Adjusting color levels

[carlangas]

How could I adjust the color levels of an image? I mean, like using Tools->Color Tools->Levels in gimp. I've tried with contrast, gamma and brightness, but it's not exactly the same. Colors get a little strange this way.

Maybe it's the same question as in this topic: http://galfar.vevb.net/imaging/smf/index.php/topic,81.0.html

[Galfar]

I could add new point transform to do the same thing as Levels.
Can you access SVN repository so you could check it out when it's done?

[carlangas]

Wonderful! Just tell me and I'll try!

[Galfar]

Check out the new canvas method AdjustColorLevels. You can set black, white, and mid point as in common image editors.

[carlangas]

I must be doing something wrong. When I run this procedure, I get an Invalid floating point operation exception. Using ModifyContrastBrightness works ok.

Code: [Select]

var
  FBitmap:TImagingBitmap;
  FImage:TMultiImage;
  FImageCanvas: TImagingCanvas;

begin
FImage:=TMultiImage.Create;
FBitmap:=TImagingBitmap.Create;
Image.Picture.Graphic:=FBitmap;
FImageCanvas:=TImagingCanvas.Create;
FImage.LoadFromFile('example.JPG');
Image.Picture.Graphic.Assign(FImage);
FImageCanvas.CreateForImage(FImage);
FImageCanvas.AdjustColorLevels(27.0,252.0,1.0);
Image.Picture.Graphic.Assign(FImage);
end;                       

[Galfar]

Black and white point values must be in the range 0.0 to 1.0, not 0 to 255.
So just divide your points by 255 and you should get correct results.
All color values in the canvas class that are of Single type use [0,1] range.

[carlangas]

Well, MidPoint, at least in The Gimp can be up to 10.0.

I get the same exception, but I have made some changes and now it runs great. The first one tries to do less operations, but I don't really know if I get that (every time I run the function it takes a different time).

Code: [Select]

procedure TImagingCanvas.AdjustColorLevels(BlackPoint, WhitePoint, MidPoint: Single);
begin
  PointTransform(TransformLevels, BlackPoint, WhitePoint-BlackPoint, 1.0/MidPoint);
end;


Code: [Select]

function TransformLevels(const Pixel: TColorFPRec; BlackPoint, Divisor, Exponente: Single): TColorFPRec;

begin
  Result.A := Pixel.A;
if Pixel.R>BlackPoint then
  Result.R := Power((Pixel.R - BlackPoint) / Divisor, Exponente)
else
  Result.R:=0.0;
if Pixel.G>BlackPoint then
  Result.G := Power((Pixel.G - BlackPoint) / Divisor, Exponente)
else
  Result.G:=0.0;
if Pixel.B>BlackPoint then
  Result.B := Power((Pixel.B - BlackPoint) / Divisor, Exponente)
else
  Result.B:=0.0;
end;

[Galfar]

MidPoint could be > 1.0, it's gamma and not color value.
Could you upload/attach the image that caused AVs here?

[carlangas]

It happened with any image. For example, the last image I was using was lazarus.jpg. You can find it in your lazarus installation, inside of components/jpeg/examples/.

The exception happened when Result was less than 0, so I just control its value, and in that case, assign 0 to this.

Didn't it happen to you?

Do you think calculating these parameters before calling the function could make it faster? As I said before, I don't get anything consistent.

[Galfar]

Oh, I did all the tests in Deplhi and it worked there.
FPC compiled exe indeeds throws "Invalid FP operation" on many images.
Your fix helps, thanks.

[carlangas]

I tried it in lazarus under linux (x86 and amd64) getting the errors I wrote before.