path: root/src/scene.cc

// Copyright 2021, 2022 Benjamin Barenblat
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.

#include "src/scene.h"

#include <math.h>
#include <stddef.h>
#include <stdint.h>

#include <Eigen/Core>
#include <algorithm>
#include <chrono>
#include <iostream>
#include <vector>

#include "res/res.h"
#include "src/astro.h"
#include "src/gl/buffer.h"
#include "src/gl/draw.h"
#include "src/gl/shader.h"
#include "src/gl/texture.h"
#include "src/mesh.h"
#include "src/webp.h"
#include "third_party/abseil/absl/strings/string_view.h"
#include "third_party/abseil/absl/types/span.h"
#include "third_party/date/include/date/date.h"
#include "third_party/date/include/date/tz.h"

namespace glplanet {

namespace {

constexpr int kSectorsPerTurn = 64;
constexpr int kSlices = 64;

Eigen::Matrix4f ModelViewProjection(const double longitude_radians,
                                    const double latitude_radians) noexcept {
  // This is the precomputed result of the following transformations, applied in
  // order:
  //
  //   1. rotation around the model z axis (turning to longitude)
  //
  //   2. rotation around the model y axis (tilting to latitude)
  //
  //   3. transformation from model coordinates to view coordinates
  //
  //   4. perspective transformation
  //
  // This was computed with Maxima using the src/mvp.maxima script; see
  // documentation there for what the variables mean.

  const double sin_beta = sin(latitude_radians);
  const double cos_beta = cos(latitude_radians);

  const double sin_gamma = sin(-longitude_radians);
  const double cos_gamma = cos(-longitude_radians);

  constexpr double z = 12.0;

  // Compute the camera field of view. We would like this to be such that we can
  // see the full Earth.
  constexpr double kEarthRadius = 1.0;
  constexpr double tan_half_phix = (1.05 * kEarthRadius) / z;
  constexpr double tan_half_phiy = (1.05 * kEarthRadius) / z;

  constexpr double zn = z - kEarthRadius;
  constexpr double zf = z;  // We can't see beyond the horizon.

  const double sin_beta_over_tan_half_phiy = sin_beta / tan_half_phiy;
  constexpr double zf_minus_zn = zf - zn;
  constexpr double clip_factor = (zf + zn) / zf_minus_zn;
  const double clip_factor_cos_beta = clip_factor * cos_beta;

  Eigen::Matrix4f mvp;
  mvp(0, 0) = sin_gamma / tan_half_phix;
  mvp(0, 1) = cos_gamma / tan_half_phix;
  mvp(0, 2) = 0;
  mvp(0, 3) = 0;
  mvp(1, 0) = -sin_beta_over_tan_half_phiy * cos_gamma;
  mvp(1, 1) = sin_beta_over_tan_half_phiy * sin_gamma;
  mvp(1, 2) = cos_beta / tan_half_phiy;
  mvp(1, 3) = 0;
  mvp(2, 0) = -clip_factor_cos_beta * cos_gamma;
  mvp(2, 1) = clip_factor_cos_beta * sin_gamma;
  mvp(2, 2) = -sin_beta * clip_factor;
  mvp(2, 3) = z * clip_factor - 2 * zf * zn / zf_minus_zn;
  mvp(3, 0) = -cos_beta * cos_gamma;
  mvp(3, 1) = cos_beta * sin_gamma;
  mvp(3, 2) = -sin_beta;
  mvp(3, 3) = z;
  return mvp;
}

void LoadMipmapsFromTableau(absl::Span<const uint8_t> webp, int max_width,
                            int max_height, int column,
                            gl::Texture2d& texture) {
  int x = column * max_width;
  if (x % 2 == 1) {
    // libwebp can't crop from an odd-numbered pixel. By convention, we've
    // pushed anything that would be on an odd boundary one pixel to the right
    // or down.
    ++x;
  }

  int y = 0;
  int width = max_width;
  int height = max_height;
  int level = 0;
  while (true) {
    std::vector<uint8_t> mipmap = DecodeWebp(webp, x, y, width, height);
    texture.LoadSubimage(width, height, gl::Texture::PixelFormat::kRgb,
                         gl::Texture::PixelType::kUnsignedByte, mipmap.data(),
                         level);

    if (width == 1 && height == 1) {
      // We just loaded the last mipmap.
      break;
    }

    y += height;
    if (y % 2 == 1) {
      // libwebp can't crop from an odd-numbered pixel. See note above about
      // shifting one to the right or down.
      ++y;
    }
    width = std::max(width / 2, 1);
    height = std::max(height / 2, 1);
    ++level;
  }
}

void LoadStandardTexture(absl::Span<const uint8_t> webp, int column,
                         gl::Texture2d& tex) {
  assert(tex.width() == 1024);
  assert(tex.height() == 512);
  LoadMipmapsFromTableau(webp,
                         /*max_width=*/1024, /*max_height=*/512, column, tex);
  tex.SetWrap(gl::Texture2d::Dimension::kS, gl::Texture::Wrap::kRepeat);
  tex.SetWrap(gl::Texture2d::Dimension::kT, gl::Texture::Wrap::kClampToEdge);
  tex.SetMinFilter(gl::Texture::MinFilter::kLinearMipmapLinear);
  tex.SetMagFilter(gl::Texture::MagFilter::kLinear);
}

void ReportShaderWarnings(const char* description,
                          absl::string_view log) noexcept {
  if (!log.empty()) {
    std::cerr << "glplanet: while " << description << ":\n" << log;
  }
}

