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    <blockquote> <p><strong>tl;dr;</strong> Use GDI+</p> </blockquote> <h3>SetWorldTransform</h3> <p>With WinAPI's <a href="http://msdn.microsoft.com/en-us/library/dd145104(v=vs.85).aspx" rel="noreferrer">SetWorldTransform</a> you can transform the space of device context: rotate, shear, offset, and scale. This is done by setting the members of a transform matrix of type XFORM. Fill its members according <a href="http://msdn.microsoft.com/en-us/library/dd145228(v=vs.85).aspx" rel="noreferrer">the documentation</a>.</p> <pre><code>procedure RotateBitmap(Bmp: TBitmap; Rads: Single; AdjustSize: Boolean; BkColor: TColor = clNone); var C: Single; S: Single; XForm: tagXFORM; Tmp: TBitmap; begin C := Cos(Rads); S := Sin(Rads); XForm.eM11 := C; XForm.eM12 := S; XForm.eM21 := -S; XForm.eM22 := C; Tmp := TBitmap.Create; try Tmp.TransparentColor := Bmp.TransparentColor; Tmp.TransparentMode := Bmp.TransparentMode; Tmp.Transparent := Bmp.Transparent; Tmp.Canvas.Brush.Color := BkColor; if AdjustSize then begin Tmp.Width := Round(Bmp.Width * Abs(C) + Bmp.Height * Abs(S)); Tmp.Height := Round(Bmp.Width * Abs(S) + Bmp.Height * Abs(C)); XForm.eDx := (Tmp.Width - Bmp.Width * C + Bmp.Height * S) / 2; XForm.eDy := (Tmp.Height - Bmp.Width * S - Bmp.Height * C) / 2; end else begin Tmp.Width := Bmp.Width; Tmp.Height := Bmp.Height; XForm.eDx := (Bmp.Width - Bmp.Width * C + Bmp.Height * S) / 2; XForm.eDy := (Bmp.Height - Bmp.Width * S - Bmp.Height * C) / 2; end; SetGraphicsMode(Tmp.Canvas.Handle, GM_ADVANCED); SetWorldTransform(Tmp.Canvas.Handle, XForm); BitBlt(Tmp.Canvas.Handle, 0, 0, Tmp.Width, Tmp.Height, Bmp.Canvas.Handle, 0, 0, SRCCOPY); Bmp.Assign(Tmp); finally Tmp.Free; end; end; </code></pre> <h3>PlgBlt</h3> <p>The <a href="http://msdn.microsoft.com/en-us/library/dd162804(v=vs.85).aspx" rel="noreferrer">PlgBlt</a> function performs a bit-block transfer from the specified rectangle in the source device context to the specified parallelogram in the destination device context. Map the corner points of the source image via the <code>lpPoint</code> parameter.</p> <pre><code>procedure RotateBitmap(Bmp: TBitmap; Rads: Single; AdjustSize: Boolean; BkColor: TColor = clNone); var C: Single; S: Single; Tmp: TBitmap; OffsetX: Single; OffsetY: Single; Points: array[0..2] of TPoint; begin C := Cos(Rads); S := Sin(Rads); Tmp := TBitmap.Create; try Tmp.TransparentColor := Bmp.TransparentColor; Tmp.TransparentMode := Bmp.TransparentMode; Tmp.Transparent := Bmp.Transparent; Tmp.Canvas.Brush.Color := BkColor; if AdjustSize then begin Tmp.Width := Round(Bmp.Width * Abs(C) + Bmp.Height * Abs(S)); Tmp.Height := Round(Bmp.Width * Abs(S) + Bmp.Height * Abs(C)); OffsetX := (Tmp.Width - Bmp.Width * C + Bmp.Height * S) / 2; OffsetY := (Tmp.Height - Bmp.Width * S - Bmp.Height * C) / 2; end else begin Tmp.Width := Bmp.Width; Tmp.Height := Bmp.Height; OffsetX := (Bmp.Width - Bmp.Width * C + Bmp.Height * S) / 2; OffsetY := (Bmp.Height - Bmp.Width * S - Bmp.Height * C) / 2; end; Points[0].X := Round(OffsetX); Points[0].Y := Round(OffsetY); Points[1].X := Round(OffsetX + Bmp.Width * C); Points[1].Y := Round(OffsetY + Bmp.Width * S); Points[2].X := Round(OffsetX - Bmp.Height * S); Points[2].Y := Round(OffsetY + Bmp.Height * C); PlgBlt(Tmp.Canvas.Handle, Points, Bmp.Canvas.Handle, 0, 0, Bmp.Width, Bmp.Height, 0, 0, 0); Bmp.Assign(Tmp); finally Tmp.Free; end; end; </code></pre> <h3>Graphics32</h3> <p><a href="http://www.graphics32.org/wiki/" rel="noreferrer">Graphics32</a> is a library especially designed for fast bitmap handling. It requires some experience to grasp its full potential, but <a href="http://graphics32.org/documentation/Docs/_Body.htm" rel="noreferrer">the documentation</a> as well as the provided examples should get you started.