Using Canvases for Drawing

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The canvas class encapsulates graphics controls at several levels, including high-level functions for drawing individual lines, shapes, and text. The Vcl.Graphics.TCustomCanvas class defines an abstract base for all canvas objects defined in the Visual Component Library. Most controls in the VCL have an associated canvas object; exception are controls that, under the hood, bind to Windows controls (such as Vcl.StdCtrls.TButton) and controls that use the parent's canvas for drawing (such as Vcl.ExtCtrls.TPaintBox).

Vcl.Graphics.TCustomCanvas has two primary descendants:

Note that all VCL controls use the Vcl.Graphics.TCanvas descendant, because most Windows operating systems do not support Direct2D canvas. The only Windows version that supports Vcl.Direct2D.TDirect2DCanvas is Windows 7.

As a concept, the canvas is split into three levels of abstraction: High level, Intermediate level, and Low level. Each level defines a functionality set. The High level functionality is available to all canvases, regardless of implementation details. The Intermediate level stays semantically identical between canvas implementations but may differ in the exposed public interface. Finally, the Low level exposes functionality that depends on the implementation.

The following table summarizes the three levels of abstraction. Consider it a reference, because there are functions defined as high level but still not implemented in all canvas descendants.

High-level functionality:

Operation Tools

Drawing lines and shapes

Methods such as MoveTo, LineTo, Rectangle, and Ellipse

Displaying and measuring text

TextOut, TextHeight, TextWidth, and TextRect methods

Filling areas

FillRect and FloodFill methods

Intermediate-level functionality:

Operation Tools

Customizing text and graphics

Pen, Brush, and Font properties

Low-level functionality:

Operation Tools

Calling Windows GDI functions

Handle property (Vcl.Graphics.TCanvas)

Calling Windows Direct2D functions (Vcl.Direct2D.TDirect2DCanvas)

RenderTarget property

Manipulating pixels

Pixels property (Vcl.Graphics.TCanvas)

Copying and merging images

Draw, StretchDraw, BrushCopy, and CopyRect methods; CopyMode property (Vcl.Graphics.TCanvas)

Advanced drawing operations

DrawEllipse, DrawGeometry, FillRectangle (Vcl.Direct2D.TDirect2DCanvas)

See Also

Code Examples