An old experiment for getting to know Cairo bindings to Lua.

squircledrop.lua

local iup = require "iuplua"
local cairo = require "lcairo"

local dlg

local sqx, sqy = 0, 0
local dragx, dragy
local mx, my = 0, 0
local sqsize = 50
local can = iup.canvas{}

---[[--- Double buffering
local canw, canh = 0, 0
local first_hdc
local back_sfc
local back_lud
local back_cr

local function makebackbuffer()
  --todo: destroy any originals
  back_sfc = cairo.win32_surface_create_with_ddb(
    first_hdc, cairo.FORMAT_RGB24, canw, canh)
  back_lud = cairo.create(back_sfc)
  back_cr = cairo.ContextFromLUD(back_lud)
end

function can:resize_cb(x,y)
  canw, canh = x, y
  --todo: remake backbuffer?
end
---]]--- Double buffering

function can:action()
  local hdc = iup.GetAttributeData(self,"HDC_WMPAINT")
  
  -- This is how I had it before I added double buffering:
  --cr = cairo.ContextFromLUD(cairo.CreateContext(hdc))
  -- This is what I had after:
  ---[[--- Double buffering
  if not first_hdc then first_hdc = hdc end
  if not back_cr then makebackbuffer() end
  local cr = back_cr
  ---]]--- Double buffering

  cr:set_source_rgb(1,1,1)
  cr:paint()
  cr:set_source_rgb(1,0,0)

  local function squircle(size, xoff, yoff)
    local hsize = size/2
    cr:save()
    cr:translate(xoff,yoff)
    cr:move_to(hsize,0)
    cr:curve_to(0,0,0,0,0,hsize);
    cr:curve_to(0,size,0,size,hsize,size);
    cr:curve_to(size,size,size,size,size,hsize);
    cr:curve_to(size,0,size,0,hsize,0);
    cr:restore()
  end

  squircle(sqsize, sqx, sqy)
  cr:fill()

  cr:move_to(20,20)
  cr:show_text(string.format(
    "sq: (%i, %i)",sqx, sqy))
  cr:move_to(20,30)
  cr:show_text(string.format(
    "m: (%i, %i)",mx, my))
  cr:move_to(20,40)
  cr:show_text(string.format(
    "drag: (%s, %s)", tostring(dragx), tostring(dragy)))

  ---[[--- Double buffering
  local cancr = cairo.CreateContext(hdc)
  cairo.set_source_surface(cancr, back_sfc, 0, 0)
  cairo.paint(cancr)
  ---]]--- Double buffering
end

function can:button_cb(but, pressed, x, y,  status)
  mx, my = x, y
  if but == iup.BUTTON1 then
    if pressed == 1 then
      local h = sqsize/2
      if    x < sqx+sqsize and x > sqx
        and y < sqy+sqsize and y > sqy then
        dragx = x-sqx
        dragy = y-sqy
      end
    else
      dragx, dragy = nil, nil
    end
  end
end

function can:motion_cb(x,y,status)
  mx, my = x, y
  if dragx and dragy then
    if iup.isbutton1(status) then
      sqx = x - dragx
      sqy = y - dragy
      iup.Update(self)
    else
      dragx, dragy = nil, nil
    end
  end
      iup.Update(self)
end

dlg = iup.dialog{
  title = "Getting to know LuaCairo",
  size = "HALFxHALF";
  can
}

dlg:show()

iup.MainLoop()