}  // namespace

Scene::Scene(const Options& options)
    : planet_month_(0),
      planet_(gl::Texture::Format::kSrgb8, 1024, 512, 11),
      clouds_(gl::Texture::Format::kSrgb8, 1024, 512, 11),
      mvp_(ModelViewProjection(options.longitude_radians, options.latitude_radians)) {
  LoadStandardTexture(glplanet_resources::CloudsWebp(), /*column=*/0, clouds_);
  SetUpShaders();
  LoadMesh();
}

void Scene::SetUpShaders() {
  gl::VertexShader vertex_shader;
  vertex_shader.SetSource(glplanet_resources::VertexGlsl());
  vertex_shader.Compile();
  ReportShaderWarnings("compiling vertex shader", vertex_shader.compile_log());
  program_.Attach(vertex_shader);

  gl::FragmentShader fragment_shader;
  fragment_shader.SetSource(glplanet_resources::FragmentGlsl());
  fragment_shader.Compile();
  ReportShaderWarnings("compiling fragment shader",
                       fragment_shader.compile_log());
  program_.Attach(fragment_shader);
  program_.SetFragmentDataLocation("out_color", 0);

  program_.Link();
  ReportShaderWarnings("linking shader program", program_.link_log());
  uniform_mvp_ = program_.active_uniform("mvp");
  uniform_planet_ = program_.active_uniform("planet");
  uniform_clouds_ = program_.active_uniform("clouds");
  uniform_sun_direction_ = program_.active_uniform("sun_direction");
}

void Scene::LoadMesh() {
  UvSphere mesh(kSectorsPerTurn, kSlices);

  vao_.SetVertexBuffer(gl::VertexBuffer(absl::MakeConstSpan(mesh.vertices),
                                        gl::Buffer::AccessFrequency::kStatic,
                                        gl::Buffer::AccessNature::kDraw));
  vao_.SetVertexAttributeFormat(
      program_.active_vertex_attribute("cartesian_position"), 3,
      gl::VertexAttributeType::kFloat, offsetof(UvSphere::Coordinates, x));
  vao_.SetVertexAttributeFormat(
      program_.active_vertex_attribute("vertex_texture_coordinate"), 2,
      gl::VertexAttributeType::kFloat, offsetof(UvSphere::Coordinates, u));

  vao_.SetElementBuffer(gl::ElementBuffer(absl::MakeConstSpan(mesh.elements),
                                          gl::Buffer::AccessFrequency::kStatic,
                                          gl::Buffer::AccessNature::kDraw));
}

void Scene::SetGlState() {
  gl::SetActiveShaderProgram(program_);

  gl::SetActiveShaderUniform(uniform_mvp_, mvp_);

  // TODO(bbarenblat@gmail.com): Assert that we have enough fragment shader
  // texture units (GL_MAX_TEXTURE_IMAGE_UNITS).

  gl::UseTextureUnit(0);
  gl::BindTexture2d(planet_);
  gl::SetActiveShaderUniform(uniform_planet_, 0);

  gl::UseTextureUnit(1);
  gl::BindTexture2d(clouds_);
  gl::SetActiveShaderUniform(uniform_clouds_, 1);

  gl::BindVertexArray(vao_);
}

void Scene::Draw(std::chrono::system_clock::time_point now) {
  if (date::month month =
          date::year_month_day(std::chrono::floor<std::chrono::days>(now))
              .month();
      month != planet_month_) {
    // The month has changed since we last drew. Reload the planet texture to
    // reflect current snow levels.
    LoadStandardTexture(glplanet_resources::EarthWebp(), unsigned{month} - 1,
                        planet_);
    planet_month_ = month;
  }

  const auto& [noon_longitude, noon_latitude] =
      HighNoonLocation(std::chrono::time_point_cast<std::chrono::milliseconds>(
          date::clock_cast<date::tai_clock>(now)));
  Eigen::Vector3f to_sun{
      static_cast<float>(cos(noon_latitude) * cos(noon_longitude)),
      static_cast<float>(cos(noon_latitude) * sin(noon_longitude)),
      sinf(noon_latitude)};
  gl::SetActiveShaderUniform(uniform_sun_direction_, to_sun);

  gl::DrawElements(vao_, gl::Primitive::kTriangles);
}  // namespace glplanet

}  // namespace glplanet