Anybody got a 2D mask shader?

[havenphillip]

I have this rotating thing I want to make a health half-circle but I need to mask half of it. How can I do it? Anyone come across a shader for that? Or can it be done in lua? I looked around but I haven't been able to find one that works for this.

Here's my code, if that's useful:

Script.player = nil -- entity "Player"
Script.pos = Vec2(242,234) -- Vec2 "Position"
Script.offset = Vec2(4,3) -- Vec2 "Offset"
Script.csize = Vec2(200,200) -- Vec2  "Size"

function Script:Start()
    self.player = self.entity:GetParent()
    self.tex = Texture:Load("Materials/Developer/bluegrid.tex")
end

function Script:PostRender(context)
    local a = Time:GetCurrent() /10
    local w = self.csize.x
    local h = self.csize.y
    local x = self.pos.x-(Math:Cos(a)*(w/2) - Math:Sin(a)*(h/2))
    local y = self.pos.y+(-Math:Sin(a)*(w/2) - Math:Cos(a)*(h/2))
    local scale = 1
    
    context:SetRotation(a)
    context:SetScale(1,1)   
        
    context:SetBlendMode(Blend.Alpha)
    context:SetColor(1,1,1,1)
    context:DrawImage(self.tex, x, y, self.csize.x, self.csize.y)
    context:SetRotation(0)
end