</p> <p>A rotation of a <code>TBitmap32</code> image is done by transforming it by one of the many available transformation classes. The <code>TAffineTransformation</code> class is needed here. First, shift the image half its size to the upper left, then rotate, and shift the result back to the lower right, possibly using the new image dimensions.</p> <pre><code>uses GR32, GR32_Transforms; procedure RotateBitmap(Bmp: TBitmap32; Degs: Integer; AdjustSize: Boolean; BkColor: TColor = clNone; Transparent: Boolean = False); overload; var Tmp: TBitmap32; Transformation: TAffineTransformation; begin Tmp := TBitmap32.Create; Transformation := TAffineTransformation.Create; try Transformation.BeginUpdate; Transformation.SrcRect := FloatRect(0, 0, Bmp.Width, Bmp.Height); Transformation.Translate(-0.5 * Bmp.Width, -0.5 * Bmp.Height); Transformation.Rotate(0, 0, -Degs); if AdjustSize then with Transformation.GetTransformedBounds do Tmp.SetSize(Round(Right - Left), Round(Bottom - Top)) else Tmp.SetSize(Bmp.Width, Bmp.Height); Transformation.Translate(0.5 * Tmp.Width, 0.5 * Tmp.Height); Transformation.EndUpdate; Tmp.Clear(Color32(BkColor)); if not Transparent then Bmp.DrawMode := dmTransparent; Transform(Tmp, Bmp, Transformation); Bmp.Assign(Tmp); Bmp.OuterColor := Color32(BkColor); if Transparent then Bmp.DrawMode := dmTransparent; finally Transformation.Free; Tmp.Free; end; end; procedure RotateBitmap(Bmp: TBitmap; Degs: Integer; AdjustSize: Boolean; BkColor: TColor = clNone); overload; var Tmp: TBitmap32; Transparent: Boolean; begin Tmp := TBitmap32.Create; try Transparent := Bmp.Transparent; Tmp.Assign(Bmp); RotateBitmapGR32(Tmp, Degs, AdjustSize, BkColor, Transparent); Bmp.Assign(Tmp); if Transparent then Bmp.Transparent := True; finally Tmp.Free; end; end; </code></pre> <h3>GDI+</h3> <p>Introduced in Windows XP, Microsoft's <a href="http://msdn.microsoft.com/en-us/library/ms533798(v=vs.85).aspx" rel="noreferrer">GDI+</a> API is more efficient then the default GDI API. For Delphi 2009 and up, the library is available <a href="http://www.bilsen.com/gdiplus/index.shtml" rel="noreferrer">from here</a>. For older Delphi versions, the library is available <a href="http://www.progdigy.com/?page_id=7" rel="noreferrer">from here</a>.</p> <p>In GDI+ the rotation is also done by a transformation matrix. Drawing works quite differently though. Create a <code>TGPGraphics</code> object and attach it to a device context with its constructor. Subsequently, drawing operations on the object are translated by the API and will be output to the destination context.</p> <pre><code>uses GDIPOBJ, GDIPAPI; // &lt; D2009 GdiPlus; // &gt;= D2009 procedure RotateBitmap(Bmp: TBitmap; Degs: Integer; AdjustSize: Boolean; BkColor: TColor = clNone); var Tmp: TGPBitmap; Matrix: TGPMatrix; C: Single; S: Single; NewSize: TSize; Graphs: TGPGraphics; P: TGPPointF; begin Tmp := TGPBitmap.Create(Bmp.Handle, Bmp.Palette); Matrix := TGPMatrix.Create; try Matrix.RotateAt(Degs, MakePoint(0.5 * Bmp.Width, 0.5 * Bmp.Height)); if AdjustSize then begin C := Cos(DegToRad(Degs)); S := Sin(DegToRad(Degs)); NewSize.cx := Round(Bmp.Width * Abs(C) + Bmp.Height * Abs(S)); NewSize.cy := Round(Bmp.Width * Abs(S) + Bmp.Height * Abs(C)); Bmp.Width := NewSize.cx; Bmp.Height := NewSize.cy; end; Graphs := TGPGraphics.Create(Bmp.Canvas.Handle); try Graphs.Clear(ColorRefToARGB(ColorToRGB(BkColor))); Graphs.SetTransform(Matrix); Graphs.DrawImage(Tmp, (Cardinal(Bmp.Width) - Tmp.GetWidth) div 2, (Cardinal(Bmp.Height) - Tmp.GetHeight) div 2); finally Graphs.Free; end; finally Matrix.Free; Tmp.Free; end; end; </code></pre> <h3>Handling transparency</h3> <p>The routines above preserve the transparent settings of the fead bitmap, with the exception of the Graphics32 solution which requires an additional <code>Transparent</code> parameter.</p> <h3>Performance and image quality</h3> <p>I wrote a test application (see full code below) to tune the performance of the various methods and to compare the resulting image quality.</p> <p>The first and most important conclusion is that GDI+ uses anti-aliasing where the others do not, resulting in the best image quality. (I unsuccessfully tried to prevent anti-aliasing by setting <code>CompositingQuality</code>, <code>InterpolationMode</code>, <code>SmoothingMode</code>, and <code>PixelOffsetMode</code>, so when anti-aliasing is not preferred, do not use GDI+.)</p> <p>Furthermore, the GDI+ solution is also the fastest method, by far.</p> <p><a href="https://i.stack.imgur.com/3LAjH.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/MR6aU.jpg" alt="Test results" title="Show original image"></a></p> <pre><code>unit RotateTestForm; interface uses Windows, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, ExtCtrls, JPEG, Math, GR32, GR32_Transforms, GDIPOBJ, GDIPAPI {, GdiPlus}; type TTestForm = class(TForm) private FImage: TImage; FOpenDialog: TOpenDialog; procedure FormPaint(Sender: TObject); public constructor Create(AOwner: TComponent); override; end; var TestForm: TTestForm; implementation {$R *.dfm} procedure RotateBitmapSWT(Bmp: TBitmap; Rads: Single; AdjustSize: Boolean; BkColor: TColor = clNone); var C: Single; S: Single; XForm: TXForm; Tmp: TBitmap; begin C := Cos(Rads); S := Sin(Rads); XForm.eM11 := C; XForm.eM12 := S; XForm.eM21 := -S; XForm.eM22 := C; Tmp := TBitmap.Create; try Tmp.TransparentColor := Bmp.TransparentColor; Tmp.TransparentMode := Bmp.TransparentMode; Tmp.Transparent := Bmp.Transparent; Tmp.Canvas.Brush.Color := BkColor; if AdjustSize then begin Tmp.Width := Round(Bmp.Width * Abs(C) + Bmp.Height * Abs(S)); Tmp.Height := Round(Bmp.Width * Abs(S) + Bmp.Height * Abs(C)); XForm.eDx := (Tmp.Width - Bmp.Width * C + Bmp.Height * S) / 2; XForm.eDy := (Tmp.Height - Bmp.Width * S - Bmp.Height * C) / 2; end else begin Tmp.Width := Bmp.Width; Tmp.Height := Bmp.Height; XForm.eDx := (Bmp.Width - Bmp.Width * C + Bmp.Height * S) / 2; XForm.eDy := (Bmp.Height - Bmp.Width * S - Bmp.Height * C) / 2; end; SetGraphicsMode(Tmp.Canvas.Handle, GM_ADVANCED); SetWorldTransform(Tmp.Canvas.Handle, XForm); BitBlt(Tmp.Canvas.Handle, 0, 0, Tmp.Width, Tmp.Height, Bmp.Canvas.Handle, 0, 0, SRCCOPY); Bmp.Assign(Tmp); finally Tmp.Free; end; end; procedure RotateBitmapPLG(Bmp: TBitmap; Rads: Single; AdjustSize: Boolean; BkColor: TColor = clNone); var C: Single; S: Single; Tmp: TBitmap; OffsetX: Single; OffsetY: Single; Points: array[0..2] of TPoint; begin C := Cos(Rads); S := Sin(Rads); Tmp := TBitmap.Create; try Tmp.TransparentColor := Bmp.TransparentColor; Tmp.TransparentMode := Bmp.TransparentMode; Tmp.Transparent := Bmp.Transparent; Tmp.Canvas.Brush.Color := BkColor; if AdjustSize then begin Tmp.Width := Round(Bmp.Width * Abs(C) + Bmp.Height * Abs(S)); Tmp.Height := Round(Bmp.Width * Abs(S) + Bmp.Height * Abs(C)); OffsetX := (Tmp.Width - Bmp.Width * C + Bmp.Height * S) / 2; OffsetY := (Tmp.Height - Bmp.Width * S - Bmp.Height * C) / 2; end else begin Tmp.Width := Bmp.Width; Tmp.Height := Bmp.Height; OffsetX := (Bmp.Width - Bmp.Width * C + Bmp.Height * S) / 2; OffsetY := (Bmp.Height - Bmp.Width * S - Bmp.Height * C) / 2; end; Points[0].X := Round(OffsetX); Points[0].Y := Round(OffsetY); Points[1].X := Round(OffsetX + Bmp.Width * C); Points[1].Y := Round(OffsetY + Bmp.Width * S); Points[2].X := Round(OffsetX - Bmp.Height * S); Points[2].Y := Round(OffsetY + Bmp.Height * C); PlgBlt(Tmp.Canvas.Handle, Points, Bmp.Canvas.Handle, 0, 0, Bmp.Width, Bmp.Height, 0, 0, 0); Bmp.Assign(Tmp); finally Tmp.Free; end; end; procedure RotateBitmapGR32(Bmp: TBitmap32; Degs: Integer; AdjustSize: Boolean; BkColor: TColor = clNone; Transparent: Boolean = False); overload; var Tmp: TBitmap32; Transformation: TAffineTransformation; begin Tmp := TBitmap32.Create; Transformation := TAffineTransformation.Create; try Transformation.BeginUpdate; Transformation.SrcRect := FloatRect(0, 0, Bmp.Width, Bmp.Height); Transformation.Translate(-0.5 * Bmp.Width, -0.5 * Bmp.Height); Transformation.Rotate(0, 0, -Degs); if AdjustSize then with Transformation.GetTransformedBounds do Tmp.SetSize(Round(Right - Left), Round(Bottom - Top)) else Tmp.SetSize(Bmp.Width, Bmp.Height); Transformation.Translate(0.5 * Tmp.Width, 0.5 * Tmp.Height); Transformation.EndUpdate; Tmp.Clear(Color32(BkColor)); if not Transparent then Bmp.DrawMode := dmTransparent; Transform(Tmp, Bmp, Transformation); Bmp.Assign(Tmp); Bmp.OuterColor := Color32(BkColor); if Transparent then Bmp.DrawMode := dmTransparent; finally Transformation.Free; Tmp.Free; end; end; procedure RotateBitmapGR32(Bmp: TBitmap; Degs: Integer; AdjustSize: Boolean; BkColor: TColor = clNone); overload; var Tmp: TBitmap32; Transparent: Boolean; begin Tmp := TBitmap32.Create; try Transparent := Bmp.Transparent; Tmp.Assign(Bmp); RotateBitmapGR32(Tmp, Degs, AdjustSize, BkColor, Transparent); Bmp.Assign(Tmp); if Transparent then Bmp.Transparent := True; finally Tmp.Free; end; end; procedure RotateBitmapGDIP(Bmp: TBitmap; Degs: Integer; AdjustSize: Boolean; BkColor: TColor = clNone); var Tmp: TGPBitmap; Matrix: TGPMatrix; C: Single; S: Single; NewSize: TSize; Graphs: TGPGraphics; P: TGPPointF; begin Tmp := TGPBitmap.Create(Bmp.Handle, Bmp.Palette); Matrix := TGPMatrix.Create; try Matrix.RotateAt(Degs, MakePoint(0.5 * Bmp.Width, 0.5 * Bmp.Height)); if AdjustSize then begin C := Cos(DegToRad(Degs)); S := Sin(DegToRad(Degs)); NewSize.cx := Round(Bmp.Width * Abs(C) + Bmp.Height * Abs(S)); NewSize.cy := Round(Bmp.Width * Abs(S) + Bmp.Height * Abs(C)); Bmp.Width := NewSize.cx; Bmp.Height := NewSize.cy; end; Graphs := TGPGraphics.Create(Bmp.Canvas.Handle); try Graphs.Clear(ColorRefToARGB(ColorToRGB(BkColor))); Graphs.SetTransform(Matrix); Graphs.DrawImage(Tmp, (Cardinal(Bmp.Width) - Tmp.GetWidth) div 2, (Cardinal(Bmp.Height) - Tmp.GetHeight) div 2); finally Graphs.Free; end; finally Matrix.Free; Tmp.Free; end; end; { TTestForm } constructor TTestForm.Create(AOwner: TComponent); begin inherited Create(AOwner); Font.Name := 'Tahoma'; Top := 0; ClientWidth := 560; ClientHeight := 915; Show; FImage := TImage.Create(Self); FOpenDialog := TOpenDialog.Create(Self); FOpenDialog.Title := 'Select an small sized image (min. 100 x 100)'; FOpenDialog.Options := FOpenDialog.Options + [ofFileMustExist]; FOpenDialog.Filter := 'JPEG|*.JPG|BMP|*.BMP'; if FOpenDialog.Execute then