CODEWITHSUNDEEP
delphiimage-processing
Mohammed Nasman
Mohammed Nasman

Reputation: 11050

Writing Transparent Text on Image

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.

Upvotes: 6

Views: 21186

Answers (6)

IceCold
IceCold

Reputation: 21154

This function is based on Dave Elsberry's idea.

What's different:

  • Draws only the shadow transparently
  • It uses almost 2 times less RAM
  • Parameters
{-------------------------------------------------------------------------------------------------------------
  DrawTextShadowBox
    Draws text in a semi-transparent rectangle with shadow text.
    The shadow text is blended to the background and then blurred.

  Variant:
     1: Draws text in a box that is as wide as the BMP and can be aligned to top or bottom
     2: Draws text in a box that is as wide as text and is placed into the image at coordinates x,y

  Parameters:
     Opacity a value from 0-255. 0 => Shadow is completelly transparent
     To set the Font color/size, the caller should do: aCanvas.Font.Size:= x

  Issues:
     The blurring function cuts too suddenly. The rectangle that was blurred is too visible. Do a blur that slowly fades at the edges.
     Might be slow becuase of the alpha blending and because of the blur.

 Important!
     The input img must be pf24bit.
     When the AlphaFormat member is AC_SRC_ALPHA, the source bitmap must be 32 bpp. If it is not, the AlphaBlend function will fail.
-------------------------------------------------------------------------------------------------------------}
procedure DrawTextShadowBox(BMP: TBitmap; CONST Text: string; AlignTop: Boolean; ShadowColor: TColor= clTextShadow; ShadowOpacity: Byte= 20; Blur: Byte= 2);
VAR
   Shadow: Vcl.Graphics.TBitmap;
   BlendFunc: BLENDFUNCTION;
   x, y: Integer;
   BmpRect: TRect; { Rectangle in the original bitmap where we want to draw the shadowed text }
   ShadowRect: TRect;
   TextWidth, TextHeight: Integer;
   OriginalColor: TColor;
begin
  Assert(BMP.PixelFormat= pf24bit, 'Wrong pixel format!!');
  OriginalColor:= bmp.Canvas.Font.Color;
  TextWidth := BMP.Canvas.TextWidth (Text);
  TextHeight:= BMP.Canvas.TextHeight(Text);

  { Write the shadow on a separate bitmap (overlay) }
  Shadow := TBitmap.Create;
  TRY
    { Bitmap setup }
    Shadow.Canvas.Font.Assign(BMP.Canvas.Font);
    Shadow.PixelFormat:= pf24bit;
    Shadow.SetSize(BMP.Width, TextHeight);

    { Bitmap rectangle as big as ShadowBMP }
    ShadowRect.Left:= 0;
    ShadowRect.Top := 0;
    ShadowRect.Right := Shadow.Width;
    ShadowRect.Bottom:= Shadow.Height;

    { Fill shadow rectangle }
    Shadow.Canvas.Brush.Color := clBlack;                                        { In AlphaBlend, Black is always 100% transparent. So, paint Shadow completely Black. }
    Shadow.Canvas.FillRect(ShadowRect);

    BmpRect.Left  := 0;
    BmpRect.Right := Shadow.Width;
    if AlignTop
    then BmpRect.Top := 0
    else BmpRect.Top := BMP.Height- TextHeight;
    BmpRect.Bottom:= BmpRect.Top+ TextHeight;

    { Blend rectangle with orig image }                                          { Use the AlphaBlend function to overlay the bitmap holding the text on top of the bitmap holding the original image. }
    BlendFunc.BlendOp    := AC_SRC_OVER;
    BlendFunc.BlendFlags := 0;
    BlendFunc.SourceConstantAlpha := ShadowOpacity;
    BlendFunc.AlphaFormat         := 0;                                          //AC_SRC_ALPHA; //  if I put this back, the shadow will be completly invisible when merged with a white source image
    WinApi.Windows.AlphaBlend(BMP.Canvas.Handle, BmpRect.Left, BmpRect.Top, BmpRect.Right, TextHeight, Shadow.Canvas.Handle, 0, 0, Shadow.Width, Shadow.Height, BlendFunc);

    { Copy the blended area back to the Shadow bmp }
    Shadow.Canvas.CopyRect(ShadowRect, BMP.Canvas, BmpRect);

    { Diagonal shadow }
    x:= (BMP.Width  - TextWidth) DIV 2;   // Find center
    Shadow.Canvas.Brush.Style:= bsClear;
    Shadow.Canvas.Font.Color := ShadowColor;
    Shadow.Canvas.TextOut(x, 0, Text);

    { Blur the shadow }
    janFX.GaussianBlur(Shadow, Blur, 1);

    { Paste it back }
    BMP.Canvas.CopyRect(BmpRect, Shadow.Canvas, ShadowRect);
  FINALLY
    FreeAndNil(Shadow);
  END;

