opengl glulookat strange behavior

Question

can someone please explain me why is the camera in my code moving on every onIdle callback? I am not changing any parameters so it seems to me that it should display the same all the time. Still cannot figure it out by myself, thanks for help

#include <iostream>
#include <math.h>
#include "gl/glut.h"
#include <windows.h>

using namespace std;

void Display(void){

    glMatrixMode(GL_PROJECTION);
    glFrustum(-0.5, 0.5, -0.5, 0.5, 0.8, 2);

    gluLookAt (0,0,1,  0,0,0,  0,1,1);

    glMatrixMode(GL_MODELVIEW);
    glClear(GL_COLOR_BUFFER_BIT| GL_DEPTH_BUFFER_BIT);

    glutWireCube(0.2);

    glFlush();

}


void onIdle(void){

    Sleep(1000);
    glutPostRedisplay();

}

int main(void){

glutInitDisplayMode(GLUT_DEPTH | GLUT_RGBA);
glEnable(GL_DEPTH_TEST);
glutCreateWindow("camera");

glutDisplayFunc(Display);
glutIdleFunc(onIdle);
glutMainLoop();
return 0;
}

score 2 · Answer 1 · answered Oct 11 '13 at 00:30

2

That is because you don't reset the matrices, after each render or at the beginning of each render.

Basically, you need to call glLoadIdentity(); that will reset the current selected matrix, but setting the matrix's values to the default values (Matrix Identity).

answered Oct 11 '13 at 00:30

vallentin

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Well, there's that and then there's also the fact that he's combining projection and modelview into one ugly matrix. And then there's the fact that the up vector is zero. This whole code is a mess :) – Andon M. Coleman Oct 11 '13 at 01:22
Alternatively (and perhaps faster) than calling `glLoadIdentity()` each frame is doing a `glPushMatrix()` before your viewing transform call (e.g. `gluLookAt()`), and then calling `glPopMatrix()` after all of your rendering is complete (i.e., say, right before calling `glFlush()` in your case). – radical7 Oct 11 '13 at 03:58
@AndonM.Coleman Err, the second "set" of parameters to `gluLookAt` is the point you're looking at, and in the OP's case is the origin, and not the up vector, if that's what you were saying. The last three parameters are the up vector, which unless I'm looking at an edited version of the post, isn't a zero vector. – radical7 Oct 11 '13 at 04:02
@radical7: Yeah, you're right. I am used to a different API, which has the forward and up vectors reversed in its `LookAt`. The good news is GLU is not part of OpenGL, so I do not care that I cannot remember the proper order for that particular API function =P. I still stand by my assertion that moving `gluLookAt (...)` to come after the switch to `GL_MODELVIEW` will fix a lot of issues in the fixed-function pipeline, however. – Andon M. Coleman Oct 11 '13 at 04:08
@radical7 An even faster and much better alternative would be to use and calculate all the Matrices yourself. – vallentin Oct 11 '13 at 07:02
@Vallentin Actually, I'd say "that depends ...". In a shader-based world, I agree with you. However, in a fixed-function pipeline world, like this post addresses, using `glLoadMatrix` and `glMultMatrix` - if that's what you're suggesting - may disable some optimizations in the driver where it tracks the type of matrices uses that information to optimize the required matrix multiplications in the pipeline (e.g., if you're only doing 2D projection, then don't do the z component math). That's why operations like `glRotate*` are better than generic matrices. – radical7 Oct 12 '13 at 17:11

opengl glulookat strange behavior

1 Answers1