glTranslatef() is moving both, my origin and my sprite

Question

Here is the deal, I'm programming a 2D framework/game engine with opengl ES. I am using VBOs and an ortho projection to draw an arrangement of sprites throughout the screen (as part of the testing), and everything was going nice and smooth until I had to play with translations and rotations. The specific problem I am having is that when I apply a translation with glTranslatef() prior to the rotation, the function does not only move the sprite, but also my origin, messing up my whole transformation. I am 100% sure it is working this way, because I used glTranslatef() to move to the right and bottom the sprite half of the size of the screen (yes, my origin is in the top left) and then apply a constant rotation and the thing just keeps mooving in a circular path around the center of the screen (actually rotating, but not as I expect.

If you want some code, here we go:

gl.glTranslatef(-(x+width/2), -(y+height/2), -layer);
gl.glRotatef(angle, 0.0f, 0.0f, -1.0f);
gl.glTranslatef(x+width/2, y+height/2, layer);

In this fragment of code, x and y are the position of the sprite, height and width are the size of the sprite, angle the angle of rotation, and layer just a form of organizing the sprites into several layers, pretty straight forward, right?

Again, my problem is that glTranslatef(); is moving both, the sprite and the origin, am I doing something wrong or misunderstanding something about the translation?

Thanks in advance.

Answer 1

you might need to use glPushMatrix and glPopMatrix since anything you do after those translations and rotations will be affected by them

but what you are describing is actually how it works, if you use a translate, that sort of becomes your new origin because once you do a translate, everything after that is affected by that translate, thats why you need to push and pop, so that you can go, push -> translate object and/or rotate -> pop, and then you can go about with whatever other translations you need to do without having that previous translation affecting everything else

its a bit confusing at first but google around and you'll see how to use them properly

http://www.khronos.org/opengles/sdk/1.1/docs/man/glPushMatrix.xml

Answer 2

I think you misunderstood how matrices work in openGL. When you do a matrix operation such as glRotatef and glTranslatef the matrices are being multiplied, resulting in affecting the base vectors.. For instance, let's say we are only drawing a point that starts at (0,0,0). If you call translate(1,0,0) the point will be in (1,0,0), after that you call rotate(90, 0, 0, 1) and your point will be on the same place as before but rotated. Now the last call is translate(-1,0,0) and your point is at (1,-1,0) (and not where you started)!

And that is what you did in your "fragment of code". The thing is you did not specify what you really want to do and how do you define your verices is relative as well.. If you want something like a view with some image that you want to control in sense of changing the position and rotation, you might want to create a square vertex buffer with values from -1 to 1 in both dimensions (or (-width/2, -height/2) to (width/2, height/2)). In this case the base center of your object is in (0,0,0) and that is probably the point you want to rotate it around (or am I wrong here?). So when you want to define the position of the object with origin point, you will need to write translatef(x+width/2,y+height/2,..).

As for the whole process of drawing in this case: If you want the origin to be at (x,y,z), with a (width, height) and rotated by (angle) here is the sequence

glTranslatef(x,y,z)
glTranslatef(width/2,height/2,0)
glScalef(width/2,height/2, 1) //only if verices defined at (-1,1)
glRotatef(angle, 0, 0, 1)

Do note in this case that since you rotate the object around its center its origin will not be at (x,y,z) anymore.

In general I would suggest to stay away from glRotate, glTranslate and glScale if possible. They tend to make things very nasty. So another way is to construct a matrix directly from base vectors: With little math you can compute all 4 points of your "square view" based on parameters such as origin, width, height and rotation.. The 4 points being (A-origin), (B-lower left point), (C-lower right point), (D-upper right point) your base vectors are (B-A), (D-A) and normalized(dotProduct((B-A), (D-A))) this 3 vectors can be inserted int top left 3x3 matrix of the GL matrix (witch is 4x4 or float[16]) and they represent both, rotation and scale so all you need to add is the translation part (just google around a bit for this approach).