begin FImage.Picture.LoadFromFile(FOpenDialog.FileName); OnPaint := FormPaint; Invalidate; end else Application.Terminate; end; procedure TTestForm.FormPaint(Sender: TObject); var Img: TBitmap; Bmp: TBitmap; Bmp32: TBitmap32; BkColor: TColor; AdjustSize: Boolean; Degs: Integer; Rads: Single; RotCount: Integer; I: Integer; Tick: Cardinal; begin Img := TBitmap.Create; Bmp := TBitmap.Create; Bmp32 := TBitmap32.Create; try BkColor := clBtnFace; Img.Canvas.Brush.Color := BkColor; Img.Width := 100; Img.Height := 100; Img.Canvas.Draw(0, 0, FImage.Picture.Graphic); AdjustSize := False; Degs := 45; Rads := DegToRad(Degs); RotCount := 1000; Canvas.TextOut(10, 10, 'Original:'); Canvas.Draw(10, 30, Img); Canvas.TextOut(10, 140, Format('Size = %d x %d', [Img.Width, Img.Height])); Canvas.TextOut(10, 160, Format('Angle = %d°', [Degs])); Canvas.TextOut(10, 250, Format('%d rotations:', [RotCount])); Canvas.TextOut(120, 10, 'SetWorldTransform:'); Bmp.Assign(Img); RotateBitmapSWT(Bmp, Rads, AdjustSize, BkColor); Canvas.Draw(120, 30, Bmp); if not AdjustSize then begin Tick := GetTickCount; for I := 0 to RotCount - 2 do RotateBitmapSWT(Bmp, Rads, AdjustSize, BkColor); Canvas.TextOut(120, 250, Format('%d msec', [GetTickCount - Tick])); Canvas.Draw(120, 140, Bmp); end; Canvas.TextOut(230, 10, 'PlgBlt:'); Bmp.Assign(Img); RotateBitmapPLG(Bmp, Rads, AdjustSize, BkColor); Canvas.Draw(230, 30, Bmp); if not AdjustSize then begin Tick := GetTickCount; for I := 0 to RotCount - 2 do RotateBitmapPLG(Bmp, Rads, AdjustSize, BkColor); Canvas.TextOut(230, 250, Format('%d msec', [GetTickCount - Tick])); Canvas.Draw(230, 140, Bmp); end; Canvas.TextOut(340, 10, 'Graphics32:'); Bmp.Assign(Img); RotateBitmapGR32(Bmp, Degs, AdjustSize, BkColor); Canvas.Draw(340, 30, Bmp); if not AdjustSize then begin Tick := GetTickCount; for I := 0 to RotCount - 2 do RotateBitmapGR32(Bmp, Degs, AdjustSize, BkColor); Canvas.TextOut(340, 250, Format('%d msec', [GetTickCount - Tick])); Canvas.Draw(340, 140, Bmp); // Without in between conversion to TBitmap: Bmp32.Assign(Img); Tick := GetTickCount; for I := 0 to RotCount - 1 do RotateBitmapGR32(Bmp32, Degs, AdjustSize, BkColor, False); Canvas.TextOut(340, 270, Format('%d msec (optimized)', [GetTickCount - Tick])); end; Canvas.TextOut(450, 10, 'GDI+ :'); Bmp.Assign(Img); RotateBitmapGDIP(Bmp, Degs, AdjustSize, BkColor); Canvas.Draw(450, 30, Bmp); if not AdjustSize then begin Tick := GetTickCount; for I := 0 to RotCount - 2 do RotateBitmapGDIP(Bmp, Degs, AdjustSize, BkColor); Canvas.TextOut(450, 250, Format('%d msec', [GetTickCount - Tick])); Canvas.Draw(450, 140, Bmp); end; finally Bmp32.Free; Bmp.Free; Img.Free; OnPaint := nil; end; end; end. </code></pre>
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    1. CONow looking at [`this library`](http://www.crossgl.com/aggpas/) internals, it could also be one of the hot favorites for this benchmark... And also the results of antialiasing [`looks impressive`](http://www.antigrain.com/doc/introduction/introduction.agdoc.html#toc0005) (I have to try it :-)
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    2. CO@TLama - Hi TLama, What are the conclusions with that library? The 'rotate' is it better (smooth, fast) than the code above? Thanks.
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    3. CO@Altar, sorry, I forgot on this. The output of AGG is great (I'm using it for every vector rendering I do). If will be faster than the above ways I don't know though.
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