OpenGL example

CMakeLists.txt

add_executable(triangle triangle.cc utility.hh)
target_link_libraries(triangle glut GLEW GL)

fragment.f.glsl

smooth out vec4 color;
uniform float loopDuration;
uniform float time;

uniform float zNear;
uniform float zFar;

const vec4 red = vec4(1, 0, 0, 1);
const vec4 green = vec4(1, 1, 1, 1);

float dist(float a, float b) {
  return abs(a - b);
}

void main() {
  float currTime = mod(time, loopDuration);
  float currLerp = currTime / loopDuration;

  float lerpValue = dist(gl_FragCoord.z / zFar, 0) ;

  color = vec4(lerpValue, lerpValue, lerpValue, 1);// mix(red, green, lerpValue);
};

triangle.cc

#include <iostream>
#include <GL/glew.h>
#include <GL/freeglut.h>
#include "utility.hh"
#include <cstring>

GLuint program;
GLuint boxVBO;
GLuint timeUniformLocation;
GLuint perspectiveMatrixUnf;
float theMatrix[16];

void init() {
  GLint compile_ok = false, link_ok = false;
  program = glCreateProgram();

  GLuint vs = CreateShader(program, "vertex.v.glsl");
  GLuint fs = CreateShader(program, "fragment.f.glsl");

  glLinkProgram(program);
  glGetProgramiv(program, GL_LINK_STATUS, &link_ok);
  if (!link_ok) {
    print_log(program);
    std::cerr << "Program linking error" << std::endl;
    exit(0);
  }

  GLuint loopDurationUnf  = glGetUniformLocation(program, "loopDuration");
  perspectiveMatrixUnf  = glGetUniformLocation(program, "perspectiveMatrix");
  GLuint frustumScaleUnf = glGetUniformLocation(program, "frustumScale");
  GLuint zNearUnf = glGetUniformLocation(program, "zNear");
  GLuint zFarUnf  = glGetUniformLocation(program, "zFar");
  timeUniformLocation = glGetUniformLocation(program, "time");

  glUseProgram(program);
  glUniform1f(loopDurationUnf, 5);
  glUniform1f(frustumScaleUnf, 1);
  glUniform1f(zNearUnf, 1);
  glUniform1f(zFarUnf, 4);
  glUseProgram(0);

  glDeleteShader(vs);
  glDeleteShader(fs);

  float fzNear = 1;
  float fzFar = 4;
  memset(theMatrix, 0, sizeof(float) * 16);

  theMatrix[0] = 1;
  theMatrix[5] = 1;
  theMatrix[10] = (fzFar + fzNear) / (fzNear - fzFar);
  theMatrix[14] = (2 * fzFar * fzNear) / (fzNear - fzFar);
  theMatrix[11] = -1.0f;

  glUseProgram(program);
  glUniformMatrix4fv(perspectiveMatrixUnf, 1, GL_FALSE, theMatrix);
  glUseProgram(0);

  GLfloat r = 0.1, n = -2;
  GLfloat box_vertices[] = {
    -r,r,n,0,
    r,r,n,0,
    r,-r,n,0,
    -r,-r,n,0,

    r,r,n,0,
    r,r,n-2,0,
    r,-r,n-2,0,
    r,-r,n-0,0,

    r,r,n-2,0,
    -r,r,n-2,0,
    -r,-r,n-2,0,
    r,-r,n-2,0,

    -r,r,n-2,0,
    -r,r,n,0,
    -r,-r,n,0,
    -r,-r,n-2,0,

    -r,r,n-2,0,
    r,r,n-2,0,
    r,r,n,0,
    -r,r,n,0,

    -r,-r,n,0,
    -r,-r,n-2,0,
    r,-r,n-2,0,
    r,-r,n,0
  };
  boxVBO = CreateVertexBuffer(box_vertices, sizeof(box_vertices));
  std::cout << box_vertices[93] << std::endl;

  GLuint vao;
  glGenVertexArrays(1, &vao);
  glBindVertexArray(vao);

  glEnable(GL_CULL_FACE);
  glCullFace(GL_BACK);
  glFrontFace(GL_CW);
}

void reshape (int w, int h)
{
  theMatrix[0] = 1 / (w / (float)h);
  theMatrix[5] = 1;

  glUseProgram(program);
  glUniformMatrix4fv(perspectiveMatrixUnf, 1, GL_FALSE, theMatrix);
  glUseProgram(0);

  glViewport(0, 0, (GLsizei) w, (GLsizei) h);
}

void display() {
  glClearColor(0.3, 0.3, 0.3, 0);
  glClear(GL_COLOR_BUFFER_BIT);
  glUseProgram(program);

