/* Minimal OpenGL code to replicate part of
 * https://bugzilla.opensuse.org/show_bug.cgi?id=1021803
 *
 * gcc -Wall -Wextra -pedantic -o missing-pixels missing-pixels.c -lglut -lGL -lGLU
 */

#include <stdlib.h>

#ifdef __APPLE__
#include <OpenGL/gl.h>
#include <OpenGL/glu.h>
#include <GLUT/glut.h>
#else
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#endif




/****************************
 * GLUT window event handlers
 ***************************/

void on_display(void)
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  /* Some context.
   * Black.
   */
  glColor3f(0.0, 0.0, 0.0);
  glBegin(GL_LINE_STRIP);
    glVertex3f(11, 10, 0);
    glVertex3f(20, 10, 0);
  glEnd();
  glBegin(GL_LINE_STRIP);
    glVertex3f(11, 20, 0);
    glVertex3f(20, 20, 0);
  glEnd();
  glBegin(GL_LINE_STRIP);
    glVertex3f(30, 11, 0);
    glVertex3f(30, 20, 0);
  glEnd();
  glBegin(GL_LINE_STRIP);
    glVertex3f(40, 11, 0);
    glVertex3f(40, 20, 0);
  glEnd();



  /* The problematic lines.
   * Problematic renderers omit the first pixel.
   * Red.
   */
  glColor3f(1.0, 0.5, 0.5);
  glBegin(GL_LINE_STRIP);
    glVertex3f(10, 20, 0);
    glVertex3f(10, 10, 0);
  glEnd();

  glBegin(GL_LINE_STRIP);
    glVertex3f(40, 10, 0);
    glVertex3f(30, 10, 0);
  glEnd();


  glFlush();
  glutSwapBuffers();
}

void on_reshape(int w, int h)
{
  /* Set up the Viewport transformation */
  glViewport(0, 0, (GLsizei) w, (GLsizei) h);

  /* Set up the Projection transformation */
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  gluOrtho2D(0, w-1, h-1, 0);

  /* Switch to Model/view transformation for drawing objects */
  glMatrixMode(GL_MODELVIEW);
}




/****************************
 * Main function
 ***************************/

int main(int argc, char** argv)
{
  glutInit(&argc, argv);
  glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGBA);
  glutInitWindowSize (500, 500);
  glutInitWindowPosition (100, 100);
  glutCreateWindow (argv[0]);

  /* Init GL */
  glClearColor(1.0, 1.0, 1.0, 0.0);
  glShadeModel(GL_FLAT);
  /* glShadeModel(GL_SMOOTH); */

  glEnable(GL_BLEND);
  glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);


  /* Init camera matrix */
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();

  /* Register callbacks */
  glutDisplayFunc(on_display);
  glutReshapeFunc(on_reshape);

  glutMainLoop();
  return 0;
}