Create and print a screen shot of a TForm

Problem/Question/Abstract:

How to create and print a screen shot of a TForm

Answer:

The following details a better way to print the contents of a form, by getting the device independent bits in 256 colors from the form, and using those bits to print the form to the printer.

In addition, a check is made to see if the screen or printer is a palette device, and if so, palette handling for the device is enabled. If the screen device is a palette device, an additional step is taken to fill the bitmap's palette from the system palette, overcoming some buggy video drivers who don't fill the palette in.

Note: Since this code does a screen shot of the form, the form must be the topmost window and the whole from must be viewable when the form shot is made.

unit Prntit;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Image1: TImage;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

uses
  Printers;

procedure TForm1.Button1Click(Sender: TObject);
var
  dc: HDC;
  isDcPalDevice: BOOL;
  MemDc: HDC;
  MemBitmap: hBitmap;
  OldMemBitmap: hBitmap;
  hDibHeader: THandle;
  pDibHeader: pointer;
  hBits: THandle;
  pBits: pointer;
  ScaleX: Double;
  ScaleY: Double;
  ppal: PLOGPALETTE;
  pal: hPalette;
  Oldpal: hPalette;
  i: integer;
begin
  {Get the screen dc}
  dc := GetDc(0);
  {Create a compatible dc}
  MemDc := CreateCompatibleDc(dc);
  {create a bitmap}
  MemBitmap := CreateCompatibleBitmap(Dc, form1.width, form1.height);
  {select the bitmap into the dc}
  OldMemBitmap := SelectObject(MemDc, MemBitmap);
  {Lets prepare to try a fixup for broken video drivers}
  isDcPalDevice := false;
  if GetDeviceCaps(dc, RASTERCAPS) and RC_PALETTE = RC_PALETTE then
  begin
    GetMem(pPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)));
    FillChar(pPal^, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)), #0);
    pPal^.palVersion := $300;
    pPal^.palNumEntries := GetSystemPaletteEntries(dc, 0, 256, pPal^.palPalEntry);
    if pPal^.PalNumEntries <> 0 then
    begin
      pal := CreatePalette(pPal^);
      oldPal := SelectPalette(MemDc, Pal, false);
      isDcPalDevice := true
    end
    else
      FreeMem(pPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)));
  end;
  {copy from the screen to the memdc/ bitmap}
  BitBlt(MemDc, 0, 0, form1.width, form1.height, Dc, form1.left, form1.top, SrcCopy);
  if isDcPalDevice = true then
  begin
    SelectPalette(MemDc, OldPal, false);
    DeleteObject(Pal);
  end;
  {unselect the bitmap}
  SelectObject(MemDc, OldMemBitmap);
  {delete the memory dc}
  DeleteDc(MemDc);
  {Allocate memory for a DIB structure}
  hDibHeader := GlobalAlloc(GHND, sizeof(TBITMAPINFO) + (sizeof(TRGBQUAD) * 256));
  {get a pointer to the alloced memory}
  pDibHeader := GlobalLock(hDibHeader);
  {fill in the dib structure with info on the way we want the DIB}
  FillChar(pDibHeader^, sizeof(TBITMAPINFO) + (sizeof(TRGBQUAD) * 256), #0);
  PBITMAPINFOHEADER(pDibHeader)^.biSize := sizeof(TBITMAPINFOHEADER);
  PBITMAPINFOHEADER(pDibHeader)^.biPlanes := 1;
  PBITMAPINFOHEADER(pDibHeader)^.biBitCount := 8;
  PBITMAPINFOHEADER(pDibHeader)^.biWidth := form1.width;
  PBITMAPINFOHEADER(pDibHeader)^.biHeight := form1.height;
  PBITMAPINFOHEADER(pDibHeader)^.biCompression := BI_RGB;
  {find out how much memory for the bits}
  GetDIBits(dc, MemBitmap, 0, form1.height, nil, TBitmapInfo(pDibHeader^),
    DIB_RGB_COLORS);
  {Alloc memory for the bits}
  hBits := GlobalAlloc(GHND, PBitmapInfoHeader(pDibHeader)^.BiSizeImage);
  {Get a pointer to the bits}
  pBits := GlobalLock(hBits);
  {Call fn again, but this time give us the bits!}
  GetDIBits(dc, MemBitmap, 0, form1.height, pBits, PBitmapInfo(pDibHeader)^,
    DIB_RGB_COLORS);
  {Lets try a fixup for broken video drivers}
  if isDcPalDevice = true then
  begin
    for i := 0 to (pPal^.PalNumEntries - 1) do
    begin
      PBitmapInfo(pDibHeader)^.bmiColors[i].rgbRed := pPal^.palPalEntry[i].peRed;
      PBitmapInfo(pDibHeader)^.bmiColors[i].rgbGreen := pPal^.palPalEntry[i].peGreen;
      PBitmapInfo(pDibHeader)^.bmiColors[i].rgbBlue := pPal^.palPalEntry[i].peBlue;
    end;
    FreeMem(pPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)));
  end;
  {Release the screen dc}
  ReleaseDc(0, dc);
  {Delete the bitmap}
  DeleteObject(MemBitmap);
  {Start print job}
  Printer.BeginDoc;
  {Scale print size}
  if Printer.PageWidth < Printer.PageHeight then
  begin
    ScaleX := Printer.PageWidth;
    ScaleY := Form1.Height * (Printer.PageWidth / Form1.Width);
  end
  else
  begin
    ScaleX := Form1.Width * (Printer.PageHeight / Form1.Height);
    ScaleY := Printer.PageHeight;
  end;
  {Just in case the printer driver is a palette device}
  isDcPalDevice := false;
  if GetDeviceCaps(Printer.Canvas.Handle, RASTERCAPS) and
    RC_PALETTE = RC_PALETTE then
  begin
    {Create palette from dib}
    GetMem(pPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)));
    FillChar(pPal^, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)), #0);
    pPal^.palVersion := $300;
    pPal^.palNumEntries := 256;
    for i := 0 to (pPal^.PalNumEntries - 1) do
    begin
      pPal^.palPalEntry[i].peRed := PBitmapInfo(pDibHeader)^.bmiColors[i].rgbRed;
      pPal^.palPalEntry[i].peGreen := PBitmapInfo(pDibHeader)^.bmiColors[i].rgbGreen;
      pPal^.palPalEntry[i].peBlue := PBitmapInfo(pDibHeader)^.bmiColors[i].rgbBlue;
    end;
    pal := CreatePalette(pPal^);
    FreeMem(pPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)));
    oldPal := SelectPalette(Printer.Canvas.Handle, Pal, false);
    isDcPalDevice := true
  end;
  {send the bits to the printer}
  StretchDiBits(Printer.Canvas.Handle, 0, 0, Round(scaleX), Round(scaleY), 0, 0,
    Form1.Width, Form1.Height, pBits, PBitmapInfo(pDibHeader)^,
    DIB_RGB_COLORS, SRCCOPY);
  {Just in case you printer driver is a palette device}
  if isDcPalDevice = true then
  begin
    SelectPalette(Printer.Canvas.Handle, oldPal, false);
    DeleteObject(Pal);
  end;
  {Clean up allocated memory}
  GlobalUnlock(hBits);
  GlobalFree(hBits);
  GlobalUnlock(hDibHeader);
  GlobalFree(hDibHeader);
  {End the print job}
  Printer.EndDoc;
end;