Billboard
From DmWiki
A billboard is a sprite that appears to always face the viewer, giving the illusion of a 3D object. Hence the object's forward vector is rotated until it points directly to the camera. There are of course (as with all things in computer graphics) approximation methods for billboarding, but true billboarding can be achieved without any hassle either. Billboards are often used for grass, particles, beams of lightning and electricity, explosions, and other special effects.
The approximation method makes the object's front vector point to the plane that is perpendicular to the camera's front vector. So, imagine the camera points out of a wall; then fake billboarding makes a billboarded object point to that wall instead of the camera. It's hard to tell the difference when the field of view is not very wide, but as the view gets wider objects start to look distorted.
The true method involves two basic steps. First, the object is rotated about its up vector (which means the object now will face in the direction of the camera, but still doesn't face the camera directly because the camera could be high up or down low). Then the object is rotated about its right vector so that it faces the camera.
See also
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