August 29, 2020

Direct2D canvas for Delphi forms

In this blog post, I will show you how easy it is the have a Direct2D canvas for your Delphi form.

What is a canvas?

In Delphi VCL technology, a canvas is an abstraction encapsulating Windows API to render content on screen. In VCL, the standard canvas is implemented using GDI. It allows the developer to draw anything on screen. The class that encapsulate a canvas is names TCanvas. Every form has a TCanvas instance. You use it from the form’s OnPaint event handler. Similarly, all VCL component which are able to render something on screen has a TCanvas instance and a Paint method you can override to draw your own content.

TCanvas has method and properties to render many graphic primitives such as lines, rectangles, ellipses, polygons, bitmaps and text. There are properties such as Brush and Pen to select how you want something to be rendered. For example, a Pen is used to select the color, width and style of the rectangle outline while a Brush is use to select the color used to paint the rectangle’s interior.

Why would you like a Direct2D canvas?

Direct2D is a Microsoft DirectX technology especially designed for high performance 2D drawing. This is an API that provides Win32 / Win64 developers with the ability to perform 2D graphics rendering tasks with superior performance and visual quality.

Delphi is exactly a development platform for Win32 / Win64 application and has everything required to use almost any Windows API.

Delphi VCL has a TCanvas based on Direct2D API. It is not created by default but you can easily create it if you need it. But why would you need it?

I see two reasons:
    1. Speed
    2. More rendering features

The aim of this blog post is not to explain all the details of Direct2D. No, it is to show you how easy it is to start using it. Actually, my previous blog post already made use od it to display images. Now, I will show you how to benefit from powerful new Direct2D API: apply transformations to your drawings.

Direct2D transformation are geometric computation inserted between you call of a drawing primitive (For example a rectangle) and the actual rendering (The rectangle appears on screen).


The demo

The demo will show a simple transformation: a rotation. The demo has a for-loop which draw the same rectangle 24 times after applying a rotation transformation with an incrementing angle. The result visible on screen is 24 rectangles rotated 15°, drawing a nice picture.

How does it works?

Instead of showing the full code as is, I will explain the steps required to build the application from scratch. You’ll then be able to apply the steps to your own application.

1. Create a VCL form application

2. Add the units Vcl.Direct2D and Winapi.D2D1 to the uses clause.

3. Add the following code to the form’s declaration:

FD2DCanvas : TDirect2DCanvas;
        function CreateD2DCanvas: Boolean;
        procedure CreateWnd; override;


 4. Implement CreateD2DCanvas method:

    function TMainForm.CreateD2DCanvas: Boolean;
FD2DCanvas    := TDirect2DCanvas.Create(Handle);
Result        := TRUE;
Result        := FALSE;

5. Implement CreateWnd method:

    procedure TMainForm.CreateWnd;

6. Add an OnResize event handler to your form:

    procedure TMainForm.FormResize(Sender: TObject);
        Size: D2D1_SIZE_U;
        // When the windows is resized, we needs to resize RenderTarget as well
        Size := D2D1SizeU(ClientWidth, ClientHeight);

7. Add a OnPaint event handler to your form:

    procedure TMainForm.FormPaint(Sender: TObject);
        Rect1 : D2D1_RECT_F;
       Angle : Single;
       I     : Integer;
        RECT_SIZE  = 50;
        ANGLE_STEP = 15.0;
            // Erase background

            // Set pen color to draw rectangle outline
            FD2DCanvas.Pen.Color   := clYellow;

            // Clear all transformations


            // Define rectangle to be drawn. Top left corner in center of window
            Rect1                  := Rect((ClientWidth  div 2),
                                           (ClientHeight div 2),
                                           (ClientWidth  div 2) + RECT_SIZE,
                                           (ClientHeight div 2) + RECT_SIZE);
            // Loop drawing the same rectangle but rotated step by step
            for I := 0 to Round(360.0 / ANGLE_STEP) do begin
                Angle := ANGLE_STEP * I;

8. Compile and run your application.

TDirect2DCanvas has almost the same methods and properties as the standard TCanvas. Porting code from standard TCanvas  to TDirect2DCanvas is very easy.

But TDirect2DCanvas is far from implementing all the features of Direct2D API. Fortunately, all the new features – such as transformations – are accessible very easily thru the property RenderTarget.
RenderTarget is an interface implemented in Direct2D DLL. When you call a method of RenderTarget, your are actually calling a Microsoft DLL!

In the demo code you see above, there are two calls to SetTransform. This is how we specify Direct2D to apply one or more transformation. Here we apply a simple rotation.

Transformations are described mathematically by a matrix of 3x2 floating point number. I will not enter the math details here. To help us, Microsoft has prebuilt several matrices for common transformations. Rotation is among them. In the call

     TD2DMatrix3x2F.Rotation(Angle, Rect1.Left, Rect1.Top);

We simply invoke a prebuilt matrix to rotate all subsequent drawings by the specified angle. The rotation take place around the point specified by the second and third arguments.

To cancel any transformation, just set a new transformation. If you want to transformation at all, you can use the matrix names “Identity” which is a kind of do-nothing. The code is:


I invite your to see the online help for TDirect2DCanvas at

Fran├žois Piette


Arnaud said...

IIRC in practice Direct2D is told to be most of the time slower than GDI, due to problems with drivers. So what is the benefit of using Direct2D if the performance impact is not consistent?

FPiette said...

That is a kind of fake news based of very old publications. As with any API, you have to use it correctly to benefit from his performance fully. And if you have poor driver with your cheap display interface, then upgrade or then complain your driver manufacturer, not Microsoft API. And if by chance you have a bad driver, you can still using Direct2D to benefit from his features.
See those links:

Anonymous said...

Looks like OnResize is leaking a canvas every resize.

Does the canvas really have to be destroyed and recreated? It can't be resized?

FPiette said...

You are right!
I updated the code in the article to resize the canvas instead of recreating it.