OpenGL hands-on tutorial

Running this tutorial on Linux desktop one requires at least the OpenGL 2.0 graphics with the OpenGL Shading Language (GLSL) 1.1 and supporting libraries GL, GLU, GLUT, GLEW. This can be verified with the following commands:

$ glxinfo |grep OpenGL.*version
OpenGL version string: 2.1 Mesa 8.0.5
OpenGL shading language version string: 1.20
$ ls /usr/include/GL/{glut.h,glew.h,gl.h,glu.h}
/usr/include/GL/glew.h  /usr/include/GL/glu.h
/usr/include/GL/gl.h    /usr/include/GL/glut.h

Legacy OpenGL

Create the following first.c using your favorite editor.

#include <GL/glut.h>

void display()
{
  glClear(GL_COLOR_BUFFER_BIT);
  glColor3f(1.0, 0.4, 1.0);
  glBegin(GL_LINES);
    glVertex2f(0.1, 0.1);
    glVertex3f(0.8, 0.8, 1.0);
  glEnd();
  glutSwapBuffers();
}

int main(int argc, char *argv[])
{
  glutInit(&argc,argv);
  glutInitDisplayMode(GLUT_DOUBLE);
  glutCreateWindow("first.c GL code");
  glutDisplayFunc(display);
  glutMainLoop();
  return 0;
}

Create Makefile to build your program.

CFLAGS=-Wall
LDFLAGS=-lGL -lGLU -lglut -lGLEW

ALL=first
default: $(ALL)

first : first.o

clean:
      rm -rf *~ *.o $(ALL)

Beware that Makefile is TAB aware. So the last line should contain TAB indentation and not spacing.

Make and run the program with

make
./first

Try the same program in Python

from OpenGL.GLUT import *
from OpenGL.GL import *
import sys

def display():
    glClear(GL_COLOR_BUFFER_BIT)
    glColor3f(1.0, 0.4, 1.0)
    glBegin(GL_LINES)
    glVertex2f(0.1, 0.1)
    glVertex3f(0.8, 0.8, 1.0)
    glEnd()
    glutSwapBuffers()

if __name__ == "__main__":
    glutInit(sys.argv)
    glutInitDisplayMode(GLUT_DOUBLE)
    glutCreateWindow("first.py GL code")
    glutDisplayFunc(display)
    glutMainLoop()

and run it with

python first.py

Exercise #1:

1. Add RGB color to vertices with glColor3f(0.0, 0.4, 1.0);.
2. Replace single line drawing in display() with the following snippet

      GLfloat vertices[][2] = {
       { -0.90, -0.90 }, // Triangle 1
       {  0.85, -0.90 },
       { -0.90,  0.85 },
       {  0.90, -0.85 }, // Triangle 2
       {  0.90,  0.90 },
       { -0.85,  0.90 }
      };
   

   and try to draw two wireframe triangles in a loop. Change primitive to GL_LINE_LOOP.
3. Draw two primitives with GL_TRIANGLES.
4. Add different color to each vertex.

    GLfloat color[][3] = {
     {1, 0, 0}, {0, 1, 0}, {0, 0, 1},
     {1, 1, 0}, {0, 1, 1}, {1, 0, 1}};
   

5. Replace loop with the following

     glVertexPointer(2, GL_FLOAT, 0, &vertices[0][0]);
     glEnableClientState(GL_VERTEX_ARRAY);
     glDrawArrays(GL_TRIANGLES, 0, 6);
     glDisableClientState(GL_VERTEX_ARRAY);
   

   How can we add color to vertices? See ​glColorPointer and ​glEnableClientState.
6. Change background to glClearColor(0.9,1,1,1.0);

Modern OpenGL

We extend previous exercise with example that introduces OpenGL 3.x techniques:

• OpenGL Shading Language where simple vertex and fragment shader are required.
• Vertex Buffer Objects stored in GPU

