Delphi fast large bitmap creation (without clearing)

Question

When using the TBitmap wrapper for a GDI bitmap from the unit Graphics I noticed it will always clear out the bitmap (using a PatBlt call) when setting up a bitmap with SetSize( w, h ). When I copy in the bits later on (see routine below) it seems ScanLine is the fastest possibility and not SetDIBits.

function ToBitmap: TBitmap;
var
    i, N, x: Integer;
    S, D:    PAnsiChar;
begin
  Result := TBitmap.Create();
  Result.PixelFormat := pf32bit;
  Result.SetSize( width, height );
  S := Src;
  D := Result.ScanLine[ 0 ];
  x := Integer( Result.ScanLine[ 1 ] ) - Integer( D );
  N := width * sizeof( longword );
  for i := 0 to height - 1 do begin
    Move( S^, D^, N );
    Inc( S, N );
    Inc( D, x );
  end;
end;

The bitmaps I need to work with are quite large (150MB of RGB memory). With these iomages it takes 150ms to simply create an empty bitmap and a further 140ms to overwrite it's contents.

Is there a way of initializing a TBitmap with the correct size WITHOUT initializing the pixels itself and leaving the memory of the pixels uninitialized (eg dirty)? Or is there another way to do such a thing. I know we could work on the pixels in place but this still leaves the 150ms of unnessesary initializtion of the pixels.

Answer 1

There's not many things you can do here - working with huge bitmaps is slow... but you can try following:

1. Set PixelFormat after calling SetSize() - this won't avoid initialization of pixels but might make it faster.

2. The fastest way I can think of is to use Win32 API functions (this or this) to create a DIB, and the assign HBITMAP handle of that DIB to a Handle of your TBitmap object.

3. Use memory-mapped files (once again requires calling API or alternatively there are some third-party libraries that can do that for you).

Answer 2

This link has some benchmarks that may interest you:

http://ksymeon.blogspot.com/2010/02/getdibits-vs-scanline-vs-pixels-in.html