[Algorithms] Keeping an object's label on the screen

[Monteleone, N. <nat...@lm...>]

I have to draw some billboard text over some shapes we've "painted" onto
our terrain via multitexturing.  Normally this is easy enough - just
plop the label in an auto-rotated billboard and float it above the
center of the geometry.  But I need to keep the label on screen at all
times if any part of the painted geometry is visible.  I.e. if I have a
square painted on the terrain, but I can only see a small corner of it,
I still need to show the whole label.

 

What's a good algorithm to figure out where to draw the label?

 

The idea I had in mind was to do something in screen space...  I'd write
out an object ID version of the projected texture, apply it, render the
terrain geometry into a low-res buffer, then compute min/max screen
coords for each object.  Then I could just draw my text label in screen
space based on that information.  Is this a sane approach?

 

Otherwise I'd have to create a geometric representation of our shapes
that conforms to the terrain and choose a 3d location for my text based
on the view frustum.  That seems painful to implement efficiently.

 

Thanks,

Nathan