Clear a Lua-based texture

[TomatoSoup]

I'm creating a texture with code like this:
SWEP.DisplayRT = GetRenderTarget("TF_DisplayRT", 512, 512, false)
SWEP.DisplayMat = CreateMaterial( "TF_DisplayMat", "UnlitGeneric", { ["$basetexture"] = "TF_DisplayRT" } )

This works and I can draw it no problem. However, I can't make it transparent. I need it to start transparent and I need to be able to clear what's on it, so it's fully transparent and blank again. What texture/shader flags do I need, and how do I clear it? Bonus points if anyone knows how to clear just sections of it.

[Gmod4phun]

I think you cannot make a transparent texture with Lua, at least not without some stencil trickery and whatnot. Don't fully take my word for it, but I am pretty sure it is not possible. You can clear the render target to a certain color, not so sure about transparency. If there is a way and someone knows, please correct me.

[SneakySquid]

Material Parameters

[TomatoSoup]

That Material Parameters link has neither examples nor the actual parameter used to create transparent effects.

The solution I came up with was this:

SWEP.DisplayRT = GetRenderTarget("TF_DisplayRT", 512, 512, false)
SWEP.DisplayMat = CreateMaterial( "TF_DisplayMat", "UnlitGeneric", { ["$basetexture"] = "TF_DisplayRT", ["$translucent"] = 1 } )

--Then, actually update it like this:

render.SetRenderTarget(self.DisplayRT)
render.OverrideAlphaWriteEnable( true, true )

render.ClearDepth()
render.Clear( 0, 0, 0, 0 )

surface.SetDrawColor(0,0,0,255)
--do more surface.Draw whatevs here

render.OverrideAlphaWriteEnable( false )

This has an interesting effect that, despite making the texture 512 by 512, it does not draw how you'd expect. Drawing a line from 0,0 to 256,256 does not go from the top left corner to the center, but drawing a line from 0,0 to ScrW()/2,ScrH()/2 does. It does something like rescaling the parameters from your screen size to the size of the rendered texture, as fractions of the whole display.