In the book 《OpenGL Programming Guide》, i run the image.c,the source code of the book,
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/* image.c
* This program demonstrates drawing pixels and shows the effect
* of glDrawPixels(), glCopyPixels(), and glPixelZoom().
* Interaction: moving the mouse while pressing the mouse button
* will copy the image in the lower-left corner of the window
* to the mouse position, using the current pixel zoom factors.
* There is no attempt to prevent you from drawing over the original
* image. If you press the 'r' key, the original image and zoom
* factors are reset. If you press the 'z' or 'Z' keys, you change
* the zoom factors.
*/
#include <GL/glut.h>
#include <stdlib.h>
#include <stdio.h>
/* Create checkerboard image */
#define checkImageWidth 64
#define checkImageHeight 64
GLubyte checkImage[checkImageHeight][checkImageWidth][3];
static GLdouble zoomFactor = 1.0;
static GLint height;
void makeCheckImage(void)
{
int i, j, c;
for (i = 0; i < checkImageHeight; i++) {
for (j = 0; j < checkImageWidth; j++) {
c = ((((i&0x8)==0)^((j&0x8))==0))*255;
checkImage[i][j][0] = (GLubyte) c;
checkImage[i][j][1] = (GLubyte) c;
checkImage[i][j][2] = (GLubyte) c;
}
}
}
void init(void)
{
glClearColor (0.0, 0.0, 0.0, 0.0);
glShadeModel(GL_FLAT);
makeCheckImage();
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
}
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glRasterPos2i(0, 0);
glDrawPixels(checkImageWidth, checkImageHeight, GL_RGB,
GL_UNSIGNED_BYTE, checkImage);
glFlush();
}
void reshape(int w, int h)
{
glViewport(0, 0, (GLsizei) w, (GLsizei) h);
height = (GLint) h;
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0, (GLdouble) w, 0.0, (GLdouble) h);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void motion(int x, int y)
{
static GLint screeny;
screeny = height - (GLint) y;
glRasterPos2i (x, screeny);
glPixelZoom (zoomFactor, zoomFactor);
glCopyPixels (0, 0, checkImageWidth, checkImageHeight, GL_COLOR);
glPixelZoom (1.0, 1.0);
glFlush ();
}
void keyboard(unsigned char key, int x, int y)
{
switch (key) {
case 'r':
case 'R':
zoomFactor = 1.0;
glutPostRedisplay();
printf ("zoomFactor reset to 1.0
");
break;
case 'z':
zoomFactor += 0.5;
if (zoomFactor >= 3.0)
zoomFactor = 3.0;
printf ("zoomFactor is now %4.1f
", zoomFactor);
break;
case 'Z':
zoomFactor -= 0.5;
if (zoomFactor <= 0.5)
zoomFactor = 0.5;
printf ("zoomFactor is now %4.1f
", zoomFactor);
break;
case 27:
exit(0);
break;
default:
break;
}
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(250, 250);
glutInitWindowPosition(100, 100);
glutCreateWindow(argv[0]);
init();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutMotionFunc(motion);
glutMainLoop();
return 0;
}
According to the explanation in the book, the image should be displayed in the bottom left corner of the window, and the image in the lower left corner and the bottom left corner of the window coincides (normal),
But the actual operation, the image into the upper left corner of the window, and the upper left corner of the image and the upper left corner of the window coincides (anomalies).
I changed glutInitDisplayMode (GLUT_SINGLE |; GLUT_RGB) into the glutInitDisplayMode (GLUT_RGB GLUT_DOUBLE |);, it is Normal again. why? I am confused!
I studied for a week, or don’t know what the problem is, I don’t know why GLUT_SINGLE single buffer, why not displaying correctly,
Who can help answer the question?
My OpenGL version is 1.3,(use the implement of Mesa Project,2.1 Mesa 9.2.0-devel) and my computer os is CentOS6.5.