transparency - Make some black pixels transparent in Love2D

Is it possible to put a black opaque rectangle over my game screen and then change transparency on some pixels?

For example I have black rectangle on screen represented by 1

1111111111111111111111
1111111111111111111111
1111111111111111111111
1111111111111111111111
1111111111111111111111
1111111111111111111111

1111111111111111111111

and then I would like to make several pixels transparent represented by 0

1111111111111111111111
1111100000000000011111
1111111100000011111111
1111111111001111111111
1111111100000011111111
1111100000000000011111
1111111111111111111111

Would that be possible using blend modes?

P.S. I'm using version 0.8.0. of Love2D

Answer

You want a stencil.

Stencils let you flexibly define regions that love.graphics operations won't affect.

Here's an example. It does this:

1. Draw some colourful circles.


2. Set an inverted stencil made of two triangles.



3. Draw a black rectangle over everything.

See how the black rectangle doesn't actually cover everything? It's cut out where the triangles were. That's the stencil.

Here's the code I for that:

love.graphics.setBackgroundColor(255,255,255)

function love.draw()
    local rect  = love.graphics.rectangle
    local circ  = love.graphics.circle

    local col   = love.graphics.setColor
    local poly  = love.graphics.polygon

    -- reset stencil
    love.graphics.setStencil(nil);
    -- draw some colorful circles
    col(255,0,0)
    circ("fill", 75,30, 20)
    col(0,0,255)
    circ("fill", 10,30, 30)

    col(255,0,255)
    circ("fill", 70,70, 20)
    col(255,255,0)
    circ("fill", 30,60, 30)

    -- set stencil to an hourglass shape
    love.graphics.setInvertedStencil(function()
        poly("fill", 10,10, 50,50, 90,10)
        poly("fill", 10,90, 50,50, 90,90)
    end)

    -- draw a black rectangle
    col(0,0,0)
    rect("fill", 0,0,100,100)
end