Rotation around Center

Newbie & Debugging Questions
#1

Hi guys,

I want to rotate a mesh around its center (kinda like in a arcball). The mesh is read in from a .obj file.
The way I have it now, the model is rotating around a point which is not its center (kinda like if you run around a house in circles).

I read that I would have to translate the object to 0,0,0 and then rotate, but I have no ideas which values I should take.

Oh, I am using JOGL btw.

Here’s the code for rendering:

	
public void display(GLAutoDrawable drawable) {
		final GL gl = drawable.getGL();		
		gl.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);
		gl.glLoadIdentity();
		
		if(zoom)
			setProjectionMatrix(gl);
		
		drawBone(gl);

		
		gl.glFlush();

	
private void drawBone(GL gl){
		gl.glPushMatrix();
		
		gl.glTranslated(0, 0,0);		
				
		gl.glRotated(rotX, 1, 0, 0);
		gl.glRotated(rotY, 0, 1, 0);
		
		if(faces != null){
			for(Face f: faces){
				f.draw(gl);
			}
		}

		gl.glPopMatrix();
	}

reshape:

	public void reshape(GLAutoDrawable drawable, int x, int y, int w, int h) {
		GL gl = drawable.getGL();
		if(h<=0){
			this.height=1;
		}
		else{
			this.height = h;
		}
		this.width = w;
		
		aspect = (float)width/(float)height;
		
		// Set the viewport to be the entire window
		gl.glViewport(0, 0, width, height);
		
		// Reset the coordinate system before modifying
		gl.glMatrixMode(GL.GL_PROJECTION);
		gl.glLoadIdentity();
		
		// Set the correct perspective.
		glu.gluPerspective(DEFAULT_ZOOM*zoomFactor, aspect, 1.0, 1000.0f);
		
		gl.glMatrixMode(GL.GL_MODELVIEW);
		gl.glLoadIdentity();
		
		glu.gluLookAt(0.0, 0.0, 5.0, 
		0.0, 0.0, 0.0, 
		0.0, 1.0, 0.0);

	
public void setProjectionMatrix(GL gl){
		GLU glu = new GLU();
		
		gl.glMatrixMode(GL.GL_PROJECTION);
		gl.glLoadIdentity();
		
		glu.gluPerspective(DEFAULT_ZOOM*zoomFactor, aspect, 1f, 100.0f);
		
		gl.glMatrixMode(GL.GL_MODELVIEW);
		gl.glLoadIdentity();
	}

Any help is much appreciated. Thank you!

#2

If you plan on rotating the object from it’s center a lot, you probably want to go ahead and center it’s vertices around a 0,0,0 origin on import.

#3

tha following have no effect

gl.glTranslated(0, 0,0);

what you probalby want to do is

gl.glTranslated(-centerX, -centerY,-centerZ);

where centerX,centerY,centerZ is the center of your object vertices in local object space => sum all your vertices and divide each result by number of vertices (inded do it only once and apply same transform to it if you apply/modify vertices of your object (zoom for example…) like if it was another vertex)

for(n=0;n<nbVertices;n++) 
{ 
centerX+=v[n].x;
centerY+=v[n].y;
etc....
} 
centerX/=nbVertices;
centerY/=nbVertices;
etc...
#4

yes, the rotation will be only done around the objects center :slight_smile: how would i center the vertices around the origin at import? with the method DzzD described below or is there a simpler way ?

thanks for your suggestion, i will try this and report back my results :slight_smile:

#5

okay, i did what you said and it seems it’s better now :slight_smile:

instead of the object flying in circles around “my head”, i can see it now in front of me and it’s rotating. but still not around its center, it looks like it’s attached to a point at its lower side and it’s going around circles there :frowning:

#6

may be verify the value of centerx,centery,centerz to see if they match the intended object center location. averaging vertices to find center may not work as expected in severals cases.

#7

hi DzzD

it might be a stupid question, but how can i find out the intended object center location coordinates ?

#8

find the bouding box : find min X,max X min Y max Y, min Z max Z of all vertices this will give you the bouding box then you got a pretty good center point with
centerX=(maxX+minX)/2;
centerY=(maxY+minY)/2;
centerZ=(maxZ+minZ)/2;

#9

Thank you very much for your answer DzzD.

The bounding box coordinates are around the same coordinates as the ones I calculated before with summing up each axis and dividing through number of points, so that seems to be okay.

getCenterVertex at : 119.68530292026117/62.29729093334656/239.88274836337095
Bounding box coordinates at : 116.318302/67.105699/248.7705065

I still don’t get why it is not rotating around the centroid :frowning:

Did I set up my view wrong ?

#10

Sure there isn’t some junk data in the original mesh which is skewing your center?

Kev

#11

I’m not really sure, no. How would I go and check this ?