  { Draw actual text at 100% opacity }
  if AlignTop
  then y := 0
  else y := BMP.Height- TextHeight;
  BMP.Canvas.Brush.Style:= bsClear;
  BMP.Canvas.Font.Color := OriginalColor;
  BMP.Canvas.TextOut(x, y, Text);
end;



procedure DrawTextShadowBox(aCanvas: TCanvas; CONST Text: string;  X, Y: Integer; ShadowColor: TColor= clTextShadow; ShadowOpacity: Byte= 20; Blur: Byte= 2);
VAR
   Shadow: Vcl.Graphics.TBitmap;
   BlendFunc: BLENDFUNCTION;
   H, W: Integer;
   OriginalColor: TColor;
   R, R2: TRect;
CONST Edge= 5;
begin
 OriginalColor:= aCanvas.Font.Color;

 { Write the shadow on a separate bitmap (overlay) }
 Shadow := TBitmap.Create;
 TRY
   { Assign font }
   Shadow.Canvas.Font.Assign(aCanvas.Font);
   Shadow.PixelFormat:= pf24bit;

   { Compute overlay size }
   W:= Shadow.Canvas.TextWidth (Text);
   H:= Shadow.Canvas.TextHeight(Text);
   Shadow.SetSize(W, H);

   { Fill shadow rectangle }
   R:= Rect(0, 0, Shadow.Width, Shadow.Height);
   Shadow.Canvas.Brush.Color := clBlack;                               { In AlphaBlend, Black is always 100% transparent. So, paint Shadow completely Black. }
   Shadow.Canvas.FillRect(R);

   { Blend rectangle with orig image }                                 { Use the AlphaBlend function to overlay the bitmap holding the text on top of the bitmap holding the original image. }
   BlendFunc.BlendOp := AC_SRC_OVER;
   BlendFunc.BlendFlags := 0;
   BlendFunc.SourceConstantAlpha := ShadowOpacity;
   BlendFunc.AlphaFormat := 0;                                         //AC_SRC_ALPHA; //  if I put this back, the shadow will be completly invisible when merged with a white source image
   WinApi.Windows.AlphaBlend(aCanvas.Handle, x, y, Shadow.Width, Shadow.Height, Shadow.Canvas.Handle, 0, 0, Shadow.Width, Shadow.Height, BlendFunc);

   { Copy the blended area back to the Shadow bmp }
   R2:= rect(x, y, x+Shadow.Width, y+Shadow.Height);
   Shadow.Canvas.CopyRect(R, aCanvas, R2);

   { Diagonal shadow }
   Shadow.Canvas.Brush.Style:= bsClear;
   Shadow.Canvas.Font.Color := ShadowColor;
   Shadow.Canvas.TextOut(0, 0, Text);

   { Blur the shadow }
   janFX.GaussianBlur(Shadow, blur, 1);

   { Paste it back }
   aCanvas.CopyRect(R2, Shadow.Canvas, R);
 FINALLY
   FreeAndNil(Shadow);
 END;

 { Draw actual text at 100% opacity }
 aCanvas.Brush.Style:= bsClear;
 aCanvas.Font.Color := OriginalColor;
 aCanvas.TextOut(x, y, Text);
end;


procedure TfrmTest.UseIt;
VAR BackgroundImage: tbitmap;
begin
 BackgroundImage := Graphics.TBitmap.Create;   
 try
   BackgroundImage.LoadFromFile('c:\test.bmp');
   DrawShadowText (BackgroundImage.Canvas, 'This is some demo text', 20, 40, 140, clRed, clSilver);
   Image1.Picture.Bitmap.Assign(BackgroundImage);
 FINALLY
   BackgroundImage.Free;
 end;
end;

Upvotes: 2

X-Ray
X-Ray

Reputation: 2846

i haven't tested it but it'll give you some idea where to go. the key is the brush style.

something like this:

img.Canvas.Brush.Style:=bsClear;
img.Canvas.Font.Color:=clBlack;
img.Canvas.TextOut(0, 0, 'hi there');

Upvotes: 2

Dave Elsberry
Dave Elsberry

Reputation: 141

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;

Upvotes: 8

skamradt
skamradt

Reputation: 15538

You could use the bitblt routines to merge an image to a common canvas, then save the image again.

Upvotes: 1

Anya Shenanigans
Anya Shenanigans

Reputation: 94614

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.

Upvotes: 4

Jim McKeeth
Jim McKeeth

Reputation: 38703

The shadow is easy:

// Bold shows up better when over an image
image1.Canvas.Font.Style := [fsBold]; 
// Write the shadow first
image1.Canvas.Brush.Style:=bsClear;
image1.Canvas.Font.Color := clGrayText;
image1.Canvas.TextOut(1, 1, 'hi there');
// Then put the text on top (slightly offset)
image1.Canvas.Brush.Style:=bsClear;
image1.Canvas.Font.Color :=clBlack;
image1.Canvas.TextOut(0, 0, 'hi there');

This is text with a transparent background. Or did you want the text itself to be simi-transparent? That is a little trickier. You would need to draw it manually. An easy way to do it instead would be to sample the average of the color of the area you are writing on the image. Then set your font color to be a little lighter and your shadow to be a little darker. Then it kind of blends in.

Upvotes: 3

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