Adjust display gamma value

I am looking into modifying brightness/contrast/gamma of my display i found an api whose purpose i think is this but i didn't had much success implementing it... here is the code

var
i,j:Integer;
buf:array [0..2,0..255] of Word;
wBright:Word;
myDC:HDC;
begin
    myDC:=GetDc(GetDesktopWindow);
    GetDeviceGammaRamp(mydc,buf);

   for i:=0 to 2 do
      for j:=0 to 255 do
        begin
            buf[i][j]:=buf[i][j] + 100; //if i don't modify the values the api works
        end;
    SetDeviceGammaRamp(mydc,buf);
end;

I will be grateful if you point me into right direction. Thanks.

The last error says : The parameter is i开发者_如何学编程ncorrect

---

The values in the array must really be a ramp, i.e. they map the possible R, G and B values to a brightness value. This way you can create funny effects too, but not with the routine below. Found something like this on the web:

uses Windows;

//    SetDisplayBrightness
//
//    Changes the brightness of the entire screen.
//    This function may not work properly in some video cards.
//
//    The Brightness parameter has the following meaning:
//
//      128       = normal brightness
//      above 128 = brighter
//      below 128 = darker

function SetDisplayBrightness(Brightness: Byte): Boolean;
var
  GammaDC: HDC;
  GammaArray: array[0..2, 0..255] of Word;
  I, Value: Integer;
begin
  Result := False;
  GammaDC := GetDC(0);

  if GammaDC <> 0 then
  begin
    for I := 0 to 255 do
    begin
      Value := I * (Brightness + 128);
      if Value > 65535 then
        Value := 65535;
      GammaArray[0, I] := Value; // R value of I is mapped to brightness of Value
      GammaArray[1, I] := Value; // G value of I is mapped to brightness of Value
      GammaArray[2, I] := Value; // B value of I is mapped to brightness of Value
    end;

    // Note: BOOL will be converted to Boolean here.
    Result := SetDeviceGammaRamp(GammaDC, GammaArray); 

    ReleaseDC(0, GammaDC);
  end;  
end;

Unfortunately, in my Win7 VM in Parallels on a Mac, I can't test his, but it should work on most normal Windows PCs.

Edit

FWIW, I ran it in my Win7 VM and the routine returns True. If I use other values, e.g.

Value := 127 * I;

the routine returns False and

ShowMessage(SysErrorMessage(GetLastError));

displays

The parameter is incorrect

Changing this to:

Value := 128 * I;

returns True again. I assume the values must form some kind of slope (or ramp). This routine creates a linear ramp. I guess you can also use other kinds, e.g. a sigmoid, to achieve other effects, like higher contrast.

I can't, of course, see any differences in brightness in the VM, sorry.

Update: But it seems to work for David Heffernan and I could just test it on my sister in law's laptop, and there it works too.