Create a thumbnail from a JPEG image

Problem/Question/Abstract:

Once I was trying to resize a jpeg image and made some Internet search. Believe or not, I couldn't find clear answers to my question, but it's very easy to do.

Answer:

The code below will reduce width and height of a chosen .jpg image.

Go to "File / New / Console Application" and paste this code. Set the SizePct (a const on the code below, but can be a variable on your program) to fit your needs. If you want a new image with 30% of the original width and height set this to 30.

All I do is load the JPEG on a TJPEGImage, create a bitmap and .StretchDraw the JPEG on the bitmap. Then I copy the bitmap to a TJPEGImage using the .Assign method, and, finally, save it.

program Project1;

{$APPTYPE CONSOLE}

uses
  Classes, Windows, SysUtils, Dialogs, JPEG, Graphics;

const
  SizePct: integer = 50; { The new image will have 50% of the original }

var
  OpenDlg: TOpenDialog;
  SaveDlg: TSaveDialog;
  oJPG: TJPEGImage;
  oBmp: TBitmap;

begin
  OpenDlg := TOpenDialog.Create(nil);
  SaveDlg := TSaveDialog.Create(nil);

  if (OpenDlg.Execute) then
  begin
    try
      begin
        oJPG := TJPEGImage.Create;
        oJPG.LoadFromFile(OpenDlg.FileName);
      end
    except
      MessageBox(
        0,
        PChar('Error while trying to open ' +
        OpenDlg.FileName +
        '.'),
        PChar('Error'),
        MB_OK or MB_ICONERROR
        );
      exit;
    end;

    oBmp := TBitmap.Create;
    oBmp.Width := Round(oJPG.Width * SizePct / 100);
    oBmp.Height := Round(oJPG.Height * SizePct / 100);
    oBmp.Canvas.StretchDraw(
      Rect(0, 0, oBmp.Width - 1, oBmp.Height - 1),
      oJPG
      );

    oJPG.Assign(oBmp);
    oJPG.Compress;

    if (SaveDlg.Execute) then
    begin
      oJPG.SaveToFile(SaveDlg.FileName);
    end;

    oBmp.Free;
    oJPG.Free;
  end;

  OpenDlg.Free;
  SaveDlg.Free;

end.