https://bugs.kde.org/show_bug.cgi?id=525130

--- Comment #1 from [email protected] ---
I'm the person from the linked conversation. Providing some details in case the
conversation gets broken by post auto-deletion or other Fediverse shenanigans
before this is discussed more fully.

When doing geometric work (pattern designs, architectural drawings, etc), it is
often useful to scale things by some specific factor, but often this factor is
something like 1/3 (33.3333...%) or 7/8 (87.5%) rather than simple percentage
(in fact, the only whole percentages I've found useful have been those
corresponding to fractions, like 25% and 150%), and when working with
layers/selections larger than a few hundred pixels, being off by a fraction of
1% leads to noticeable misalignments. Using the transform tool's handles can
get more precise scaling, but these can be difficult to control precisely as
well. In particular, the handles require visual landmarks or guides for
precision, but in situations calling for a particular scale factor, often there
simply aren't any.

Concrete example from today: I wanted to resize a scale drawing by 187/188
because I noticed I'd made a mistake some days ago when upscaling from an
earlier draft. That's about half a percent difference, but at ~4k pixels in
width, it was causing serious misalignments. Because I already had some layers
drawn at the correct scale, I couldn't use Image Scale, so selecting just the
mis-scaled layers and transforming them by my desired factor should've been the
answer. Instead, I had to either make a second document to use Scale Image and
then copy the already-correct layers into it (which is what I did, and it broke
my Clone Layers despite copying them together with their parents, a separate
issue), or draw some rectangles (e.g. 1880px and 1870px wide) to use as visual
landmarks for using the transform tool's handles. Both of these options are
much more tedious than entering 187/188 or 99.4681% would've been.

1/3 and 2/3 scales show up weirdly frequently even outside of overtly geometric
work for symmetrical compositions. 33% and 67% don't cut it, the scaled
shapes/panels/whatever often end up with noticeable gaps or width differences.
Sqrt 2 (141.4214%) and 1/sqrt2 (70.7107%) scales show up regularly when working
on isometric and RPG-view gamedev art, and not always in contexts where grids
and guides can readily help.

4 decimal places should be enough for percentages. 3 is 1px precision at 100k
pixels, 4k is 1px precision at 1m pixels - in practice they actually make a
difference at much lower resolutions than that due to anti-aliasing, which adds
256 more levels of precision per pixel (at 8-bit colour depth), which means 3
decimal points makes a potentially perceptible difference at 390 pixels! and 4
at around 3.9k).
5+ places isn't likely to have practical benefits over 4 at the sorts of
resolutions Krita can reliably work with, but being able to enter a precise
value and not have it get rounded *feels* nice even if it doesn't change the
result xP

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