Android OpenGL ES 1.0 中的广告牌

Question

I'm trying to get billboarding to work, but having trouble with the last step.

After following these directions from NeHe's tutorials (http://nehe.gamedev.net/data/articles/article.asp?article=19) I have my look, right, and up vectors, and I have already translated the modelview matrix to the centerpoint of the billboard by using glTranslatef().

float[] m = {right.x,right.y,right.z,0f,
            up.x,up.y,up.z,0f,
            look.x,look.y,look.z,0f,
            pos.x,pos.y,pos.z,1f}; //pos is the centerpoint position
gl.glMultMatrixf(m, 0);

When I try to create a multiplication matrix out of these like so, the billboards are displayed all over the place in the wrong positions and orientations.

I guess my problem is that I don't know how to correctly create and multiply the matrix. I tried doing this instead, but then half the lines (the ones that need to rotate counter-clockwise) are rotated in the wrong direction:

//normal is the vector that the billboard faces before any manipulations.
float angle = look.getAngleDeg(normal); //returns angle between 0 and 180.
gl.glRotatef(angle, up.x,up.y,up.z);

Answer 1

Got it, using my second method. Calculating the angle between vectors (arccos of the dot product) can only give an angle between 0 and 180, so half the time you want to negate the angle so the rotation is in the opposite direction.

This is easy to check...since I already have the right vector, I can just check if the angle between the right vector and the normal is acute. If it's acute, then you want to negate the original angle.