Rotate specific area

OpenGL OpenGL: Basic Coding
1

Hi,
How can i rotate a single area in opengl? tks

2

Define “area” : image, portion of screen, object(s), … ?

3

i have a polygon consisting of several polygons, and I want to rotate one of these polygons. I think i have to use glLoadMatrix but im not sure/dont know how

thanks in advance

4

matrix it is. glLoadMatrix specifies current matrix in current matrix stack f.e glMatrixMode(GL_MODELVIEW). whatever value you put in there your vertices will be “transformed” by this matrix. if you need to rotate objects you need to upload a rotation matrix then. also check glRotate().

5

but how do I define this matrix to put in the glLoadMatrix ? I have a 6 vertice polygon that I want rotated, I know the world coordinates of its vertices and I was successful on rotating the entire polygon, but rotating this single polygon I cant do

6

this is advanced because you either need your own matrix class which performs matrix operations and stores the output in say GLfloat[16] array so you can just pass it to glLoadMatrix, but for writing a matrix class you need excellent math knowledge or you can reuse some matrix classes. there are plenty of them in open source 3d engines.

7

Why do you want to use glLoadMatrix?
It would go something like this without glLoadMatrix
[source]
glPushMatrix();
glTranslatef(x, y, z);
RenderSomeThings();
glPushMatrix();
glRotatef(45.0, 0.0, 1.0, 0.0);
RenderMyArea();
glPopMatrix(); //Restore matrix
glPopMatrix(); //Restore matrix
[/source]

I recommend the Red book. It explains the fundamentals of GL.