Aseprite Dithering on currently selected layer.

Dithering-By-Luke.lua

-- Dithering (In Aseprite)
-- Dithers the selected layer using a basic algo.
-- By Luke Parker (https://twitter.com/MrLParker)

if app.apiVersion < 1 then
    return app.alert("This script requires Aseprite v1.2.10+")
end

local cel = app.activeCel
if not cel then
    return app.alert("There is no active image")
end

-- Currently we don't need to select a layer as the script runs on the selected layer
-- though that may change in future versions of aseprite.
-- Grab the image function.
local img = cel.image

-- Reference the rgba functions from app.pixelColor
local rgba = app.pixelColor.rgba
local rgbaR = app.pixelColor.rgbaR
local rgbaG = app.pixelColor.rgbaG
local rgbaB = app.pixelColor.rgbaB

-- Loop through all pixels keeping the colour, but applying 0 to alpha.
for it in img:pixels() do
    if it.x % 1 == 0 and it.y % 2 == 0 then
        it(rgba(
            rgbaR(it()),
            rgbaG(it()),
            rgbaB(it()),
            0))
    end
    if it.x % 2 == 0 and it.y % 1 == 0 then
        it(rgba(
            rgbaR(it()),
            rgbaG(it()),
            rgbaB(it()),
            0))
    end
end

-- Put updated image back.
cel.image = img

-- Don't need to refresh the image but it's better to be safe.
app.refresh()