initialize_global_viewport = function()
	local minx, miny, maxx, maxy = surface_bounds(Definitions)
	-- ensure 1px of white background all around
	-- just in case the viewport ever goes
	-- outside canvas bounds while animating
	minx,miny, maxx,maxy = minx-1,miny-1, maxx+2,maxy+1
	local zoomx = App.screen.width/(maxx-minx)
	local zoomy = App.screen.height/(maxy-miny)
	local zoom, cw, ch
	if zoomx < zoomy then
		zoom = zoomx
		cw = maxx-minx
		ch = cw*App.screen.height/App.screen.width
	else
		zoom = zoomy
		ch = maxy-miny
		cw = ch*App.screen.width/App.screen.height
	end
	Global_viewport = {x=minx, y=miny, w=cw, h=ch, zoom=zoom}
end