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 !

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 !