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viorik
02-01-2014, 05:06 PM
Hi,

I am totally beginner with opengl. I have a very simple task: given a 3d model, I need to rotate a camera around it on a semi-sphere (y>0) and get the views from every angle.
I use gluLookAt, with spherical coordinates for eyex,eyey,eyez. The object is in (0,0,0) and up vector is (0,1,0). The camera moves on a sphere of radius 2.
My camera starts near (0,0,2) and walks around the object. The problem is when eyez gets <=0. My object gets flipped and "goes back"; it does not do the full 360 rotation. And the images I obtain when eyez <=0 are actually mirror versions of my object. The other images are ok.
I checked the eye coordinates, they are correct; also in the modelview matrix, the vectors seem coherent, so I don't understand why the flipping.
I am putting here some code, maybe someone can spot the error(s).

This is the loop where I do the camera orbiting:


glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(15.0f,1.0f,0.1f,4.0f);
step = 10;
theta = 80;
while( theta > 0 )
{
phi = 0;
while( phi < 360 )
{
get_pos( radius, theta, phi, camera_pos );
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt( camera_pos[0], camera_pos[1], camera_pos[2], 0, 0, 0, 0, 1, 0 ); */
draw_triangles();
glutSwapBuffers();

glGetDoublev(GL_MODELVIEW_MATRIX, modelview);
glGetDoublev(GL_PROJECTION_MATRIX, projection);
glGetIntegerv(GL_VIEWPORT, viewport);

write_image( imname , compression_params );
phi += step;
}
theta -= step;
}


To get the EYE coordinates I have:


void get_pos( double radius, double theta, double phi, double *camera_pos )
{
/* get angles in radians */
double th = theta*0.0174532;
double ph = phi*0.0174532;
camera_pos[0] = radius * sin(th) * sin(ph);
camera_pos[1] = radius * cos(th);
camera_pos[2] = radius * sin(th) * cos(ph);
}

If anyone could give me a hint of what I'm doing wrong, that would be really great. I doubt it's a gimbal lock problem, I suspect more a silly beginner error.

Thanks in advance !