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  1. PO
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    <blockquote> <p>I assume this is because the text will rotate around the rectangle's TopLeft position</p> </blockquote> <p>No, the rotation is centered around the current origin of the canvas. By default, that is coordinate <code>0, 0</code>, but could be altered by the currently set deformation matrix. The typical way to go is: choose a rotation center, move the origin to that center, rotate, move back to the changed origin, and then draw. See <code>TControl.MatrixChanged</code> for reference. But there are many other ways.</p> <p>Hereby an example of how to paint text from the lower left to the upper right within a form:</p> <p><img src="https://i.stack.imgur.com/rXHKj.png" alt="enter image description here"></p> <pre><code>procedure TForm1.FormPaint(Sender: TObject; Canvas: TCanvas; const ARect: TRectF); var Angle: Single; R: TRectF; S: String; H: Single; Matrix: TMatrix; begin Canvas.Fill.Color := TAlphaColors.Black; Angle := -ArcTan2(ClientHeight, ClientWidth); R := ClientRect; S := 'Text from bottom-left...'; H := Canvas.TextHeight(S); Matrix := CreateRotationMatrix(Angle); Matrix.m31 := Sin(Angle) * (ClientHeight - H); Matrix.m32 := ClientHeight * (1 - Cos(Angle)); Canvas.SetMatrix(Matrix); Canvas.FillText(R, S, False, 1, [], TTextAlign.taLeading, TTextAlign.taTrailing); S := '...to top-right'; Matrix.m31 := ClientWidth * (1 - Cos(Angle)) + Sin(Angle) * H; Matrix.m32 := -Sin(Angle) * ClientWidth; Canvas.SetMatrix(Matrix); Canvas.FillText(R, S, False, 1, [], TTextAlign.taTrailing, TTextAlign.taLeading); end; </code></pre> <h3>Update:</h3> <p>This code does not yet take an already shifted origin into account.</p> <p>In response to your comment, the following code draws text from coordinate <code>50, 100</code> down, 90° rotated around that point, using the method explained above, on a PaintBox which is arbitrarily positioned on the form.</p> <pre><code>procedure TForm1.PaintBox1Paint(Sender: TObject; Canvas: TCanvas); const S = 'Hello World'; var R: TRectF; OriginalMatrix: TMatrix; ShiftMatrix: TMatrix; RotationMatrix: TMatrix; ShiftBackMatrix: TMatrix; Matrix: TMatrix; begin PaintBox1.Canvas.Fill.Color := TAlphaColors.Black; R.Right := 50; R.Bottom := 100; R.Left := R.Right - 5000; R.Top := R.Bottom - 5000; OriginalMatrix := PaintBox1.Canvas.Matrix; ShiftMatrix := IdentityMatrix; ShiftMatrix.m31 := -R.Right; ShiftMatrix.m32 := -R.Bottom; RotationMatrix := CreateRotationMatrix(DegToRad(-90)); ShiftBackMatrix := IdentityMatrix; ShiftBackMatrix.m31 := R.Right; ShiftBackMatrix.m32 := R.Bottom; Matrix := MatrixMultiply(RotationMatrix, ShiftBackMatrix); Matrix := MatrixMultiply(ShiftMatrix, Matrix); Matrix := MatrixMultiply(Matrix, OriginalMatrix); PaintBox1.Canvas.SetMatrix(Matrix); PaintBox1.Canvas.FillText(R, S, False, 1, [], TTextAlign.taTrailing, TTextAlign.taTrailing); PaintBox1.Canvas.SetMatrix(OriginalMatrix); end; </code></pre> <p>Which can be reduced to:</p> <pre><code>procedure TForm1.PaintBox1Paint(Sender: TObject; Canvas: TCanvas); const S = 'Hello World'; var R: TRectF; SaveMatrix: TMatrix; Matrix: TMatrix; begin PaintBox1.Canvas.Fill.Color := TAlphaColors.Black; R := RectF(-Canvas.TextWidth(S), -Canvas.TextHeight(S), 0, 0); SaveMatrix := PaintBox1.Canvas.Matrix; Matrix := CreateRotationMatrix(DegToRad(-90)); Matrix.m31 := 50; Matrix.m32 := 100; PaintBox1.Canvas.MultyMatrix(Matrix); PaintBox1.Canvas.FillText(R, S, False, 1, [], TTextAlign.taTrailing, TTextAlign.taTrailing); PaintBox1.Canvas.SetMatrix(SaveMatrix); end; </code></pre> <p>Which in turn evolves into this general routine:</p> <pre><code>procedure DrawRotatedText(Canvas: TCanvas; const P: TPointF; RadAngle: Single; const S: String; HTextAlign, VTextAlign: TTextAlign); var W: Single; H: Single; R: TRectF; SaveMatrix: TMatrix; Matrix: TMatrix; begin W := Canvas.TextWidth(S); H := Canvas.TextHeight(S); case HTextAlign of TTextAlign.taCenter: R.Left := -W / 2; TTextAlign.taLeading: R.Left := 0; TTextAlign.taTrailing: R.Left := -W; end; R.Width := W; case VTextAlign of TTextAlign.taCenter: R.Top := -H / 2; TTextAlign.taLeading: R.Top := 0; TTextAlign.taTrailing: R.Top := -H; end; R.Height := H; SaveMatrix := Canvas.Matrix; Matrix := CreateRotationMatrix(RadAngle); Matrix.m31 := P.X; Matrix.m32 := P.Y; Canvas.MultyMatrix(Matrix); Canvas.FillText(R, S, False, 1, [], HTextAlign, VTextAlign); Canvas.SetMatrix(SaveMatrix); end; procedure TForm1.PaintBox1Paint(Sender: TObject; Canvas: TCanvas); begin PaintBox1.Canvas.Fill.Color := TAlphaColors.Black; DrawRotatedText(PaintBox1.Canvas, PointF(50, 100), DegToRad(-90), 'Hello world', TTextAlign.taTrailing, TTextAlign.taTrailing); end; </code></pre>
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    1. COFirst, what is the difference between coordinates 0,0 on the current canvas and TopLeft of the rectangle? Then, what should I do if I do not know the length of the text?
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    2. COTopLeft of a rectangle could be anything, doesn't it? Well, in the above example width of text isn't important, but you can obtain it with Canvas.TextWidth. What do you want to do?
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    3. COI want to be able to rotate any given text at any point around the TopRight point of the canvas. Example: I have text 'hello world' which start at point 50;100. And want to rotate the text 90 degrees at the bottom right of the text (that is 90 degrees around the 'd').
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