Thanks Karim for explaining floating point in an easy and useful manner. Can I ask, when you export an image from DT for further editing in programs like GIMP would you suggest 32 bit FP or is there little benefit?
On Thu, 3 Sep 2020 at 07:04, Top Rock Photography < [email protected]> wrote: > Not really a bug, per se, (and not in Dt) > > The application which is being used to do tho merge is using integer > arithmetic. (Dt uses floating point). When an int goes beyond its range, it > gets clipped to the maximum. Any correction then gets evenly reduced. So > for an integer between 0 and 255, a value of 255 gets increased by 30%, it > remains at 255. If reduced by nineteen percent, it becomes 206. The clipped > areas in your “bright” image got reduced to an even grey. > > With fp values, nothing is really “out of range” (technically possible, > but nevermind that), meaning that if the value is to fall somewhere between > 0 and 1, but it gets computed to 1.3 (or even 17.9), the variable can still > hold the number. If these numbers are then later reduced accordingly, and, > if reduced by nineteen percent, is still 1.053, or still at saturation > (>=1). > > So, with the two images, the application does some maths on a certain > value near zero, and a certain value at 255, and it results in another > value, (somewhere in the vicinity of > 127). But with the tree images, at > some point, the high value got clipped, and then reduced, resulting in the > even grey circles. One can say, “but I used a 16-bit format.” Sure, but if > it was a 16-bit integer format, one still has the same problem, except now, > the values are 0-65535, and 65535 times 30% is still 65535, reduced by 19 % > is still 53083. > > With a 16-bit fp, or a 32-bit fp, the answer is still 1.053; the only > difference is to what decimal place accuracy. > > Perhaps, if the images were sent to the algorithm in a different order, > it would not have resulted in the clipping. Pipeline order is important, > especially with integer mathematics. > > Talk to the developers of the program doing the merge, to consider using a > fp pixel pipeline in their next major upgrade. (That is no easy feat. It > may involve a total rewrite, and a few years. Just ask the developers of > The GIMP). > > Sincerely, > > Karim Hosein > Top Rock Photography > 754.999.1652 > > > > On Tue, 1 Sep 2020 at 15:49, Michael <[email protected]> wrote: > >> I merged three files of a room and got a round discoloration. I then >> merged The lighter and darkest image and the pic was normal. If it is >> a bug please fix it. If it isn't a bug nevermind:) >> -- >> :-)~MIKE~(-: >> images >> https://drive.google.com/drive/u/2/folders/0B2xvsVTZy4y1ODR6dTk5UldpR3c > > > > ____________________________________________________________________________ > darktable user mailing list to unsubscribe send a mail to > [email protected] > -- Dr Terry Pinfold Cytometry & Histology Lab Manager Lecturer in Flow Cytometry University of Tasmania 17 Liverpool St, Hobart, 7000 Ph 6226 4846 or 0408 699053 ____________________________________________________________________________ darktable user mailing list to unsubscribe send a mail to [email protected]
