On Nov 23, 2007 6:03 AM, GSR - FR <famrom@xxxxxxxxxxxxxxxxxxxx> wrote: > Hi, > 00ai99@xxxxxxxxx (2007-11-22 at 1544.46 +1030): > > Hi Liam, > > > > On Nov 22, 2007 9:45 AM, Liam R E Quin <liam@xxxxxxxxxxx> wrote: > > > > > > Evidence that auto levels loses details -- > > > take a photograph (or a scanned engraving, or whatever) and open > > > Levels, and press autl. Note that the little triangles marking > > > the end-points are not under the ends of the black part - in > > > > The 'Value' controls are not effected by 'Auto'. > > Look at the R,G,B,(A) controls instead. > > And then you see it changes them, in a destructive way. > > > All 'Auto' does, is: > > 1. Set the input range to min(CHANNEL), max(CHANNEL) -- ie the > > lowest used value of that channel in the picture, and the highest used > > value of that channel in the picture. > > No, it does not. It sets min and max "inside" the histrogram used > range (play with Lin/Log setting if this is not clear). There is a way > anybody can test it: > > 1 Create new image, 512*512 > 2 Render plasma, with seed 0 and turbulence 1 > 3 Run Levels, click the auto button, look at all the channels.... OOPS! > > You get: > R 9 1.0 229 | 0 255 > G 12 1.0 194 | 0 255 > B 25 1.0 241 | 0 255 > > Those settings are destructive, Red 0-9 becomes 0, Red 229-255 becomes > 255, and so on. You can look for such pixels before applying the > operation, mark with guides or points, then look what they are after. > > You can also look at the code and see how it iterates over the > histrogram until it decides it has "eaten" enough. It does not stop > when it reaches the first non zero, which would be non destructive (as > in any input channel value gets mapped to a new, different, output > value, rounding and precission issues aside). Okay, well this is not what I expected and I won't be using Levels Auto in the future - I hate clipping. I had thought that Auto was supposed to perform the same function as Colors->Auto->Stretch contrast -- to stretch the current R,G,B range ends to 0,255, clearly it doesn't. It would be nice to have an Auto button in Curves that did achieve this result. > > > 2. Set the output range to 0, 255 > > for each CHANNEL in R,G,B (and possibly A) > > > > Because the output range is full, there is literally no way that this > > operation can reduce detail. > > If the input range is not full, there is loss, as shadows and > highlights are lost and become flattened, grouped in the same result. > > GSR > > _______________________________________________ > Gimp-developer mailing list > Gimp-developer@xxxxxxxxxxxxxxxxxxxxxx > https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer > _______________________________________________ Gimp-developer mailing list Gimp-developer@xxxxxxxxxxxxxxxxxxxxxx https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
