https://gmod.facepunch.com/f/gmoddev/oaxt/Blue-s-Masks-and-Shadows/1/#postevegt

bluesmasks.lua

--This code can be improved a lot.
--Feel free to improve, use or modify in any way although credit would be appreciated.

if BMASKS == nil then
BMASKS = {} --Global table, access the functions here

BMASKS.Materials = {} --Cache materials so they dont need to be reloaded
BMASKS.Masks = {} --A table of all active mask objects, you should destroy a mask object when done with it

--The material used to draw the render targets
BMASKS.MaskMaterial = CreateMaterial("!bluemask","UnlitGeneric",{
	["$translucent"] = 1,
	["$vertexalpha"] = 1,
	["$alpha"] = 1,
})

--Creates a mask with the specified options
--Be sure to pass a unique maskName for each mask, otherwise they will override each other
BMASKS.CreateMask = function(maskName, maskPath, maskProperties)
	local mask = {}

	--Set mask name
	mask.name = maskName

	--Load materials
	if BMASKS.Materials[maskPath] == nil then
		BMASKS.Materials[maskPath] = Material(maskPath, maskProperties)
	end

	--Set the mask material
	mask.material = BMASKS.Materials[maskPath]

	--Create the render target
	mask.renderTarget = GetRenderTargetEx("BMASKS:"..maskName, ScrW(), ScrH(), RT_SIZE_FULL_FRAME_BUFFER, MATERIAL_RT_DEPTH_NONE, 2, CREATERENDERTARGETFLAGS_UNFILTERABLE_OK, IMAGE_FORMAT_RGBA8888)

	BMASKS.Masks[maskName] = mask

	return maskName
end

--Call this to begin drawing with a mask.
--After calling this any draw call will be masked until you call EndMask(maskName)
BMASKS.BeginMask = function(maskName)
	--FindMask
	if BMASKS.Masks[maskName] == nil then 
		print("Cannot begin a mask without creating it first!") 
		return false
	end

	--Store current render target
	BMASKS.Masks[maskName].previousRenderTarget = render.GetRenderTarget() 
	
	--Confirgure drawing so that we write to the masks render target
	render.PushRenderTarget(BMASKS.Masks[maskName].renderTarget)
	render.OverrideAlphaWriteEnable( true, true )
	render.Clear( 0, 0, 0, 0 ) 
end

--Ends the mask and draws it
--Not calling this after calling BeginMask will cause some really bad effects 
--This done return the render target used, using this you can create other effects such as drop shadows without problems
--Passes true for dontDraw will result in it not being render and only returning the texture of the result (which is ScrW()xScrH())
BMASKS.EndMask = function(maskName, x, y, sizex, sizey, opacity, rotation, dontDraw)

	dontDraw = dontDraw or false
	rotation = rotation or 0
	opacity = opacity or 255

	--Draw the mask
	render.OverrideBlendFunc( true, BLEND_ZERO, BLEND_SRC_ALPHA, BLEND_DST_ALPHA, BLEND_ZERO )
	surface.SetDrawColor(Color(255,255,255,opacity))
	surface.SetMaterial(BMASKS.Masks[maskName].material)
	if rotation == nil or rotation == 0 then
		surface.DrawTexturedRect(x, y, sizex, sizey) 
	else
		surface.DrawTexturedRectRotated(x, y, sizex, sizey, rotation) 
	end
	render.OverrideBlendFunc(false)
	render.OverrideAlphaWriteEnable( false )
	render.PopRenderTarget() 

    --Update material
	BMASKS.MaskMaterial:SetTexture('$basetexture', BMASKS.Masks[maskName].renderTarget)

	--Clear material for upcoming draw calls
	draw.NoTexture()

	--Only draw if they want is to
	if not dontDraw then
		--Now draw finished result
		surface.SetDrawColor(Color(255,255,255,255)) 
		surface.SetMaterial(BMASKS.MaskMaterial) 
		render.SetMaterial(BMASKS.MaskMaterial)
		render.DrawScreenQuad() 
	end

	return BMASKS.Masks[maskName].renderTarget
end
end

Made by CODE BLUE

https://forum.facepunch.com/t/blues-masks-and-shadows/238492