Direct2D (11) : 画刷之 ID2D1LinearGradientBrush

TDirect2DCanvas.Brush 的类型是 TDirect2DBrush，它和 GDI 的 TBrush 的区别主要有两点：
1、只保留了 TBrush.Style 中的 bsSolid、bsClear 选项，弃用了：bsHorizontal、bsVertical、bsFDiagonal、bsBDiagonal、bsCross、bsDiagCross。
2、其 Handle 属性是 Direct2D 中的 ID2D1Brush。

Direct2D 提供有四种画刷：

ID2D1SolidColorBrush     //纯色画刷；TDirect2DBrush 默认使用了它
ID2D1LinearGradientBrush //线性渐变画刷
ID2D1RadialGradientBrush //放射渐变画刷
ID2D1BitmapBrush         //位图画刷

ID2D1Brush //是它们的父接口，它提供了透明色和 2D 变换的基础能力。

{相关方法}
TDirect2DCanvas.RenderTarget.CreateSolidColorBrush();     //建立纯色画刷
TDirect2DCanvas.RenderTarget.CreateLinearGradientBrush(); //建立线性渐变画刷
TDirect2DCanvas.RenderTarget.CreateRadialGradientBrush(); //建立放射渐变画刷
TDirect2DCanvas.RenderTarget.CreateBitmapBrush();         //建立位图画刷

TDirect2DCanvas.RenderTarget.CreateGradientStopCollection(); //建立画刷需要的 ID2D1GradientStopCollection 对象
  //ID2D1GradientStopCollection 中包含渐变需要的颜色及位置数组和位图排列规则参数等

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ID2D1LinearGradientBrush : 线性渐变画刷

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{建立函数}
function CreateLinearGradientBrush(
  const linearGradientBrushProperties: TD2D1LinearGradientBrushProperties; //描述渐变轴的结构体
  brushProperties: PD2D1BrushProperties;               //描述透明度及 2D 变换的结构指针
  gradientStopCollection: ID2D1GradientStopCollection; //通过该接口传递 TD2D1GradientStop 结构(颜色及位置)的数组
  out linearGradientBrush: ID2D1LinearGradientBrush    //输出建立的对象
): HResult; stdcall;

{描述渐变轴的结构体}
TD2D1LinearGradientBrushProperties = record
  startPoint: D2D1_POINT_2F;
  endPoint: D2D1_POINT_2F;
end;
{描述透明度及 2D 变换的结构体}
TD2D1BrushProperties = record
  opacity: Single;
  transform: D2D1_MATRIX_3X2_F;
end;
{描述位置与颜色的结构体}
TD2D1GradientStop = record
  position: Single;
  color: D2D1_COLOR_F;
end;

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测试代码：

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uses Direct2D, D2D1;

procedure TForm1.FormPaint(Sender: TObject);
var
  cvs: TDirect2DCanvas;
  iBrush: ID2D1LinearGradientBrush;
  R: TRect;
  rLinear: TD2D1LinearGradientBrushProperties;
  arrGradientStop: array[0..2] of TD2D1GradientStop;
  iGradientStops: ID2D1GradientStopCollection;
begin
  cvs := TDirect2DCanvas.Create(Canvas, ClientRect);

  R := ClientRect;
  InflateRect(R, -ClientWidth div 6, -ClientHeight div 6);

  {对角线轴}
  rLinear.startPoint := D2D1PointF(R.Left, R.Top);
  rLinear.endPoint := D2D1PointF(R.Right, R.Bottom);

  {横向轴}
//  rLinear.startPoint := D2D1PointF(R.Left, ClientHeight / 2);
//  rLinear.endPoint := D2D1PointF(R.Right, ClientHeight / 2);

  arrGradientStop[0].position := 0.0;
  arrGradientStop[0].color := D2D1ColorF(clRed);
  arrGradientStop[1].position := 0.5;
  arrGradientStop[1].color := D2D1ColorF(clYellow);
  arrGradientStop[2].position := 1.0;
  arrGradientStop[2].color := D2D1ColorF(clGreen);

  cvs.RenderTarget.CreateGradientStopCollection(
    @arrGradientStop[0], Length(arrGradientStop), D2D1_GAMMA_2_2, D2D1_EXTEND_MODE_CLAMP, iGradientStops
  );

  cvs.RenderTarget.CreateLinearGradientBrush(rLinear, nil, iGradientStops, iBrush);
  cvs.Brush.Handle := iBrush;

  cvs.BeginDraw;
  cvs.Rectangle(R);
  cvs.EndDraw;
  cvs.Free;
end;

procedure TForm1.FormResize(Sender: TObject);
begin
  Repaint;
end;

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效果图：

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