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, globalw, globalh
	if zoomx < zoomy then
		zoom = zoomx
		globalw = maxx-minx
		globalh = globalw*App.screen.height/App.screen.width
	else
		zoom = zoomy
		globalh = maxy-miny
		globalw = globalh*App.screen.width/App.screen.height
	end
	zoom = math.max(zoom, --[[disallow font size of 0; not sure why 1 doesn't work]] 2 / --[[baseline font size at zoom 1]] 20)
	local global_viewport = {x=minx, y=miny, zoom=zoom}
	return global_viewport, globalw, globalh
end