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2. What is Billboarding
Billboarding is a simple concept, but can be difficult to implement. A billboard is a flat object, usually a quad (square), which faces the camera. This direction usually changes constantly during runtime as the object and camera move, and the object needs to be rotated each frame to point in that direction.- There are two types of billboarding: point and axis.
- Point sprites, or point billboards, are a quad that is centered at a point and the billboard rotates about that central point to face the user.
- Axis billboards come in two types: axis aligned and arbitrary.
- The axis-aligned (AA) billboards always have one local axis that is aligned with a global axis, and they are rotated about that axis to face the user.
- The arbitrary axis billboards are rotated about any axis to face the user.
Billboards will always face the camera, but this technique can also be used to face other objects as well. For instance, this can be used to rotate a turret to face a target, or to have the head on a character follow another character.
The concepts in the document use the right handed coordinate system and assume that the billboards you are drawing will face down the +z axis, up is the +y axis, and right is the +x axis. I found this site: http://www.evl.uic.edu/ralph/508S98/coordinates.html has a good explanation.
3. Terms
Point sprite | A quad billboard rotated about a central point that faces the camera. |
Axis Aligned billboard | A billboard that rotates about an axis (x, y, z). The up vector for the billboard is along that axis. |
Axis billboard | A billboard that is aligned along an arbitrary axis, this can include axis aligned billboards. The up vector for the billboard is along the arbitrary axis. |
Look vector | The direction the object is facing. |
Up vector | The up direction of the object. |
Right vector | The right direction of the object. |
Global axis | The axis system for the entire application as defined by OpenGL with no transformations applied. |
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To be Continued....For now, is busy with Transform-Feedback :-)
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