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sgibbons

OpenGL Hidden Surface Removal

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sgibbons    122
I've been reading the OpenGL redbook to learn OpenGL, and I found that it lacks much coverage of hidden surface removal. In fact, I couldn't find much on the topic (specific to opengl) anywhere on the internet. I can't seem to make it work, even though I've followed all of the instructions that I've found, which amount to: 1) glutInitDisplayMode with GLUT_DEPTH 2) glEnable(GL_DEPTH_TEST) 3) glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT) whenever drawing a new frame Alas, this has failed me. I'm starting to think it could be a problem with my video card, a Radeon 9200, since whenever I start any program that I've written in openGL it complains: "libGL warning: 3D driver claims to not support visual 0x4b" Is there something important I could be missing here? Does anyone else have any experience with this problem?

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Lord_Evil    680
What is the error / false result you get?

Maybe you are looking for back face culling to remove polygons that don't face the camera.

The error you get seems to be a linux driver error. I personally don't use linus but a quick search on google gives me the impression that the error should not affect the OpenGL rendering engine.

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sgibbons    122
When I move objects around, and redisplay the scene, they are always drawn in the same order, even if something is behind something else.

Here's my code. I think I may have problems with back face culling as well, but it seems like this problem goes beyond that.

Here's my code (it's just a slightly modified example from the red book).

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
static int year = 0, day = 0;
void init(void)
{
glClearColor (0.0, 0.0, 0.0, 0.0);
glShadeModel (GL_SMOOTH);
}
void display(void)
{
// glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f (1.0, 0.0, 0.0);
glPushMatrix();
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//gluLookAt (0.0, 2.5, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glutSolidSphere(1.0, 20, 16); /* draw sun */
glRotatef ((GLfloat) year, 0.0, 1.0, 0.0);
glTranslatef (2.0, 0.0, 0.0);
glRotatef ((GLfloat) day, 0.0, 1.0, 0.0);
glColor3f(0.0, 1.0, 0.0);
glutSolidSphere(0.2, 10, 8); /* draw smaller planet */
glRotatef((GLfloat) year, 0.0, 1.0, 0.0);
glTranslatef (2.0, 0.0, 0.0);
glRotatef((GLfloat) day, 0.0, 1.0, 0.0);
glutSolidSphere(0.2, 10, 8);
glPopMatrix();
glutSwapBuffers();
}
void reshape (int w, int h)
{
glViewport (0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode (GL_PROJECTION);
glLoadIdentity ();
gluPerspective(60.0, (GLfloat) w/(GLfloat) h, 1.0, 20.0);
gluLookAt (0.0, 2.5, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}

void keyboard (unsigned char key, int x, int y)
{
switch (key) {
case 'd':
day = (day + 10) % 360;
glutPostRedisplay();
break;
case 'D':
day = (day - 10) % 360;
glutPostRedisplay();
break;
case 'y':
year = (year + 5) % 360;
glutPostRedisplay();
break;
case 'Y':
year = (year - 5) % 360;
glutPostRedisplay();
break;
default:
break;
}
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glEnable(GL_DEPTH_TEST);
//glDepthFunc(GL_GREATER);
glutInitWindowSize (500, 500);
glutInitWindowPosition (100, 100);
glutCreateWindow (argv[0]);
init ();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutMainLoop();
return 0;
}

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Lord_Evil    680
First, gluLookAt should be called in MODELVIEW mode, since it defines the camera's position in the world. In your example it affects the projection matrix but the camera is still centered at the origin (like the sun thus ending up inside the sun).

Second, do you intend to rotate the first planet around the sun and the second planet around the first one? If so you should probably exclude the 'day' rotation of the first planet from the rotation matrix of the second planet and decrease translation.x. In your example the second planet is rotating by planet1.day + planet2.year around the first planet with the same translation distance. So if you have a 'year' angle of 180 degrees and a zero-degree 'day' angle the first planet is rotated by 180 degrees around the sun, the second is rotated by 180 degrees around the sun (same transformation as planet1) then rotated by another 180 degrees around planet1, so ending up with a 360 degree rotation and a zero-translation (translation1 = (2/0/0) * rotation by 180 around (0/1/0) = (-2/0/0); translation2 = (2/0/0) * * rotation1 by 360 around (0/1/0) = (2/0/0); translation = translation1+ translation2 = (0/0/0)). This means the second planet would be drawn inside the sun in that special case ... intended?

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