  // provide time for shaders
  glUniform1f(timeUniformLocation, glutGet(GLUT_ELAPSED_TIME)/1000.f);

  glBindBuffer(GL_ARRAY_BUFFER, boxVBO);
  glEnableVertexAttribArray(0);
  glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);
  glDrawArrays(GL_QUADS, 0, 96);
  glBindBuffer(GL_ARRAY_BUFFER, 0);
  glDisableVertexAttribArray(0);

  glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, (void*)0);
  glDrawArrays(GL_TRIANGLES, 0, 36);
  glBindBuffer(GL_ARRAY_BUFFER, 0);
  glDisableVertexAttribArray(0);

  glUseProgram(0);

  glutSwapBuffers();
  glutPostRedisplay();
}

void NormalKeyboardEvent(unsigned char key, int x, int y) {
  if (key == 27)
    glutLeaveMainLoop();
}

void free_resources() {
  glDeleteProgram(program);
}

int main(int argc, char** argv) {
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH);
  glutInitWindowSize(400, 400);
  glutCreateWindow("My triangle");
  glutSetOption(GLUT_ACTION_ON_WINDOW_CLOSE, GLUT_ACTION_CONTINUE_EXECUTION);

  GLenum glew_status = glewInit();
  if (glew_status != GLEW_OK) {
    std::cerr << "Error: " << glewGetErrorString(glew_status) << std::endl;
    return 1;
  }

  init();
  glutDisplayFunc(display);
  glutKeyboardFunc(NormalKeyboardEvent);
  glutReshapeFunc(reshape);
  glutMainLoop();

  free_resources();
  return 0;
}

utility.hh

#include <fstream>
#include <sstream>
#include <string>
#include <cmath>

std::string ReadFile(std::string name) {
  std::stringstream contents;
  std::ifstream file(name.c_str());
  if (file.is_open()) {
    std::string line;
    while (file.good()) {
      getline(file, line);
      contents << line << std::endl;
    }
    file.close();
  }
  return contents.str();
}

void print_log(GLuint object)
{
  GLint log_length = 0;
  if (glIsShader(object))
    glGetShaderiv(object, GL_INFO_LOG_LENGTH, &log_length);
  else if (glIsProgram(object))
    glGetProgramiv(object, GL_INFO_LOG_LENGTH, &log_length);
  else {
    std::cerr << "printlog: Not a shader or a program" << std::endl;
    return;
  }

  char* log = new char[log_length];

  if (glIsShader(object))
    glGetShaderInfoLog(object, log_length, NULL, log);
  else if (glIsProgram(object))
    glGetProgramInfoLog(object, log_length, NULL, log);

  std::cerr << log << std::endl;
  delete log;
}

GLuint CreateShader(GLuint program, std::string filename)
{
  GLenum type;
  std::string ending = filename.substr(filename.size()-7, filename.size());
  if (ending == ".v.glsl")
    type = GL_VERTEX_SHADER;
  else if (ending == ".f.glsl")
    type = GL_FRAGMENT_SHADER;

  std::string bare_source = ReadFile(filename);
  if (bare_source.empty()) {
    std::cerr << "Error opening " << filename << std::endl;
    return 0;
  }

  GLuint shader = glCreateShader(type);
  const char *full_source[2] = {
    "#version 330\n",
    bare_source.c_str()
  };
  glShaderSource(shader, 2, full_source, NULL);

  glCompileShader(shader);
  GLint compile_ok = GL_FALSE;
  glGetShaderiv(shader, GL_COMPILE_STATUS, &compile_ok);
  if (compile_ok == GL_FALSE) {
    std::cerr << filename << ":";
    print_log(shader);
    glDeleteShader(shader);
    return 0;
  }

  glAttachShader(program, shader);

  return shader;
}

template <typename T>
GLuint CreateVertexBuffer(T data[], GLsizeiptr size) {
  GLuint position;
  glGenBuffers(1, &position);
  glBindBuffer(GL_ARRAY_BUFFER, position);
  glBufferData(GL_ARRAY_BUFFER, size, data, GL_STATIC_DRAW);
  glBindBuffer(GL_ARRAY_BUFFER, 0);

  return position;
}

void ComputePositionOffset(float &x, float &y) {
  const float loopDuration = 5.0f;
  const float scale = 3.14159f * 2.0f / loopDuration;

  float elapsedTime = glutGet(GLUT_ELAPSED_TIME) / 1000.0f;

  float currentTimeThroughLoop = fmodf(elapsedTime, loopDuration);

  x = cosf(currentTimeThroughLoop * scale) * 0.5f;
  y = sinf(currentTimeThroughLoop * scale) * 0.5f;
}

vertex.v.glsl

layout(location = 0) in vec4 position;

uniform float loopDuration;
uniform float time;

uniform float zNear;
uniform float zFar;
uniform float frustumScale;

void main() {
  float timeScale = 3.14159f * 2.0f / loopDuration;
  float currTime = mod(time, loopDuration);
  vec4 offset = vec4(
    sin(currTime * timeScale),
    cos(currTime * timeScale),
    0, 0
  );

  vec4 cameraPos = position + vec4(offset.x, offset.y, 0.0, 0.0);
  vec4 clipPos;

  clipPos.xy = cameraPos.xy * frustumScale;

  clipPos.z = cameraPos.z * (zNear + zFar) / (zNear - zFar);
  clipPos.z += 2 * zNear * zFar / (zNear - zFar);

  clipPos.w = -cameraPos.z;

  gl_Position = clipPos;
}