/*  readpixels.c   */

/* E. Angel, Interactive Computer Graphics */
/* A Top-Down Approach with OpenGL, Third Edition */
/* Addison-Wesley Longman, 2003 */

/* reads pixels values from display and prints out RGB */
#include <stdlib.h>#include <stdio.h>#include <GL/glut.h>int n=300;int m=300;void display(){    glClear(GL_COLOR_BUFFER_BIT);    glColor3ub(255, 255, 127);    glRecti(0,0,n,m);    glFlush(); }void myReshape(int w, int h){    glViewport(0, 0, w, h);    glMatrixMode(GL_PROJECTION);    glLoadIdentity();    gluOrtho2D(0.0, (GLfloat) w, 0.0, (GLfloat) h);    glMatrixMode(GL_MODELVIEW);}void mouse(int state, int button, int x, int y){	int i,j;	GLubyte pixels[3];	glutPostRedisplay();	glReadPixels(x,m-y,1,1,GL_RGB,GL_UNSIGNED_BYTE, pixels);	for(i=0;i<2;i++) for(j=0;j<2;j++)	{		printf("%d %d %d\n", pixels[0], pixels[1], pixels[2]);	}}void myinit(){    glClearColor (0.0, 0.5, 1.0, 1.0);    gluOrtho2D(0.0, 0.0, (GLfloat) n, (GLfloat) m);    glReadBuffer(GL_BACK);     glPixelStorei(GL_PACK_ALIGNMENT, 1);    /* glDisable(GL_DITHER); */}main(int argc, char *argv[]){    glutInit(&argc, argv);    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB );    glutInitWindowSize(500, 500);    glutCreateWindow("image");    myinit();    glutReshapeFunc(myReshape);    glutMouseFunc(mouse);    glutDisplayFunc(display);    glutMainLoop();}