Touch Demo Part I

Over the next few posts I am going to walk you through building one of the touch demos I used in my CodeRage 4 Session. This demo demonstrates using Direct2D and the WM_TOUCH message. Before we get started here is a brief explanation of these two technologies.

Since Windows Vista GDI has become a legacy drawing API supported with software rendering on top of Direct3D. Direct3D is great but it is very raw. This is where Direct2D shines for applications. Direct2D has all the performance benefits of Direct3D and adds support for bitmaps and text and support for GDI Interop all with a much simpler API. And Delphi 2010 has made it even simpler by wrapping it up with a TDirect2DCanvas that is very similar to TCanvas. Direct2D supports remote rendering through Remote Desktop which is very cool.

WM_TOUCH is the raw multi-touch message that provides various states of contact with the input digitizer for hot sizzling touch applications.

Now that we got that out of the way, create yourself a VCL Application. We want to start painting with Direct2D so add Direct2D and D2D1 to the uses and a TDirect2DCanvas property to your form like this:

  private
    FCanvas: TDirect2DCanvas;
  ...
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;

    property Canvas: TDirect2DCanvas read FCanvas
      write FCanvas;
  end;
...
constructor TTouchForm.Create(AOwner: TComponent);
begin
  inherited;
  FCanvas := TDirect2DCanvas.Create(Handle);
end;

destructor TTouchForm.Destroy;
begin
  FCanvas.Free;
  inherited;
end;

This creates a Direct2D surface (TDirect2DCanvas) that we can draw to instead of the traditional GDI surface (TCanvas).

Depending on the type of application you build you may experience some flicker. To avoid this handle the message WM_ERASEBKGND and returns true.

procedure TTouchForm.WMEraseBkgnd(
  var Message: TWMEraseBkgnd);
begin
  Message.Result := 1;
end;

Next override Paint and just draw a line:

procedure TTouchForm.Paint;
begin
  Canvas.BeginDraw;
  try
    Canvas.Pen.Color := clRed;
    Canvas.MoveTo(10, 10);
    Canvas.LineTo(40, 40);
  finally
    Canvas.EndDraw;
  end;
end;

If you run your application at this point and resize the form you will noticed that the line changes size. This is because the view port is resizing to scale to the window and your line will do the same. To prevent this override the Resize method and add the following:

procedure TTouchForm.Resize;
var
  HwndTarget: ID2D1HwndRenderTarget;
begin
  inherited;
  if Supports(Canvas.RenderTarget,
    ID2D1HwndRenderTarget, HwndTarget) then
    HwndTarget.Resize(D2D1SizeU(ClientWidth, ClientHeight));
  Invalidate;
end;

Now you have a form that renders everything drawn to the Canvas through a Direct2D surface.

UPDATE: Added Invalidate to Resize because when resizing the window larger it will force a redraw but smaller will not because typically a redraw isn't required.