offscreen bitmaps

I want to construct a line offscreen and then display it over a bitmap with the picture showing through.
I always end up with a white background covering the bitmap.  Why?
eg.,

source:=rect(0,0,600,360);
dest:=rect(0,0,600,360);
offscreen:=tbitmap.create;
try
offscreen.width:=600;
offscreen.height:=360;
with offscreen.canvas do
begin
pen.color:=clred;
brush.color:=clwhite;
brush.style:=bsclear;
moveto(60,60);
lineto(250,300);
end;
paintbox1.canvas.copyrect(dest,offscreen.canvas,source);
finally
offscreen.free;
end;
niftyAsked:
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JimBob091197Commented:
Do you mean that your offscreen bitmap must have a transparent background?  If so, you have to make it draw tranparently onto your other bitmap.  If this is your case, I can give you code to draw transparently.

JB
ZifNabCommented:
Yeah JimBob, I guess that's what nifty wants.
JimBob091197Commented:
If that's the case, then this should be the answer:

const
  TransparentMask = $02000000;

function Min(i1, i2: Integer): Integer;
begin
      if (i1 < i2) then
            Result := i1
      else
            Result := i2;
end;

function Max(i1, i2: Integer): Integer;
begin
      if (i1 > i2) then
            Result := i1
      else
            Result := i2;
end;

function WidthOf(R: TRect): Integer;
begin
      Result := R.Right - R.Left;
end;

function HeightOf(R: TRect): Integer;
begin
      Result := R.Bottom - R.Top;
end;

procedure AdjustRect(var R: TRect; LeftChange, TopChange, RightChange, BottomChange: Integer);
begin
      Inc(R.Left, LeftChange);
      Inc(R.Top, TopChange);
      Inc(R.Right, RightChange);
      Inc(R.Bottom, BottomChange);
end;

procedure DrawTransparentBitmapRect(DC: HDC; Bitmap: HBitmap; xStart, yStart,
  Width, Height: Integer; Rect: TRect; TransparentColor: TColorRef);
var
      BM: Windows.TBitmap;
      cColor: TColorRef;
  bmAndBack, bmAndObject, bmAndMem, bmSave: HBitmap;
  bmBackOld, bmObjectOld, bmMemOld, bmSaveOld: HBitmap;
  hdcMem, hdcBack, hdcObject, hdcTemp, hdcSave: HDC;
  ptSize, ptRealSize, ptBitSize, ptOrigin: TPoint;
begin
  hdcTemp := CreateCompatibleDC(DC);
  SelectObject(hdcTemp, Bitmap);      { Select the bitmap    }
  GetObject(Bitmap, SizeOf(BM), @BM);
  ptRealSize.x := Min(Rect.Right - Rect.Left, BM.bmWidth - Rect.Left);
  ptRealSize.y := Min(Rect.Bottom - Rect.Top, BM.bmHeight - Rect.Top);
  DPtoLP(hdcTemp, ptRealSize, 1);
  ptOrigin.x := Rect.Left;
  ptOrigin.y := Rect.Top;
  DPtoLP(hdcTemp, ptOrigin, 1);       { Convert from device  }
                                      { to logical points    }
  ptBitSize.x := BM.bmWidth;          { Get width of bitmap  }
  ptBitSize.y := BM.bmHeight;         { Get height of bitmap }
  DPtoLP(hdcTemp, ptBitSize, 1);
  if (ptRealSize.x = 0) or (ptRealSize.y = 0) then begin
    ptSize := ptBitSize;
    ptRealSize := ptSize;
  end
  else ptSize := ptRealSize;
  if (Width = 0) or (Height = 0) then begin
    Width := ptSize.x;
    Height := ptSize.y;
  end;

  { Create some DCs to hold temporary data }
  hdcBack   := CreateCompatibleDC(DC);
  hdcObject := CreateCompatibleDC(DC);
  hdcMem    := CreateCompatibleDC(DC);
  hdcSave   := CreateCompatibleDC(DC);
  { Create a bitmap for each DC. DCs are required for a number of }
  { GDI functions                                                 }
  { Monochrome DC }
  bmAndBack   := CreateBitmap(ptSize.x, ptSize.y, 1, 1, nil);
  bmAndObject := CreateBitmap(ptSize.x, ptSize.y, 1, 1, nil);
  bmAndMem    := CreateCompatibleBitmap(DC, Max(ptSize.x, Width), Max(ptSize.y, Height));
  bmSave      := CreateCompatibleBitmap(DC, ptBitSize.x, ptBitSize.y);
  { Each DC must select a bitmap object to store pixel data }
  bmBackOld   := SelectObject(hdcBack, bmAndBack);
  bmObjectOld := SelectObject(hdcObject, bmAndObject);
  bmMemOld    := SelectObject(hdcMem, bmAndMem);
  bmSaveOld   := SelectObject(hdcSave, bmSave);
  { Set proper mapping mode }
  SetMapMode(hdcTemp, GetMapMode(DC));

