love.graphics.stencil

Available since LÖVE 0.10.0
Together with love.graphics.setStencilTest, it has replaced love.graphics.setStencil.

Draws geometry as a stencil.

The geometry drawn by the supplied function sets invisible stencil values of pixels, instead of setting pixel colors. The stencil buffer (which contains those stencil values) can act like a mask / stencil - love.graphics.setStencilTest can be used afterward to determine how further rendering is affected by the stencil values in each pixel.

Stencil values are integers within the range of [0, 255].

Starting with version 11.0, a stencil buffer must be set or requested in love.graphics.setCanvas when using stencils with a Canvas. love.graphics.setCanvas{canvas, stencil=true} is an easy way to use an automatically provided temporary stencil buffer in that case.

Function

Synopsis

love.graphics.stencil( stencilfunction, action, value, keepvalues )

Arguments

function stencilfunction: Function which draws geometry. The stencil values of pixels, rather than the color of each pixel, will be affected by the geometry.
StencilAction action ("replace"): How to modify any stencil values of pixels that are touched by what's drawn in the stencil function.
number value (1): The new stencil value to use for pixels if the "replace" stencil action is used. Has no effect with other stencil actions. Must be between 0 and 255.
boolean keepvalues (false): True to preserve old stencil values of pixels, false to re-set every pixel's stencil value to 0 before executing the stencil function. love.graphics.clear will also re-set all stencil values.

Returns

Nothing.

Notes

It is possible to draw to the screen and to the stencil values of pixels at the same time, by using love.graphics.setColorMask inside the stencil function to enable drawing to all color components.

Examples

Drawing circles masked by a rectangle

local function myStencilFunction()
   love.graphics.rectangle("fill", 225, 200, 350, 300)
end
 
function love.draw()
    -- draw a rectangle as a stencil. Each pixel touched by the rectangle will have its stencil value set to 1. The rest will be 0.
    love.graphics.stencil(myStencilFunction, "replace", 1)
 
   -- Only allow rendering on pixels which have a stencil value greater than 0.
    love.graphics.setStencilTest("greater", 0)
 
    love.graphics.setColor(1, 0, 0, 0.45)
    love.graphics.circle("fill", 300, 300, 150, 50)
 
    love.graphics.setColor(0, 1, 0, 0.45)
    love.graphics.circle("fill", 500, 300, 150, 50)
 
    love.graphics.setColor(0, 0, 1, 0.45)
    love.graphics.circle("fill", 400, 400, 150, 50)
 
    love.graphics.setStencilTest()
end

Using an Image as a stencil mask

-- a black/white mask image: black pixels will mask, white pixels will pass.
local mask = love.graphics.newImage("mymask.png")
 
local mask_shader = love.graphics.newShader[[
   vec4 effect(vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords) {
      if (Texel(texture, texture_coords).rgb == vec3(0.0)) {
         // a discarded pixel wont be applied as the stencil.
         discard;
      }
      return vec4(1.0);
   }
]]
 
local function myStencilFunction()
   love.graphics.setShader(mask_shader)
   love.graphics.draw(mask, 0, 0)
   love.graphics.setShader()
end
 
function love.draw()
    love.graphics.stencil(myStencilFunction, "replace", 1)
    love.graphics.setStencilTest("greater", 0)
    love.graphics.rectangle("fill", 0, 0, 256, 256)
    love.graphics.setStencilTest()
end

Allow drawing everywhere except where multiple circles intersect

local function myStencilFunction()
   -- Draw four overlapping circles as a stencil.
   love.graphics.circle("fill", 200, 250, 100)
   love.graphics.circle("fill", 300, 250, 100)
   love.graphics.circle("fill", 250, 200, 100)
   love.graphics.circle("fill", 250, 300, 100)
end
 
function love.draw()
   -- Each pixel touched by each circle will have its stencil value incremented by 1.
   -- The stencil values for pixels where the circles overlap will be at least 2.
   love.graphics.stencil(myStencilFunction, "increment")
 
   -- Only allow drawing in areas which have stencil values that are less than 2.
   love.graphics.setStencilTest("less", 2)
 
   -- Draw a big rectangle.
   love.graphics.rectangle("fill", 0, 0, 500, 500)
 
   love.graphics.setStencilTest()
end

The love.graphics.setStencilTest wiki page includes more examples.