Function pointers that are called at certain times in the view rendering
or lite-C function can
to any of those pointers for drawing additional objects onto the screen,
calling draw_ functions at a certain time during the render process.
render_sky is called at the beginning of every view render cycle before
render_world is called after drawing level blocks and before
render_solid is called after drawing non-transparent level entities
and before drawing sky entities.
render_entities is called at the end of of every view render cycle after drawing sky entities, transparent entities, and particles.
render_layer is called after rendering all views, and before rendering panels and texts. 8.04
Except for render_layer,
the functions are called during scene
rendering. Dont' call BeginScene or EndScene
function draw_horizon(); // dll or lite-C function
render_sky = draw_horizon;