OpenGL Rotation Origin Problems

Question

Hi guys,
I've had a browse around and can't seem to find any mention of this, i'm starting to make progress with learning OpenGL, but theres one thing thats totally throwing me off for the time being. Maybe someone will be kind enough to shed some light on this for me.
Currently, i am simply rendering a textured rectangle to the screen. I was thumbing through tutorials and wanted to improve my rendering by adding in some rotation and possibly throw in some alpha blending when i came across this post http://www.iphonedevsdk.com/forum/iphone-sdk-development/2060-how-extend-texture 2d-opacity.html
Huzzah i thought, as i happily modded my blitting method to include rotations and was presented with a slightly rotated object, but all was not as expected. For some reason (which i'm sure is totally obvious), the rectangle is rotated about the top left point, ie: 0.0, 0.0 which is really what i would expect, 0.0, 0.0 being the origin and all... but i want my object to be rotated around its center point, not the origin of the screen.
I have set up my glOrthof and glViewport for landscape orientation as such:
glViewport(0.0, 0.0, backingWidth, backingHeight);
//other stuff here
glOrthof(0.0, backingHeight, backingWidth, 0.0, -1.0, 1.0);
This gives me a landscape orientation with the top left coords being 0.0, 0.0 and bottom right being 480.0, 320.0 which is my prefered coordinate system. As of such, this appears to be the main origin for all object rotations, which isn't really what i want. Is there any way i can translate an objects origin to rotate around its center point?
for example, if i were to render the rect:
my_rect = CGRectMake(backingWidth / 4, backingHeight / 4, backingWidth / 2, backingHeight / 2);
a small rect half the width and height of the device would be presented directly in the center of the screen. Any rotations made to this however would be about the origin of 0.0, 0.0 of the screen and not the object itself. Is there any way to fix this so that the object would rotate around its center piont, in this example being the center of the screen?
I dont mind changing the coordinate system if needs be, but i would still prefer to use the current 0.0, 0.0, 480.0, 320.0 landscape orientation as opposed to -1.0, 1.0, 1.0, -1.0 unless of course it would be a feasible endeavour to write a utility function to convert between the two coordinate systems when rendering?
It seems that rotations must be made around the origin of the screen/device (0.0, 0.0) but surely that can't be the case as not every object/model rendered is required to rotate around the origin, but rather its own origin, if that makes sense?
Is there a way to set the rotation origin, or translate the object to the screen origin, rotate and back again?
Using the coordinate system 0.0, 0.0, 480.0, 320.0 for landscape orientation works perfectly, but obviously this issue with rotating objects around the origin is completely holding up any progress i can make.
Please, if anyone knows of a method, or any tutorials that can outline what must be done, i would be very grateful for any help you guys can throw my way
Just for completeness, the code i'm using for rotation is:
glRotate(rotation, 0.0, 0.0, 1.0);
Simple as it may seem, that one single line has caused me a weekend of headaches and very little progress. I'm positive there must be a way to fix this, can anyone point me in the right direction please?
Any help would be greatly appreciated, thanks for reading

Answer 1

Sorry to bump the post, but this is really starting to grind my gears. If anyone can provide a push in the right direction, i really, really would appreciate it
I'm simply modding the Texture2D class from Apple's CrashLanding app to suit my needs but have hit this one stumbling block.
The code i currently have is taken from Texture2D and modifed slightly, as follows:
- (void)drawInRect:(CGRect)rect withRotation:(GLfloat)rotation
GLfloat coords[] = {
0.0, _maxT,
_maxS, _maxT,
0.0, 0.0,
_maxS, 0.0
GLfloar vertices[] = {
rect.origin.x, rect.origin.y, 0.0,
rect.origin.x + rect.size.width, rect.origin.y, 0.0,
rect.origin.x, rect.origin.y + rect.size.height, 0.0,
rect.origin.x + rect.size.width, rect.origin.y + rect.size.height, 0.0
glPushMatrix();
//this line is my assumed fix, but needs refining
glTranslate(0.0, 0.0, 0.0);
//rotate
glRotate(rotation. 0.0, 0.0, 1.0);
glBindTexture(GL_TEXTURE_2D, _name);
glVertexPointer(3, GL_FLOAT, 0, vertices);
glTexCoordPointer(2, GL_FLOAT, 0, coords);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glPopMatrix();
The above code renders my rect properly with the right dimensions, at the right coordinates.
The problem im having is that any rotations are made around the top left corner of the rect, which is the top left corner of the screen. I've set my viewport and glOrtho to present in landscape orientation which works fine, but obviosuly the rotations aren't what i desire.
Surely there must be a way to "move" the origin about which the texture is rotated, ideally to the center of the texture itself. However, no matter what params i pass to glTranslatef(); it always rotates around the top left point of the rect itself.
Please, please, can anyone help with this as it is really starting to drive me mad. Any help would be fantastic, a tutorial, a few lines of code or improvement upon what i posted above. Anything will do, i really want to get this solved so i can move on with things.
Thanks for taking the time to read, i look forward to any replies