Scrolling an InkCollector

As I mentioned elsewhere, I’m working on a DXCore plugin to enable (Tablet PC) ink drawing on the Visual Studio editor surface. A problem I stumbled upon in this regard was the scrolling functionality. Generally this is really easy to implement, using a transformation with the ink renderer. So I had this method:

void UpdateScrollPosition(TextView textView) {
  Point p = new Point(textView.ColumnWidth * textView.HorizontalScrollPosition,
    textView.LineHeight * textView.VerticalScrollPosition);

  collector.Renderer.PixelToInkSpace(textView.Graphics, ref p);
  Matrix matrix = new Matrix();
  matrix.Translate(-p.X, -p.Y);

  collector.Renderer.SetViewTransform(matrix);
}

After wondering for a while why this didn’t work, I put in some logging and found that the value passed in as -p.Y was actually growing for every call into that method, regardless how I was dragging the vertical scrollbar. The explanation was simple in the end, as these explanations tend to be: the PixelToInkSpace method takes any current transformations into account, so that any calculation is relative to an origin set earlier — in the last update, in fact. So the solution was to use an offset for the calculation and everything started working:

void UpdateScrollPosition(TextView textView) {
  Point p = new Point(textView.ColumnWidth * textView.HorizontalScrollPosition,
    textView.LineHeight * textView.VerticalScrollPosition);
  Point origin = new Point(0, 0);

  collector.Renderer.PixelToInkSpace(textView.Graphics, ref p);
  collector.Renderer.PixelToInkSpace(textView.Graphics, ref origin);
  p.Offset(-origin.X, -origin.Y);
  Matrix matrix = new Matrix();
  matrix.Translate(-p.X, -p.Y);

  collector.Renderer.SetViewTransform(matrix);
}