src/trivial/line-xor.c

   1 /*
   2  * Draw lines in XOR mode.
   3  * Note that the last pixel in a line segment should not be drawn by GL
   4  * so that pixels aren't touched twice at the shared vertex of connected lines.
   5  *
   6  * Brian Paul
   7  * 7 Oct 2010
   8  */
   9
  10
  11 #include <stdio.h>
  12 #include <string.h>
  13 #include <stdlib.h>
  14 #include "glut_wrap.h"
  15
  16
  17 static GLboolean xor = GL_TRUE;
  18
  19
  20 static void
  21 Reshape(int width, int height)
  22 {
  23    glViewport(0, 0, (GLint)width, (GLint)height);
  24
  25    glMatrixMode(GL_PROJECTION);
  26    glLoadIdentity();
  27    glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
  28    glMatrixMode(GL_MODELVIEW);
  29 }
  30
  31
  32 static void
  33 Key(unsigned char key, int x, int y)
  34 {
  35    if (key == 'x') {
  36       xor = !xor;
  37       printf("XOR mode: %s\n", xor ? "on" : "off");
  38    }
  39    else if (key == 27)
  40       exit(0);
  41    glutPostRedisplay();
  42 }
  43
  44
  45 static void
  46 Draw(void)
  47 {
  48    glClear(GL_COLOR_BUFFER_BIT);
  49
  50    glLogicOp(GL_XOR);
  51    if (xor)
  52       glEnable(GL_COLOR_LOGIC_OP);
  53    else
  54       glDisable(GL_COLOR_LOGIC_OP);
  55
  56    glColor3f(0, 1, 0);
  57
  58    /* outer line rect */
  59    glBegin(GL_LINE_LOOP);
  60    glVertex2f(-0.9, -0.9);
  61    glVertex2f( 0.9, -0.9);
  62    glVertex2f( 0.9,  0.9);
  63    glVertex2f(-0.9,  0.9);
  64    glEnd();
  65
  66    /* middle line rect */
  67    glBegin(GL_LINE_STRIP);
  68    glVertex2f(-0.8, -0.8);
  69    glVertex2f( 0.8, -0.8);
  70    glVertex2f( 0.8,  0.8);
  71    glVertex2f(-0.8,  0.8);
  72    glVertex2f(-0.8, -0.8);
  73    glEnd();
  74
  75    /* inner line rect */
  76    glBegin(GL_LINES);
  77    glVertex2f(-0.7, -0.7);
  78    glVertex2f( 0.7, -0.7);
  79
  80    glVertex2f( 0.7, -0.7);
  81    glVertex2f( 0.7,  0.7);
  82
  83    glVertex2f( 0.7,  0.7);
  84    glVertex2f(-0.7,  0.7);
  85
  86    glVertex2f(-0.7,  0.7);
  87    glVertex2f(-0.7, -0.7);
  88    glEnd();
  89
  90    /* inner + pattern */
  91    glBegin(GL_LINES);
  92    glVertex2f(-0.6,  0.0);
  93    glVertex2f( 0.6,  0.0);
  94    glVertex2f( 0.0,  0.6);
  95    glVertex2f( 0.0, -0.6);
  96    glEnd();
  97
  98
  99    glutSwapBuffers();
 100 }
 101
 102
 103 int
 104 main(int argc, char **argv)
 105 {
 106    glutInit(&argc, argv);
 107
 108    glutInitWindowSize(250, 250);
 109
 110    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
 111    glutCreateWindow(argv[0]);
 112    glutReshapeFunc(Reshape);
 113    glutKeyboardFunc(Key);
 114    glutDisplayFunc(Draw);
 115
 116    fprintf(stderr, "GL_RENDERER   = %s\n", (char *) glGetString(GL_RENDERER));
 117    fprintf(stderr, "GL_VERSION    = %s\n", (char *) glGetString(GL_VERSION));
 118    fprintf(stderr, "GL_VENDOR     = %s\n", (char *) glGetString(GL_VENDOR));
 119    fprintf(stderr, "NOTE: there should be no pixels missing at the corners"
 120            " of the line loops.\n");
 121    fprintf(stderr, "There should be a missing pixel at the center of the '+'.\n");
 122    fprintf(stderr, "Resize the window to check for any pixel drop-outs.\n");
 123    fprintf(stderr, "Press 'x' to toggle XOR mode on/off.\n");
 124
 125    glutMainLoop();
 126
 127    return 0;
 128 }