Create triangle.c and update Makefile with new target

#include <stdio.h>
#include <stdlib.h>
#include <GL/glew.h>
#include <GL/glut.h>

static const GLchar * vertex_shader[] =
  {"void main()"
   "{"
   "  gl_Position = ftransform();"
   "}"
  };
static const GLchar * fragment_shader[] =
  {"void main()"
   "{"
   " gl_FragColor = vec4(0.4,0.4,0.8,1.0);"
   "}"
  };

void setShaders()
{
  GLuint v, f, p;

  v = glCreateShader(GL_VERTEX_SHADER);
  f = glCreateShader(GL_FRAGMENT_SHADER);
  glShaderSource(v, 1, vertex_shader, NULL);
  glShaderSource(f, 1, fragment_shader, NULL);
  glCompileShader(v);
  glCompileShader(f);
  p = glCreateProgram();
  glAttachShader(p,f);
  glAttachShader(p,v);
  glLinkProgram(p);
  glUseProgram(p);
}


enum VAO_IDs { Triangles, NumVAOs };
enum Buffer_IDs {ArrayBuffer,NumBuffers};
enum Attrib_IDs { vPosition = 0 };
GLuint VAOs[NumVAOs];
GLuint Buffers[NumBuffers];
#define NumVertices  6 

void init(void)
{
  glGenVertexArrays(NumVAOs, VAOs);
  glBindVertexArray(VAOs[Triangles]);
  GLfloat vertices[NumVertices][2] = {
    { -0.90, -0.90 }, // Triangle 1
    {  0.85, -0.90 },
    { -0.90,  0.85 },
    {  0.90, -0.85 }, // Triangle 2
    {  0.90,  0.90 },
    { -0.85,  0.90 }
  };
  glGenBuffers(NumBuffers, Buffers);
  glBindBuffer( GL_ARRAY_BUFFER, Buffers[ArrayBuffer]);
  glBufferData( GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
  glVertexAttribPointer(vPosition, 2, GL_FLOAT, GL_FALSE, 0, (const void*)0);
  glEnableVertexAttribArray(vPosition);
}


void display(void)
{
  glClear(GL_COLOR_BUFFER_BIT);
  glBindVertexArray(VAOs[Triangles]);
  glDrawArrays(GL_TRIANGLES, 0, NumVertices);
  glutSwapBuffers();
}

int main(int argc, char **argv)
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE);
  glutCreateWindow("GLSL Intro");
  glutDisplayFunc(display);
  glewInit();
  if (!glewIsSupported("GL_VERSION_2_0"))
   {
     printf("GLSL not supported\n");
     exit(EXIT_FAILURE);
   }
  glClearColor(0.9,1.0,1.0,1.0);
  init();
  setShaders();
  glutMainLoop();
  return 0;
}

Exercises #2

1. To be able to continue and not get lost introduce shader compiler logs in case of compilation errors by adding the following code into {{{setShaders()}} right at after vertex shader compilation:

    GLint compiled;
    glGetShaderiv(v, GL_COMPILE_STATUS, &compiled );
    if ( !compiled ) {
      GLsizei len;
      glGetShaderiv( v, GL_INFO_LOG_LENGTH, &len );
      GLchar* log = malloc(sizeof(GLchar)*(len+1));
      printf("Shader compilation failed: %s\n", log);
      free(log);
    }
   

   Do not forget to repeat the same thing for fragment shader.
2. Add linker debugging

    GLint linked;
    glGetProgramiv(p, GL_LINK_STATUS, &linked );
    if ( !linked ) {
      GLsizei len;
      glGetProgramiv( p, GL_INFO_LOG_LENGTH, &len );
      GLchar* log = malloc(sizeof(GLchar)*(len+1));
      glGetProgramInfoLog( p, len, &len, log );
      printf("Shader linking failed: %s\n", log);
      free(log);
    }
   

   Create some error to verify if it works.
3. For general (core) OpenGL errors we can use the following utility at suspicious places.

      GLenum errCode;
      if ((errCode = glGetError()) != GL_NO_ERROR) {
         const GLubyte *errString = gluErrorString(errCode);
         fprintf (stderr, "OpenGL Error: %s\n", errString);
      }