render.SetStencilPassOperation
Example
This uses the pass operation to blank out everything but what we just drew (from Lex's Stencil Tutorial).
hook.Add( "PostDrawOpaqueRenderables", "Stencil Tutorial Example", function()
-- Reset everything to known good
render.SetStencilWriteMask( 0xFF )
render.SetStencilTestMask( 0xFF )
render.SetStencilReferenceValue( 0 )
render.SetStencilCompareFunction( STENCIL_ALWAYS )
render.SetStencilPassOperation( STENCIL_KEEP )
render.SetStencilFailOperation( STENCIL_KEEP )
render.SetStencilZFailOperation( STENCIL_KEEP )
render.ClearStencil()
-- Enable stencils
render.SetStencilEnable( true )
-- Set the reference value to 1. This is what the compare function tests against
render.SetStencilReferenceValue( 1 )
-- Only draw things if their pixels are NOT 1. Currently this is everything.
render.SetStencilCompareFunction( STENCIL_NOTEQUAL )
-- If something draws to the screen, set the pixels it draws to 1
render.SetStencilPassOperation( STENCIL_REPLACE )
-- Draw our entities. They will draw as normal
for _, ent in ipairs( ents.FindByClass( "prop_physics" ) ) do
ent:DrawModel()
end
-- At this point, we cannot draw on top of anything that we have already drawn.
-- So, if we flush the screen, our entities will still be there.
render.ClearBuffersObeyStencil( 0, 148, 133, 255, false )
-- Let everything render normally again
render.SetStencilEnable( false )
end )
Output: 
