[GR32] What's wrong with my Gamma()?

[WojTec]

This is my version of gamma:

Delphi-Quellcode:

procedure Gamma(ASource: TBitmap32; AFactor: Single);
var
  F: Single;
  I: Integer;
  LUT: TLUT8;
  B: TBitmap32;
begin
  AFactor := EnsureRange(AFactor, 0.1, 5);

  B := TBitmap32.Create;
  try
    B.Width := ASource.Width;
    B.Height := ASource.Height;

    for I := 0 to 255 do
    begin
      F := I / 255;
      F := Power(F, AFactor);

      LUT[I] := EnsureRange(Round(F * 255), 0, 255);
    end;

    ApplyLUT(B, ASource, LUT, True);

    ASource.Assign(B);
    ASource.Changed;
  finally
    B.free;
  end;
end;

Generally for values <0.1; 1.0) makes image lighter, for (1.0; 5.0> darker... What I did wrong?

[jfheins]

Zitat von WojTec:

Generally for values <0.1; 1.0) makes image lighter, for (1.0; 5.0> darker... What I did wrong?

In my eyes: nothing. What do you expect?

Maybe the function is correct, but your expectations are not

[WojTec]

If you increase gamma, output should be lighter, aren't they? Please look on image.

[jfheins]

Try to use the inverse of dthe factor:F := Power(F, 1/AFactor);

[WojTec]

Of course! I'm stupilo...

Ok, I have another question. To TBitmap32 I'm loading also PNGs. Why after function image lost alpha channel (last param in ApplyLUT() is True)?

-- Added --

I'm really stupid. In above procedure in B bitmap A channel is $FF for any pixel. To correctly apply LUT to this bitmap simply have to set alpha, for example by copying original bitmap. Sorry guys, next questions will be brainy.