Hi Richard -- Could you open a bug report at bugzi...@gimp.org about this issue, and attach the example images ?
js -><- On 3 June 2012 17:30, Richard Gitschlag <strata_ran...@hotmail.com> wrote: > (Apologies if this is a duplicate post; I tried sending it the other day but > it didn't seem to go through) > > I've found some obscure conditions under which the Hue-Saturation tool > produces incorrect results up to and including current 2.8.0 . > > The first one I encountered "in the wild" sometime in 2.6: I had recently > finished a pencil drawing and scanned it into an image file, but the "pink" > areas of the drawing were not a warm enough shade for my preference. At the > time I felt the easiest method to fix this was via Hue-Saturation tool, so I > went to the tool, slid the Overlap to 100%, then adjusted the Magenta hue by > about +10º. (I chose the Magenta range instead of Red because the image > also contained colors in the red/orange region, and I didn't want them > affected.) Suddenly my pink pixels were now magenta and blue! As if GIMP > was calculating it around the wrong side of the HSV wheel, but that was > apparently reported, patched and fixed four years ago (bug #527085) so the > explanation can't be that simple. > > I've also found another condition that, while it's so improbable a user > might never encounter it in the wild, it is still incorrect: > > - Adjust the Cyan channel hue by +100º ( -> Magenta/blue) > - Adjust the Blue channel hue by -100º ( -> Cyan/green) > - Set Overlap to 100% > > So if a hue falls between Cyan and Blue ranges, it should get mapped to a > Magenta -> Blue -> Cyan -> Green range, right? > But instead, GIMP maps them to a Magenta -> Red -> Yellow -> Green range; > here it IS going the wrong way around the circle. > > This is because of the way GIMP calculates the hue adjustment: > > mapped_primary_hue = (input_hue + primary_hue_adjustment) > mapped_secondary_hue = (input_hue + secondary_hue_adjustment) > ... > final_hue = (mapped_primary_hue * primary_intensity) + (mapped_secondary_hue > * secondary_intensity) > > A.k.a. it maps both ranges independently then interpolates the result > between them. Meanwhile, GIMP ensures that mapped_primary_hue and > mapped_secondary_hue are kept inside the (0.0 - 1.0) range, and if there is > more than a 180º difference between them, GIMP wraps mapped_secondary_hue > again to yield a "shortest circle" route. This is correct 99% of the time, > but in the above case, it fails because there's a 200º difference between > mapped_primary_hue and mapped_secondary_hue (regardless of the actual input > value or master hue adjustement); GIMP assumes it is going the wrong way > around the HSV circle when it actually isn't (the actual difference between > blue and cyan after these adjustments is 140º, not 200º). > > Another testcase to confirm why it's a bug: > - Cyan hue +90 > - Blue hue -90 > Result: Correct (overlap fades from magenta/blue -> cyan/green). > > - Cyan hue +91 > - Blue hue -91 > Result: Incorrect (overlap fades from blue/magenta -> red -> yellow -> > green/cyan) > > Who knew a humble 2º made such difference? The patch that fixed #527085 > cannot tell whether a difference of > 180º is due to crossing the > red/magenta wraparound or if that was deliberate on the part of the user. > (And it's not the tool's job to question whether the user's adjustments are > sane.) > > I can submit a patch for this that may fix the issue permanently - but let > me know if my algebra is correct first: > > Given that: > mapped_primary_hue = input_hue + (master_hue_adjustment + > primary_hue_adjustment) > mapped_secondary_hue = input_hue + (master_hue_adjustment + > secondary_hue_adjustment) > (primary_intensity + secondary_intensity) = 1 > > And: > final_hue = mapped_primary_hue * primary_intensity + mapped_secondary_hue * > secondary_intensity > > THEN: > final_hue = (input_hue + master_hue_adjusment + primary_hue_adjusment) * > primary_intensity + (input_hue + master_hue_adjusment + > secondary_hue_adjustment) * secondary_intensity > = (input_hue + master_hue_adjustment) * (primary_intensity + > secondary_intensity) + primary_hue_adjustment * primary_intensity + > secondary_hue_adjustment * secondary_intensity > = input_hue + master_hue_adjusment + (primary_hue_adjustment * > primary_intensity + secondary_hue_adjustment + secondary_intensity) > > In other words, when dealing with pixels in an overlap region the tool > should interpolate the hue adjustment from the respective primary and > secondary ranges, THEN map that adjustment to the pixel. And since the > output value is subsequently converted from HSL back to RGB space with > essentially no further processing, there's no need to worry about crossing > the red/magenta wraparound at all. > > -- Stratadrake > strata_ran...@hotmail.com > -------------------- > Numbers may not lie, but neither do they tell the whole truth. > > > _______________________________________________ > gimp-developer-list mailing list > gimp-developer-list@gnome.org > https://mail.gnome.org/mailman/listinfo/gimp-developer-list > _______________________________________________ gimp-developer-list mailing list gimp-developer-list@gnome.org https://mail.gnome.org/mailman/listinfo/gimp-developer-list
