void __fastcall DrawClientWindow (HDC &Hdc)
{
  // this routine paints a gradient background that slowly
  // changes from StartColor at the top to black at the bottom
  const int    NumGradients = 64;   // number of shading levels
  const TColor StartColor = clBlue; // initial color

  TRect rect ;                      // calculate size of backgnd
  ::GetClientRect (Form1->Handle, (RECT *) &rect) ;

  // painting a shaded gradient is slow, and can cause flickering
  // eliminate flicker by using memory bitmaps and BitBlit
  Graphics::TBitmap *MemBitmap = new Graphics::TBitmap;
  MemBitmap->Width = rect.Right - rect.Left;
  MemBitmap->Height= rect.Bottom- rect.Top;
  MemBitmap->Canvas->Brush->Style = bsSolid;
  MemBitmap->Canvas->Brush->Color = clBlack;
  MemBitmap->Canvas->FillRect(Rect(0,0,
                                   MemBitmap->Width,
                                   MemBitmap->Height));
  // calculate the height of each little gradient section
  int GradientHeight = MemBitmap->Height / NumGradients;

  // calculate individual RGB intensities of the starting color.
  int InitialRed   = GetRValue(StartColor);
  int InitialGreen = GetGValue(StartColor);
  int InitialBlue  = GetBValue(StartColor);
  int RedIncrement  = (InitialRed + 1)  / NumGradients; // calculate
  int GreenIncrement= (InitialGreen + 1)/ NumGradients; // increment of
  int BlueIncrement = (InitialBlue + 1) / NumGradients; // each gradient

  TColor FillColor;
  // the for loop draws each gradient section. the loop stops one pass
  // away from black because black was drawn by FillRect above
  for (int j=0;j< NumGradients;j++)
  {
    rect.Bottom = rect.Top + GradientHeight;
    FillColor = (TColor) RGB( InitialRed   -(RedIncrement * j),
                              InitialGreen -(GreenIncrement* j),
                              InitialBlue  -(BlueIncrement * j));
    MemBitmap->Canvas->Brush->Color = FillColor;
    MemBitmap->Canvas->FillRect(rect);
    rect.Top += GradientHeight;
  }
  // Use API BitBlt to copy pixels to the screen.
  ::BitBlt(Hdc,0,0,MemBitmap->Width, MemBitmap->Height,
           MemBitmap->Canvas->Handle,0,0,SRCCOPY);
  delete MemBitmap; // delete the temporary bitmap.
}
void __fastcall TForm1::Button1Click(TObject *Sender)
{
  DrawClientWindow(Canvas->Handle);
}
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