  { Save the bitmap sent here, because it will be overwritten }
  BitBlt(hdcSave, 0, 0, ptBitSize.x, ptBitSize.y, hdcTemp, 0, 0, SRCCOPY);
  { Set the background color of the source DC to the color,         }
  { contained in the parts of the bitmap that should be transparent }
  cColor := SetBkColor(hdcTemp, TransparentColor);
  { Create the object mask for the bitmap by performing a BitBlt()  }
  { from the source bitmap to a monochrome bitmap                   }
  BitBlt(hdcObject, 0, 0, ptSize.x, ptSize.y, hdcTemp, ptOrigin.x, ptOrigin.y,
    SRCCOPY);
  { Set the background color of the source DC back to the original  }
  { color                                                           }
  SetBkColor(hdcTemp, cColor);
  { Create the inverse of the object mask }
  BitBlt(hdcBack, 0, 0, ptSize.x, ptSize.y, hdcObject, 0, 0,
    NOTSRCCOPY);
  { Copy the background of the main DC to the destination }
  BitBlt(hdcMem, 0, 0, Width, Height, DC, xStart, yStart,
    SRCCOPY);
  { Mask out the places where the bitmap will be placed }
  StretchBlt(hdcMem, 0, 0, Width, Height, hdcObject, 0, 0,
    ptSize.x, ptSize.y, SRCAND);
  {BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdcObject, 0, 0, SRCAND);}
  { Mask out the transparent colored pixels on the bitmap }
  BitBlt(hdcTemp, ptOrigin.x, ptOrigin.y, ptSize.x, ptSize.y, hdcBack, 0, 0,
    SRCAND);
  { XOR the bitmap with the background on the destination DC }
  StretchBlt(hdcMem, 0, 0, Width, Height, hdcTemp, ptOrigin.x, ptOrigin.y,
    ptSize.x, ptSize.y, SRCPAINT);
  {BitBlt(hdcMem, 0, 0, ptSize.x, ptSize.y, hdcTemp, ptOrigin.x, ptOrigin.y,
    SRCPAINT);}
  { Copy the destination to the screen }
  BitBlt(DC, xStart, yStart, Max(ptRealSize.x, Width), Max(ptRealSize.y, Height),
    hdcMem, 0, 0, SRCCOPY);
  { Place the original bitmap back into the bitmap sent here }
  BitBlt(hdcTemp, 0, 0, ptBitSize.x, ptBitSize.y, hdcSave, 0, 0, SRCCOPY);

  { Delete the memory bitmaps }
  DeleteObject(SelectObject(hdcBack, bmBackOld));
  DeleteObject(SelectObject(hdcObject, bmObjectOld));
  DeleteObject(SelectObject(hdcMem, bmMemOld));
  DeleteObject(SelectObject(hdcSave, bmSaveOld));
  { Delete the memory DCs }
  DeleteDC(hdcMem);
  DeleteDC(hdcBack);
  DeleteDC(hdcObject);
  DeleteDC(hdcSave);
  DeleteDC(hdcTemp);
end;

procedure InternalDrawTransBmpRect(Dest: TCanvas; X, Y, W, H: Integer; Rect: TRect; Bitmap: TBitmap; TransparentColor: TColor);
var
  MemImage: TBitmap;
  R: TRect;
begin
  MemImage := TBitmap.Create;
  try
    R := Bounds(0, 0, Bitmap.Width, Bitmap.Height);
    if TransparentColor = clNone then begin
                  if (WidthOf(Rect) <> 0) and (HeightOf(Rect) <> 0) then R := Rect;
      MemImage.Width := WidthOf(R);
                  MemImage.Height := HeightOf(R);
      MemImage.Canvas.CopyRect(Bounds(0, 0, MemImage.Width, MemImage.Height),
        Bitmap.Canvas, R);
      if (W = 0) or (H = 0) then Dest.Draw(X, Y, MemImage)
      else Dest.StretchDraw(Bounds(X, Y, W, H), MemImage);
    end
    else  begin
      MemImage.Width := WidthOf(R);
                  MemImage.Height := HeightOf(R);
      MemImage.Canvas.CopyRect(R, Bitmap.Canvas, R);
      DrawTransparentBitmapRect(Dest.Handle, MemImage.Handle, X, Y, W, H,
        Rect, ColorToRGB(TransparentColor and not TransparentMask));
    end;
  finally
    MemImage.Free;
  end;
end;

procedure DrawBitmapTransparent(Dest: TCanvas; XOrigin, YOrigin: Integer; Bitmap: TBitmap; TransparentColor: TColor);
begin
      InternalDrawTransBmpRect(Dest, XOrigin, YOrigin, 0, 0, Rect(0, 0, 0, 0), Bitmap, TransparentColor);
end;



To use this code, suppose you have a TImage on your form called imgTest, with an existing picture inside it:

procedure DoStuff;
var
  Bmp: TBitmap;
begin
  Bmp := TBitmap.Create;
  Bmp.Width := imgTest.Width;
  Bmp.Height := imgTest.Height;
  with Bmp.Canvas do
    begin
      Brush.Color := clYellow;  // This is the transparent color.
      FillRect(Rect(0, 0, imgTest.Width, imgTest.Height));
      Pen.Color := clRed;
      MoveTo(0, 0);
      LineTo(imgTest.Width, imgTest.Height);
    end;
  // Use the same transparent color as above, in this case yellow.
  DrawBitmapTransparent(imgTest.Picture.Bitmap.Canvas, 0, 0, Bmp, clYellow);
  imgTest.Invalidate;
  Bmp.Free;
end;

Regards,
JB

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niftyAuthor Commented:
Thanks JB. As you can see I am fairly new at programming (but having a ball all the same)so expect to see postings from me often.I have bought several books and I understand them perfectly
after the 'problem' has been solved, isn't it always the same!
Once again thanks to all at expert exchange.....Nifty.
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