PixelSearch is very inefficient, how can I optimize it?

Question

I am currently taking screenshots of the area in a loop to then search for 4 pixels in it. These pixels have the same color - red or $ 001300FF. The variables used are defined and initialized in the OnCreate event:

//The variables for the area:
ScanL := 500; // Left
ScanR := 800; // Right
ScanT := 180; // Top
ScanB := 400; // Bottom

screenshot: TBitMap;
canvas : TCanvas;

To take screenshots, I use the following function:

procedure TFormMain.GetSCREENSHOT(var a: TBitMap);
var
  Locked: Boolean;
begin
  Locked := Canvas.TryLock;
  try
    screenshot.Canvas.CopyRect(screenshot.Canvas.ClipRect, Canvas, Rect(ScanL, ScanT, ScanR, ScanB)); 
  finally
    if Locked then
      Canvas.Unlock;
  end;
end;

The variable "screenshot: TBitMap", globally defined, is passed to the GetSCREENSHOT function. To find these 4 pixels, I just did what a newbie would do:

   function TFormMain.findImage : Boolean;
    var
      x,y : Integer;
    begin
      Result := false;
      for x := 0 to screenshot.Width-10 do
      begin
        for y := 0 to screenshot.Height-10 do
        begin
          if screenshot.Canvas.Pixels[x,y] = $001300FF then
          begin
            if screenshot.Canvas.Pixels[x,y+1] = $001300FF then
              if screenshot.Canvas.Pixels[x,y+2] = $001300FF then
                if screenshot.Canvas.Pixels[x,y+3] = $001300FF then
                begin
                  FoundPixelX := ScanL + x;
                  FoundPixelY := ScanT + Y;
                  Result := True;
                  Exit;
                end;
          end;
        end;
      end;
    end;

Since it was so bad, I measured how long it took to run the function:

  QueryPerformanceFrequency(freq);
  QueryPerformanceCounter(startTime);

  findImage;

  QueryPerformanceCounter(endTime);
  ShowMessage('the function needs about ' + IntToStr((endTime - startTime) * 1000 div freq) + 'ms');

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Answer 1

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function TFormMain.findImage : Boolean;
var
  ScanLine, NextScanLine: PInteger;
  Stride: integer;
  MaxX: integer;
const
  MinX = 0;
  BytesPerPixel = SizeOf(integer);
  MagicColor = $001300FF; 
begin
  MaxX:= Screenshot.Width - 10;
  Assert(Screenshot.PixelFormat = pf32bit);
  Result := false;
  ScanLine:= Screenshot.ScanLine[0];
  Stride:= (NativeInt(Screenshot.ScanLine[1]) - NativeInt(ScanLine)) div BytesPerPixel; 
  y := 0
  repeat
    NextScanLine:= @ScanLine[Stride]; 
    for x:= MinX to MaxX do begin
      if (ScanLine[0] = MagicColor) then begin
        if (ScanLine[stride] = MagicColor) then begin
          if (ScanLine[stride*2] = MagicColor) then begin
            if (ScanLine[stride*3] = MagicColor) then begin
              FoundPixelX := ScanL + x;
              FoundPixelY := ScanT + Y;
              Exit(True);
            end;
          end;
        end;
      end;
      Inc(ScanLine);
    end; {for x}
    ScanLine:= NextScanLine;
    Inc(y);
  until (y > (Height - 10));
end;

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//Start with ScanLine[3]: the fourth scanline {we start at 0}
NextScanLine:= @ScanLine[Stride*4]; 
for x:= MinX to MaxX do begin
  if (ScanLine[0] = MagicColor) then begin
    Count:=   (integer(ScanLine[-stride*3] = MagicColor) * 1 
             + integer(ScanLine[-stride*2] = MagicColor) * 2
             + integer(ScanLine[-stride*1] = MagicColor) * 4
             + 8; //The line itself
    case Count of  
      1+2+4+8: begin
        FoundPixelX := ScanL + x;
        FoundPixelY := ScanT + Y-3;
        Exit(True);
      end;
      4+8+16: if (ScanLine[stride] = MagicColor) then begin
        FoundPixelX := ScanL + x;
        FoundPixelY := ScanT + Y-2;
        Exit(True);
      end;
      8+16, 1+8+16: if (ScanLine[stride] = MagicColor) and
                       (ScanLine[stride*2] = MagicColor) then begin
        FoundPixelX := ScanL + x;
        FoundPixelY := ScanT + Y-1;
        Exit(True);
      end;
    end; {case}
    if   (ScanLine[stride] = MagicColor) and
         (ScanLine[stride*2] = MagicColor) and
         (ScanLine[stride*3] = MagicColor) then begin
      FoundPixelX := ScanL + x;
      FoundPixelY := ScanT + Y;
      Exit(True);
    end;
  end;
  Inc(ScanLine);
end; {for x}
ScanLine:= NextScanLine;
Inc(y);

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