Aseprite script that generates SDF (Signed Distance Field)

Generate SDF.lua

----------------------------------------------------------------------
-- Generate SDF
--
-- It works only for RGB color mode.
----------------------------------------------------------------------

if app.apiVersion < 1 then
    return app.alert("Unsupported version")
end

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

local dlg = Dialog()
dlg:slider{ id="width",
            label="Distance",
            min=1,
            max=16,
            value=16 }
dlg:button{ id="confirm", text="Generate" }
dlg:button{ id="cancel", text="Cancel" }
dlg:show()

if dlg.data.confirm == false then
    return
end

local width = dlg.data.width

local function processLayer(layer)
    local sprite = app.activeSprite
    local newLayerName = layer.name .. "_SDF"
    local newLayer = nil
    for i, layer in ipairs(sprite.layers) do
        if layer.name == newLayerName then
            newLayer = layer
        end
    end
    if newLayer == nil then
        newLayer = sprite:newLayer()
        newLayer.name = newLayerName
    end
    
    for i, cel in ipairs(layer.cels) do
        local frame = cel.frame
    
        local img = Image(cel.image.width + width * 2, cel.image.height + width * 2)
        local position = cel.position
    
        if img.colorMode == ColorMode.RGB then
            local rgba = app.pixelColor.rgba
            local rgbaA = app.pixelColor.rgbaA
            for it in img:pixels() do
                local x = it.x - width
                local y = it.y - width
                local s = false 
                if x < 0 or y < 0 or x >= cel.image.width or y >= cel.image.height then
                    s = true
                else
                    s = rgbaA(cel.image:getPixel(x, y)) < 255
                end

                local nearest = width
                for dx = -width, width do
                    for dy = -width, width do
                        local px = x + dx
                        local py = y + dy
                        local ps = false
                        if px < 0 or py < 0 or px >= cel.image.width or py >= cel.image.height then
                            ps = true
                        else
                            ps = rgbaA(cel.image:getPixel(px, py)) < 255
                        end

                        if ps ~= s then
                            local d = math.sqrt(dx * dx + dy * dy)
                            if d < nearest then
                                nearest = d
                            end
                        end
                            
                    end
                    
                end

                if s then
                    nearest = -nearest
                end

                local value = 128 + nearest / width * 128

                local color = rgba(value, value, value, 255)
                it(color)
            end
        elseif img.colorMode == ColorMode.GRAY then
            return app.alert("This script is only for RGB Color Mode")
        elseif img.colorMode == ColorMode.INDEXED then
            return app.alert("This script is only for RGB Color Mode")
        end
        local newCel = sprite:newCel(newLayer, frame, img, Point(position.x - width, position.y - width))
    end
end

for i, layer in ipairs(app.range.layers) do
    processLayer(layer)
end
app.refresh()