Delphi 设备描述表 ;TCanvas...
设备描述表与TCanvas—evice Contexts and TCanvas）
Windows使用术语设备描述表（device context，后面简称为DC）描述用户可在其上画图的画布。DC可用来在许多表面上画图：
窗口客户区或框架（To a window's client area or frame）桌面（To the desktop） 内存（To memory） 打印机或其他输出设备（To a printer or other output device）这些仅仅是一些列子，还有许多其他的不太为人所知的DC（如菜单），但是以上列举都是用户非常感兴趣的DC。
在API级别处理DC要复杂些。首先必须从Windows中的到DC处理程序。然后选择各种对象进入DC中（笔pen、刷子brush、字体font），然后便可在DC中画图，画图完毕后，必须确保进入DC的各种对象在清除之后再删除DC。如果忘记清除DC中的各种对象，应用程序将使内存溢出。
幸运的是VCL提供有TCanvas类以便较容易地处理DC。举一个简单的例子，下面的程序使用Windows API在屏幕上绘制圆，内圆为红色，边框为蓝色。
procedure TForm1.Button1Click(Sender: TObject); var DC: HDC; Brush, OldBrush: HBRUSH; Pen, OldPen: HPEN;begin DC := GetDC(Handle); Brush := CreateSolidBrush(RGB(255, 0, 0)); Pen := CreatePen(PS_SOLID, 1, RGB(0, 0, 255)); OldBrush := SelectObject(DC, Brush); OldPen := SelectObject(DC, Pen); Ellipse(DC, 20, 20, 120, 120); SelectObject(DC, OldBrush); SelectObject(dc, OldPen); ReleaseDC(Handle, DC);end;
以上代码是不是太差，但是当处理完DC之后，使人容易忘记恢复对象，如果忘记了，其应用程序将会资源泄露。下面的代码与上面代码等价，但是用VCL写的：
procedure TForm1.Button2Click(Sender: TObject); begin Canvas.Brush.Color := clRed; Canvas.Pen.Color := clBlue; Canvas.Ellipse(20, 20, 120, 120); end;
该代码不但短小易读，而且也很健壮。
TCanvas根据需要负责释放资源，因此不必担心资源问题。
TCanvas是一种比使用API更简单有效的方法。
TCanvas继承关系如下：
TCanvas类有许多属性和方法。之后的讲解中将会使用一些属性和方法，下面的表格中列出了TCanvas主要的属性和方法。
表1 TCanvas主要属性属性描述Brush刷子颜色或图案，用于填图。—etermines the color and pattern for filling graphical shapes and backgrounds..）Delphi syntax:property Brush: TBrush;
ClipRect画布的当前剪贴矩形，任何图画均被限制在该矩形中。该属性为只读属性。（Specifies the boundaries of the clipping rectangle.）Delphi syntax:property ClipRect: TRect;
CopyMode决定图画如何表现（正图、反图等）（Specifies how a graphical image is copied onto the canvas. ）Delphi syntax:property CopyMode: TCopyMode;
Font为绘制文本画布使用的字体（Specifies the font to use when writing text on the image.）Delphi syntax:property Font: TFont;
Handle画布的处理程序，用于直接访问Windows API的时候使用。（Specifies the handle for this canvas.）Delphi syntax:property Handle: HDC;
Pen决定画布上所画线条的类型和颜色。（Specifies the kind of pen the canvas uses for drawing lines and outlining shapes.）Delphi syntax:property Pen: TPen;
PenPos在X,Y坐标系中的当前图画位置。（Specifies the current drawing position of the Pen. ）Delphi syntax:property PenPos: TPoint;
Pixels数组画布的像素（Specifies the color of the pixels within the current ClipRect.）Delphi syntax:property Pixels[X, Y: Integer]: TColor;
表2 TCanvas主要方法方法描述Arc利用当前笔在画布上画弧。Draws an arc on the image along the perimeter of the ellipse bounded by the specified rectangle.Delphi syntax:procedure Arc(X1, Y1, X2, Y2, X3, Y3, X4, Y4: Integer);
BrushCopy显示具有透明背景的位图。Copies a portion of a bitmap onto a rectangle on the canvas, replacing one of the colors of the bitmap with the brush of the canvas.Delphi syntax:procedure BrushCopy(const Dest: TRect; Bitmap: TBitmap; const Source: TRect; Color: TColor);
CopyRect拷贝图像某一部分到画布上。Copies part of an image from another canvas into the canvas.Delphi syntax: procedure CopyRect(const Dest: TRect; Canvas: TCanvas; const Source: TRect);
Draw把图像从内存中拷贝到画布上。Renders the graphic specified by the Graphic parameter on the canvas at the location given by the coordinates (X, Y).Delphi syntax: procedure Draw(X, Y: Integer; Graphic: TGraphic);
Ellipse利用当前画笔画椭圆并在画布上以当前刷子填满。Draws the ellipse defined by a bounding rectangle on the canvas.Delphi syntax:
procedure Ellipse(X1, Y1, X2, Y2: Integer); overload;
FloodFill用当前刷子填充画布的一个区域。Fills an area of the canvas using the current brush.
