Scale TBitmap while preserving alpha channel

To scale a TBitmap while preserving the alpha channel (transparency), you can use the Graphics32 library in Delphi. Graphics32 is a powerful library for image processing, and it provides functionality to handle alpha channels during image scaling.

Here's how you can scale a TBitmap with Graphics32:

1. Add Graphics32 Library:
   Download and install the Graphics32 library from the official website or through package managers like GetIt.

2. Include Graphics32 Units:
   In your Delphi project, include the necessary Graphics32 units:

delphiCopy code
uses

  GR32, GR32_Image, GR32_Layers, GR32_Transforms;

1. Scale TBitmap with Alpha Channel:
   Write a function to scale a TBitmap while preserving the alpha channel:

delphiCopy code
function ScaleBitmapWithAlpha(const SrcBitmap: TBitmap; ScaleFactor: Double): TBitmap;

var

  Bitmap32: TBitmap32;

  R: TRect;

  DstWidth, DstHeight: Integer;

begin

  // Calculate the destination size

  DstWidth := Round(SrcBitmap.Width * ScaleFactor);

  DstHeight := Round(SrcBitmap.Height * ScaleFactor);



  // Create a new Bitmap32 to work with

  Bitmap32 := TBitmap32.Create;

  try

    // Assign the source TBitmap to the Bitmap32

    Bitmap32.Assign(SrcBitmap);



    // Set the bitmap transparency properties

    Bitmap32.DrawMode := dmBlend;

    Bitmap32.MasterAlpha := $FF; // Set to 255 for fully opaque



    // Define the rectangle for the destination size

    R := Rect(0, 0, DstWidth, DstHeight);



    // Scale the bitmap using the Graphics32 transformation

    Bitmap32.Resampler := TNearestResampler.Create;

    Bitmap32.SetSize(DstWidth, DstHeight);

    Bitmap32.DrawMode := dmBlendWithAlpha;

    Bitmap32.DrawTo(SrcBitmap.Canvas.Handle, R);



    // Create a new TBitmap to store the scaled image

    Result := TBitmap.Create;

    Result.Assign(Bitmap32);

  finally

    Bitmap32.Free;

  end;

end;

1. Usage:
   Call the ScaleBitmapWithAlpha function and pass the TBitmap you want to scale and the desired scale factor:

delphiCopy code
procedure TForm1.Button1Click(Sender: TObject);

var

  ScaledBitmap: TBitmap;

begin

  ScaledBitmap := ScaleBitmapWithAlpha(Image1.Picture.Bitmap, 2.0); // Scale factor of 2.0

  try

    // Do something with the scaled bitmap, e.g., display it in an Image component

    Image2.Picture.Bitmap.Assign(ScaledBitmap);

  finally

    ScaledBitmap.Free;

  end;

end;

The ScaleBitmapWithAlpha function uses Graphics32 to perform the scaling while preserving the alpha channel. The resulting scaled TBitmap is returned for further use or display.