ModelView Vs Projection

Hi everyone

First, I need to say that I just searched but my problem still stood still !
I wrote my own transformation and rotation functions and I called them elRotateX() for example for rotating around X,y,z axis.


void elRotateX()
{
GLfloat mat[] = { 
1,	0,	0,	0,	// X Column
0,	cos(xRot),	-sin(xRot),	0,	// Y Column
0,	sin(xRot),	 cos(xRot),	0,	// Z Column
0,	0,	 0,	1	// Trans
};		
glMatrixMode(GL_MODELVIEW);
glLoadMatrixf(mat);
}

I also wrote my own


void elLoadIdentity()
{
GLfloat mat[] = {  
1,	0,	0,	0,	// X Column
0,	1,	0,	0,	// Y Column 
0,	0,	1,	0,	// Z Column
0,	0,	0,	1	// Trans
};
glMatrixMode(GL_MODELVIEW);
glLoadMatrixf(mat);
}


and this is the imporatant part of my resize function:


	glMatrixMode( GL_PROJECTION );
	elLoadIdentity();
	gluPerspective(25,(float)w/(float)h,1,200);
	glMatrixMode(GL_MODELVIEW);
	elLoadIdentity();


My rotation variables (xRot, yRot, zRot) are increased or decreased by Arrow keys.
When I use elRotate functions as above just rotates around Z axis. I pushed and popped matrixes and I replaced GL_MODELVIEWs
with GL_PROJECTION but I can’t rotate all three axises together.(for example x,y rotates but z doesn’t)
I read some Matrix concepts and I found out MODELVIEW you need to move whole scene with the inverse transformation.
but i didn’t get what exactly Projection does.

In my case how can I rotate all three axises?

This might help. Here is my replacement for glRotatef. It comes from the formula on this page http://www.opengl.org/documentation/specs/man_pages/hardcopy/GL/html/gl/rotate.html

Just replace BaseType with float if you are unfamiliar with templates:


		/**
		 * @param angle The angle is specified in radians. 
		 */
		template<typename BaseType>
		static	Matrix	MakeRotationMatrix(typename BaseType::Type angle,Vec<BaseType> const&axis)
		{
			assert(BaseType::mComponentCount==(N-1));
			assert(N==4);
			
			Vec<BaseType> normAxis=axis.Normalized();
			typename BaseType::Type x=normAxis.x,y=normAxis.y,z=normAxis.z;
			typename BaseType::Type c=::cos(angle);
			typename BaseType::Type s=::sin(angle);
			
			Matrix ret;
			ret[0][0]=x*x*(1-c)+c;
			ret[0][1]=x*y*(1-c)-z*s;
			ret[0][2]=x*z*(1-c)+y*s;
			
			ret[1][0]=y*x*(1-c)+z*s;
			ret[1][1]=y*y*(1-c)+c;
			ret[1][2]=y*z*(1-c)-x*s;
			
			ret[2][0]=x*z*(1-c)-y*s;
			ret[2][1]=y*z*(1-c)+x*s;
			ret[2][2]=z*z*(1-c)+c;
			
			return ret;
		}

  • Sean

Thanx spurserh.

I’ll surely try that.
But something tells me that there is something wrong with my glMatriXMode() function.
The thing is I don’t know exactly where to use GL_PROJECTION or GL_MODELVIEW.

The projection matrix should only hold the 2 transformations: elLoadIdentity() and gluPerspective(…) . Nothing more!
The camera (view) and stuff are all put inside the modelview matrix.

The problem you’re facing is caused by these 2 lines inside your elRotateX() :
glMatrixMode(GL_MODELVIEW); // unnecessary
glLoadMatrixf(mat); // uhoh, you’re overwriting!

In elRotateX():

  1. remove the redundant glMatrixMode(GL_MODELVIEW);
  2. replace glLoadMatrixf(mat) with glMultMatrixf(mat)

In elLoadIdentity():

  1. remove the line glMatrixMode(GL_MODELVIEW);
    you’re good to go.

P.S. for some fun, see how OpenGL behaves if you put all transformations into only one of the 2 matrices: either in GL_PROJECTION or GL_MODELVIEW… the results are identical :smiley: (as long as you don’t rely on vertex-normals) . This is because in the end both matrices get internally concatenated into a “ModelViewProjection” matrix.

It works now perfectly.

Thanks Ilian