Writing Transparent Text on Image

Question

How can I write a semi transparent text on an Image (Jpg,Bmp), or a transparent text (color as same background Image) but with a shadow, something I want to do to watermark the images.

I want to accomplish that using Delphi win32.

Answer 1

One option is to use the AlphaBlend function in Windows.pas unit. Something like this will produce semi-transparent text (with a drop shadow - building on Jim McKeeth's response) overlayed on an image:

  
uses Windows, Graphics;
.
.
.
var
  BackgroundImage: Graphics.TBitmap; { need to call out specifically for Graphics.TBitmap
                                       because the Windows unit also has a TBitmap
                                       declaration }
  TextImage: Graphics.TBitmap;
  BlendFunc: BLENDFUNCTION;
begin
  BlendFunc.BlendOp := AC_SRC_OVER;
  BlendFunc.BlendFlags := 0;
  BlendFunc.SourceConstantAlpha := $C0; { a hex value from $00-$FF (0-255).
                                          Represents the percent of opaqueness:
                                          $00 is completely transparent, 
                                          $FF is completely opaque.
                                          $C0 is 75% opaque }
  BlendFunc.AlphaFormat := AC_SRC_ALPHA;

    { BackgroundImage is for holding the image you want to overlay text onto }
    BackgroundImage := Graphics.TBitmap.Create;
    try
      BackgroundImage.LoadFromFile('yourimagehere.bmp');

      { Create another TBitmap to hold the text you want to overlay }
      TextImage := Graphics.TBitmap.Create;
      try
        { Set this bitmap to have the same dimensions as the
          background image you want the text to appear on. }
        TextImage.Height := BackgroundImage.Height;
        TextImage.Width := BackgroundImage.Width;

        { In my limited experience with AlphaBlend, Black is always 100%
          transparent. So, paint TextImage completely Black. Play around
          with this to see the effect it has on the final outcome. }
        TextImage.Canvas.Brush.Color := clBlack;
        TextImage.Canvas.FloodFill(0, 0, clNone, fsBorder);

        TextImage.Canvas.Font.Style := [fsBold];

        { Write the shadow first }
        TextImage.Canvas.Brush.Style := bsClear;
        TextImage.Canvas.Font.Color  := clDkGray;
        TextImage.Canvas.TextOut(11, 11, 'Test');

        { Then put the text on top (slightly offset) }
        TextImage.Canvas.Brush.Style := bsClear;
        TextImage.Canvas.Font.Color  := clMaroon;
        TextImage.Canvas.TextOut(10, 10, 'Test');

        { Use the AlphaBlend function to overlay the bitmap holding the text
          on top of the bitmap holding the original image. }
        Windows.AlphaBlend(BackgroundImage.Canvas.Handle, 0, 0,
                           TextImage.Width, TextImage.Height,
                           TextImage.Canvas.Handle, 0, 0, TextImage.Width,
                           TextImage.Height, BlendFunc);

        { Assign the now updated BackgroundImage to a TImage control for display }  
        Image1.Picture.Bitmap.Assign(BackgroundImage);
      finally
        TextImage.Free;
      end;
    finally
      BackgroundImage.Free;
    end;
  end;

Answer 2

I presume what you're trying to accomplish is a little more complicated than simply writing text with a transparent background; i.e. you are trying to get some form of alpha-blended text written on the image.
The simplest method would be to make use of the GDI+ routines. They are encapsulated for delphi and available for download from http://www.progdigy.com/. There are many examples there which should be usable as an example.