Update

billboard.md

@@ -5,7 +5,7 @@ In [3D](3d.md) [computer graphics](graphics.md) billboard is a flat image placed
 There are two main types of billboards:
 
 - Ones **rotating only about vertical axis**, i.e. billboards that change only their [yaw](yaw.md), they only face the camera in a top-down view of the scene. Such sprite may deform on the screen (when the camera is at different height level) just like 3D models do and when viewed completely from above will disappear completely. This may in some situations look better than other options (e.g. in [games](game.md) enemies won't appear lying on their back when seen from above).
-- **Freely rotating** ones, i.e. ones that change all three [Euler angles](euler_angle.md) so that they ALWAYS face the camera from any possible angle. There may further be other two subtypes: billboards that align themselves with the camera's projection plane (they simply rotate themselves in the same way as the camera) which always end up on the screen as an undeformed and unrotated image, and billboards that face themselves towards the camera's position and copy the camera's [roll](roll.md) (though these may seem like two same things, they are not, for the latter we need to know the camera and billboard's positions, for the former we only need the camera's rotation). The former is simpler to implement and may also look better, so we normally don't even consider the latter.
+- **Freely rotating** ones, i.e. ones that change all three [Euler angles](euler_angle.md) so that they ALWAYS face the camera from any possible angle. There may further be other two subtypes: billboards that align themselves with the camera's projection plane (they simply rotate themselves in the same way as the camera) which always end up on the screen as an undeformed and unrotated image, and billboards that face themselves towards the camera's position and copy the camera's [roll](roll.md) (though these may seem like two same things, they are not, for the latter we need to know the camera and billboard's positions, for the former we only need the camera's rotation). For simplicity we usually choose to implement the former, though the latter may result in look closer to that which would be produced by an actual 3D model of the object (e.g. a sphere projected to an ellipse by perspective).
 
 Some billboards also choose their image based on from what angle they're viewed (e.g. an enemy in a game viewed from the front will use a different image than when viewed from the side, as seen e.g. in [Doom](doom.md)). Also some billboards intentionally don't scale and keep the same size on the screen, for example health bars in some games.