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Delphi - how to crop a bitmap "in place"?

If I have TBitmap and want to get the cropped image from this bitmap, can I do the in-place cropping operation? for example, if I have a 800x600 bitmap, how can I reduce (crop) it so that it contains a 600x400 image in the center, that is, the resulting TBitmap is 600x400 and consists of a rectangle bounded by (100, 100) and (700, 500 ) in the original image?

Do I need to go through another bitmap or perform this operation in the source raster file?

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2 answers

You can use the BitBlt function

try this code.

 procedure CropBitmap(InBitmap, OutBitMap : TBitmap; X, Y, W, H :Integer); begin OutBitMap.PixelFormat := InBitmap.PixelFormat; OutBitMap.Width := W; OutBitMap.Height := H; BitBlt(OutBitMap.Canvas.Handle, 0, 0, W, H, InBitmap.Canvas.Handle, X, Y, SRCCOPY); end;

and you can use that way

 Var Bmp : TBitmap; begin Bmp:=TBitmap.Create; try CropBitmap(Image1.Picture.Bitmap, Bmp, 10,0, 150, 150); //do something with the cropped image //Bmp.SaveToFile('Foo.bmp'); finally Bmp.Free; end; end;

If you want to use the same bitmap, try this version of the function

 procedure CropBitmap(InBitmap : TBitmap; X, Y, W, H :Integer); begin BitBlt(InBitmap.Canvas.Handle, 0, 0, W, H, InBitmap.Canvas.Handle, X, Y, SRCCOPY); InBitmap.Width :=W; InBitmap.Height:=H; end;

And use this way

 Var Bmp : TBitmap; begin Bmp:=Image1.Picture.Bitmap; CropBitmap(Bmp, 10,0, 150, 150); //do somehting with the Bmp Image1.Picture.Assign(Bmp); end;
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I know that you already have your accepted answer, but since I wrote my version (which uses the VCL shell instead of the GDI call), I will post it here, and not just throw it away.

 procedure TForm1.FormClick(Sender: TObject); var Source, Dest: TRect; begin Source := Image1.Picture.Bitmap.Canvas.ClipRect; { desired rectangle obtained by collapsing the original one by 2*2 times } InflateRect(Source, -(Image1.Picture.Bitmap.Width div 4), -(Image1.Picture.Bitmap.Height div 4)); Dest := Source; OffsetRect(Dest, -Dest.Left, -Dest.Top); { NB: raster data is preserved during the operation, so there is not need to have 2 bitmaps } Image1.Picture.Bitmap.Canvas.CopyRect(Dest, Image1.Picture.Bitmap.Canvas, Source); { and finally "truncate" the canvas } Image1.Picture.Bitmap.Width := Dest.Right; Image1.Picture.Bitmap.Height := Dest.Bottom; end;
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