itch.io

That is a good question, if I understand you right. The above program keeps the top-left corner of the viewport fixed. Is that what you meant?

Since then I made a change to it to keep the centroid between fingers fixed:

function car.touchmoved(id, x,y, ...)
  if start[id] then
    curr[id] = {x=x, y=y}
    if s then
      local oldzoom = v.zoom
      v.zoom = dist(curr[f], curr[s])/dist(start[f], start[s])*initzoom
      adjust_viewport(oldzoom, v.zoom)
    elseif f then
      v.x = initpos.x + iscale(start[f].x - x)
      v.y = initpos.y + iscale(start[f].y - y)
    end end end
function adjust_viewport(oldzoom, zoom)
  -- ensure centroid of fingers remains in view
  local c = centroid(curr[f], curr[s])
  v.x = v.x + c.x/oldzoom - c.x/zoom
  v.y = v.y + c.y/oldzoom - c.y/zoom
end
function centroid(a, b)
  return{x=(a.x+b.x)/2, y=(a.y+b.y)/2}
end
function iscale(d) return d/v.zoom end