OpenGL Rotation Origin Problems

[CaptainRedmuff]

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/ip...d-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?

Any help would be greatly appreciated, thanks for reading

[CaptainRedmuff]

Sorry to bump the post, but i could really use some help with this one.

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?

Thanks for reading.

[Norbert]

I think you have to save the current transformation matrix (CTM) (glPushMatrix?), load the identity matrix, translate your object into the origin, rotate it, translate it back into its original position and restore the CTM (glPopMatrix?).