Hi,
This function is just a very simple way to modify Bright, Contrast, Hue, Saturation and white point.
It works on Lab colorspace, so there is still a lot of room for improvement. However, it works fine for most situations.
The mechanics are simple, a Lab -> Lab profile is computed by using following sequence
1) Input Lab is converted to LCh
2) LCh is modified by using: Lout = Lin * Contrast + Brightness Cout = Cin + Saturation hout = hin + hue
3) The resulting LCh is converted back to Lab and that is the output.
So, the range of parameters would be
Contrast : Something between 0 ... 2, 1 for don't change
Brightness: Something between -100 and 100. Those are extreme values, so probably -25 to 25 is more adequate. 0 for don't change
Saturation: Strictly, that is Colorfulness, -25..25 or more. 0 for don't change
Hue: can be up to -180..180 but those are huge values. Small endings would make more sense, something like -15 .. 15. 0 for don't change.
For the whitepoints, both are the temperature in �K of the blackbody locus. 5000=D50, 6504 = D65 and so.
Once you have obtained the abstract profile, you can concatenate into workflow by using Multiprofile transforms. i.e.
sRGB -> Abstract -> sRGB
And in this way you can modify Bright, Contrast, etc of sRGB values. This works on other spaces, like CMYK too.
Hope this helps -- Marti Maria The littlecms project. www.littlecms.com
----- Original Message ----- From: "Dirk Str�ker" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Wednesday, May 11, 2005 9:19 PM
Subject: [Lcms-user] Value range for BCHSWabstractProfile
Hi,
I would like to work with the "cmsCreateBCHSWabstractProfile"-function in
order to use it as an in-between profile to change contrast, hue etc. But I
wonder what the value range for "Bright", "Contrast" and "Saturation" would
be? For "hue" I think it is 0..360 degrees, am I right?
In the API it is noted, that negativ values are possible, what does this mean,
how does it work? Do I lower the brightness, for example, with negative
values for "Bright"?
Greetings Dirk
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