{x + 0.5, y + 0.5, fragment.depth}.homogeneous()))
                  .hnormalized();
          spdlog::trace("Screen space potions: ({}, {}, {}), camera space "
                        "position: ({}, {}, {})",
                        x + 0.5, y + 0.5, fragment.depth, position.x(),
                        position.y(), position.z());

          const auto &triangle = primitives[fragment.triangle.triangle_index];
          const auto &[alpha, beta, gamma] =
              calculate_barycentric_coordinate(triangle, position.head(2));
          spdlog::trace("barycentric coordinate:  ({}, {}, {})", alpha, beta,
                        gamma);

          std::vector<Point2d> triangle_uv;
          std::vector<Point3d> triangle_normal;
          for (const auto vertex_index : triangle.get_vertex_index()) {
            triangle_uv.push_back(mesh.get_texture_coordinate(vertex_index));
            triangle_normal.push_back(mesh.get_normal_vector(vertex_index));
          }

          Point2d uv{alpha * triangle_uv[0].x() + beta * triangle_uv[1].x() +
                         gamma * triangle_uv[2].x(),
                     alpha * triangle_uv[0].y() + beta * triangle_uv[1].y() +
                         gamma * triangle_uv[2].y()};
          spdlog::trace("UV: ({}, {}, {})", uv.x(), uv.y());

          const auto &normal = Point3d{alpha * triangle_normal[0].x() +
                                           beta * triangle_normal[1].x() +
                                           gamma * triangle_normal[2].x(),
                                       alpha * triangle_normal[0].y() +
                                           beta * triangle_normal[1].y() +
                                           gamma * triangle_normal[2].y(),
                                       alpha * triangle_normal[0].z() +
                                           beta * triangle_normal[1].z() +
                                           gamma * triangle_normal[2].z()}
                                   .normalized();
          spdlog::trace("Normal: ({}, {}, {})", normal.x(), normal.y(),
                        normal.z());

          auto &material = mesh.get_material();
          pixel_color =
              material->shade(Vertex{position, normal, uv}, Point3d{});
        }
      }
    }
  }
  return color_image;
}