face the camera and say cheese..

OpenGL Development JOGL Development
#1

I need certain billboard-type objects in my game to always face toward the camera. I’ve tried some rotation calls involving arctangents but I can’t get it quite right. If the object is at coordinates x, y, z, originally facing in vector (0, 0, -1) and the camera is at a, b, c, could someone less confused at this math that I am please post the few lines of glRotate calls that will rotate the object so that it’s exactly facing the camera (assuming it’s already been translated to the origin). Thanks!

#2

This is the code I uses. It’s LWJGL, not jogl. But I hope you will understand it anyway.


/**
* @param eye the position of the camera
* @param pos the position of the billboard.
*/
public void drawBillboard(Vector3f eye, Vector3f pos) {
  GL.glPushMatrix();
  Vector3f zAxis = new Vector3f(pos);
  zAxis.sub(eye);
  zAxis.normalize();
  Vector3f yAxis = new Vector3f(0, 1, 0);
  // Build the X axis vector based on the two existing vectors
  Vector3f xAxis = new Vector3f();
  xAxis.cross(yAxis, zAxis);
  xAxis.normalize();
  // Correct the Y reference vector
  yAxis.cross(xAxis, zAxis);
  yAxis.normalize();
  yAxis.scale(-1);
  float m[] = new float[16];
  m[0] = xAxis.x; m[1] = xAxis.y; m[2] = xAxis.z; m[12] = pos.x;
  m[4] = yAxis.x; m[5] = yAxis.y; m[6] = yAxis.z; m[13] = pos.y;
  m[8] = zAxis.x; m[9] = zAxis.y; m[10] = zAxis.z; m[14] = pos.z;
  m[15] = 1;

  FloatBuffer matBuff = ByteBuffer.allocateDirect(16*4).order(ByteOrder.nativeOrder()).asFloatBuffer();
  matBuff.rewind();
  matBuff.put(m);
  matBuff.flip();

  GL.glMultMatrixf(matBuff);

  //...draw a quad between (-2, -2, 0) and (2, 2, 0)

  GL.glPopMatrix();
}
#3

hmmm… tried your code but apparently something went wrong somewhere, because the objects flew all over the place instead of rotating in place. I used your exact code, only adapting it for JOGL, so I’m not sure why it didn’t work… ???

#4

I forgot to mention of to use the function. The top of the matrix stack must contain the “camera matrix”. So before drawBillboard I call:
“GLU.gluLookAt(eye.x, eye.y, eye.z, target.x, target.y, target.z, 0, 1, 0);”

Where eye is the position of the camera and the same as I pass to drawBillboard.

Beside this I’m clueless to why it doesn’t work. Sorry!! :frowning:

#5

Interesting… I haven’t even heard of the function gluLookAt. Then again, I haven’t used glu so far anywhere in my game, whose transformations consist entirely of glTranslatef and glRotatef calls… forgive me if I’m somewhat new at this.