procedure Ellipse(const Rect: TRect); overload;Delphi syntax: procedure FloodFill(X, Y: Integer; Color: TColor; FillStyle: TFillStyle);
LineTo从当前图画位置到X,Y位置参数指定的位置画线。Draws a line on the canvas from PenPos to the point specified by X and Y, and sets the pen position to (X, Y).Delphi syntax: procedure LineTo(X, Y: Integer);
MoveTo设置当前图画位置。Changes the current drawing position to the point (X,Y).Delphi syntax: procedure MoveTo(X, Y: Integer);
Pie在画布上画圆饼形。Draws a pie-shaped the section of the ellipse bounded by the rectangle (X1, Y1) and (X2, Y2) on the canvas.Delphi syntax: procedure Pie(X1, Y1, X2, Y2, X3, Y3, X4, Y4: Integer);
Polygon根据一组点画菱形于画布上，并以当前刷子填充。Draws a series of lines on the canvas connecting the points passed in and closing the shape by drawing a line from the last point to the first point.Delphi syntax: procedure Polygon(Points: array of TPoint);
PolyLine使用当前笔，根据一组点在画布上画线，这些点不是自动封闭的。Draws a series of lines on the canvas with the current pen, connecting each of the points passed to it in Points.Delphi syntax: procedure Polyline(Points: array of TPoint);
Rectangle画矩形于画布上，周围使用当前笔，矩形内使用当前刷子填充。Draws a rectangle on the canvas.Delphi syntax:
procedure Rectangle(X1, Y1, X2, Y2: Integer); overload;
RoundRect圆角矩形。Draws a rectangle with rounded corners on the canvas.
procedure Rectangle(const Rect: TRect); overload;Delphi syntax: procedure RoundRect(X1, Y1, X2, Y2, X3, Y3: Integer);
StretchDraw将位图从内存中拷贝到画布上，位图根据目标矩形大小扩大或缩小。Draws the graphic specified by the Graphic parameter in the rectangle specified by the Rect parameter.Delphi syntax: procedure StretchDraw(const Rect: TRect; Graphic: TGraphic);
TextExtent返回文本参数中输入的字符串像素的宽度和高度，利用当前画布字体计算其宽度。Returns the width and height, in pixels, of a string rendered in the current font.Delphi syntax: function TextExtent(const Text: string): TSize;
TextHeight返回文本参数中输入的字符串像素的高度，其宽度利用当前画布字体计算。Returns the height, in pixels, of a string rendered in the current font.Delphi syntax: function TextHeight(const Text: string): Integer;
TextOut利用当前字体在画布指定位置上绘制文本。Writes a string on the canvas, starting at the point (X,Y), and then updates the PenPos to the end of the string.Delphi syntax: procedure TextOut(X, Y: Integer; const Text: string);
TextRect在剪贴矩形框中绘制文本。Writes a string inside a clipping rectangle.Delphi syntax: procedure TextRect(Rect: TRect; X, Y: Integer; const Text: string);
信不信由你，这些属性和方法仅仅只是体现了Windows设备描述环境的一小部分功能。好在这些属性和方法覆盖了处理图像时需要完成工作的80%。然后，在详细论述TCanvas类之前，需要论述用于Windows程序设计中的图形对象。