Openframeworks - rotate an image from its center through OpenGL calls

问题

I'm using openframeworks (which renders through OpenGL) and I'm trying to rotate an image from it's center.

I know I should use ofRotate() and ofTranslate() but I haven't managed to figure it out by myself. Here's what I've tried so far:

ofPushMatrix();
ofRotate(ofRandom(10),0.0,0.0,1.0);
leafImg.draw(100,100);
ofPopMatrix();

回答1:

Without doing too much math you can use a nested coordinate system offset the image so that when you rotate, you rotate from the centre. In short you'll be doing this:

Move the coordinate system to centre of the image
Rotate from there
Within that coordinate system do one level deeper and translate back by half the image size so you're offset back to '0,0'

In code this would be:

ofPushMatrix();
    ofTranslate(leafImg.width/2, leafImg.height/2, 0);//move pivot to centre
    ofRotate(ofGetFrameNum() * .01, 0, 0, 1);//rotate from centre
    ofPushMatrix();
        leafImg.draw(-leafImg.width/2,-leafImg.height/2);//move back by the centre offset
    ofPopMatrix();
ofPopMatrix();

I've used the indenting to make it more obvious how the coordinate systems nest.

It's the same as:

ofPushMatrix();
    ofTranslate(leafImg.width/2, leafImg.height/2, 0);//move pivot to centre
    ofRotate(ofGetFrameNum() * .01, 0, 0, 1);//rotate from centre
    ofPushMatrix();
        ofTranslate(-leafImg.width/2,-leafImg.height/2,0);//move back by the centre offset
        leafImg.draw(0,0);
    ofPopMatrix();
ofPopMatrix();

It's fairly simple to rotate to centre as you can see. In your spare time try to work out how to rotate against an arbitrary point.

Behind the scenes there's a bit of linear algebra going on, but push/pop matrix calls basically handle the nitty gritty matrix multiplications for you. Still you need to understand and practice working with pushMatrix/popMatrix calls. Even though this is for Processing, the principle is exactly the same and the syntax is very similar, so I recommend this article: 2D Transformations

来源：https://stackoverflow.com/questions/12516550/openframeworks-rotate-an-image-from-its-center-through-opengl-calls

标签

c++

opengl

openframeworks