#!/usr/bin/luajit

local vips = require "vips"

local function add_drop_shadow(img, shadow_colour, shadow_offset, shadow_radius, background_colour)
    -- Default parameters
    shadow_colour = shadow_colour or { 100, 100, 100 }
    shadow_offset = shadow_offset or 10
    shadow_radius = shadow_radius or 5
    background_colour = background_colour or { 0, 0, 0 }

    -- Add alpha channel if missing
    if img:bands() < 4 then
        img = img:addalpha()
    end

    -- Extract alpha channel for shadow shape
    local shadow_alpha = img:extract_band(img:bands() - 1)

    -- Create shadow alpha with padding and blur
    local padded_alpha = shadow_alpha:embed(
        shadow_radius,
        shadow_radius,
        shadow_alpha:width() + 2 * shadow_radius,
        shadow_alpha:height() + 2 * shadow_radius,
        { extend = vips.ExtendBackground }
    )
    local blurred_alpha = padded_alpha:gaussblur(shadow_radius / 2)

    -- Create shadow RGB from color
    local shadow_rgb = blurred_alpha:new_from_image(shadow_colour)
    local shadow = shadow_rgb:bandjoin(blurred_alpha)

    -- Create background canvas
    local extra_border = math.max(0, math.abs(shadow_offset) - shadow_radius)
    local background = shadow:new_from_image(background_colour)
    background = background:embed(
        0,
        0,
        background:width() + extra_border,
        background:height() + extra_border,
        { extend = "copy" }
    )

    -- Calculate final composite positions
    local final_offset = math.max(0, shadow_offset - shadow_radius)
    local image_offset = final_offset - shadow_offset + shadow_radius

    -- Composite layers (background first, then shadow, then image)
    local result = background:composite(
        { shadow, img },
        "over",
        { x = { final_offset, image_offset }, y = { final_offset, image_offset } }
    )

    return result
end

local image = vips.Image.new_from_file(arg[1])
local with_shadow = add_drop_shadow(image, {255, 0, 0}, 10, 10, {0, 0, 255})
with_shadow:write_to_file(arg[2])