local canvas = require "canvas"

local function trunc(x) return x >= 0 ? math.floor(x) : math.ceil(x) end

local function circle(c, cx, cy, r, col)
    local d = trunc((5 - r * 4) / 4)
    local x = 0
    local y = r
    repeat
        c:set(cx + x, cy + y, col)
        c:set(cx + x, cy - y, col)
        c:set(cx - x, cy + y, col)
        c:set(cx - x, cy - y, col)
        c:set(cx + y, cy + x, col)
        c:set(cx + y, cy - x, col)
        c:set(cx - y, cy + x, col)
        c:set(cx - y, cy - x, col)
        if d < 0 then
            d += 2 * x + 1
        else
            d += 2 * (x - y) + 1
            y -= 1
        end
        ++x
    until x > y
end

local c = canvas.new(400, 400)
c:fill(0xffffff)  -- white background

-- Draw a blue circle with centre at (200, 200) and radius 100.
circle(c, 200, 200, 100, 0xff0000)
io.contents("mybmp.bmp", c:tobmp()) -- save to a bitmap file