Skeuomorphic POV mode (or physical scale printouts?)

[1j01]

It can be fun to play based on scales on the screen without looking at the guitar you're holding, but when you need to (especially when the frets get closer together), looking between your guitar and the app with its representation and holding in mind the relation could be easier,
especially since the strings at the top on the screen are lower physically (assuming right handed guitar/tuning/player) unless you're holding it weird.
It could be easier with: literal "skew-o-morphism"!
(skewing it into perspective so it's more like what you're seeing)
of course augmented reality would be better...
or maybe... get this... printing out scales to slip under the strings of the neck of the guitar
they would need to avoid the frets, and they might affect the sound if it's not flush enough, but it's doable; just need some slits for the frets (which could be a pain to cut out), and you'd need to specify measurements
and most printer paper isn't long enough for a full size guitar so it should let you split it up (it should still only need one A4(?) piece of paper, just two pieces)
yeah, guitar scale sleeves, I want to try that

[1j01]

Prior Art

So apparently my initial google search wasn't good enough (I was searching for "inlays" rather than "decals", and maybe should have put "learning" in there); there are actually several approaches.

Here's a breakdown:

NeckNotes Guitar Trainer

Pros: Pitch classes are visually equivalized, that is, an A looks like any other A, so you can easily see octave jumps. Nice clean look.
Cons: I can't directly see the scale I want.* Costs money probably.**

AxeTape (that's a brand name)

Pros: Probably quite simple to apply. Designed specifically for holding the guitar down, which might help learning to play by feel.
Cons: Cluttered with patent information and an ugly broken-up URL. Not great information design IMO. And the connection between the notes (physical frettings) and the representation (the diagram decal) is not direct, which is the main thing I'm looking for. (It lines up in the fret axis, the other axis is abstracted.) I can't directly see the scale I want.* Costs money probably.**

DapNote Fretmap

Pros: If metro maps are your aesthetic? ...Maybe this shows you a scale tho? Okay yeah, this blues version seems reasonable.
Cons: Cluttered information design. Shows open notes differently from the held notes. Costs money probably.**

Fretnotes

Pros: Handles the case of open frets nicely and simply. Tilted note names to match your perspective.
Cons: Leaning into the whole accidental/natural thing, just accepting the ugliness of "accidentals" that have two possible names and everything. I can't directly see the scale I want.* Costs money probably.**

LeftyFretz (that's a site name)

Pros: Free! They originally sold some sticker decals, but later opened it up as a printout you can download. Thought given to the placement of the decals: place them closer back to avoid wear and tear, and to improve visibility since you should be fretting closer to the fret (although this does put it further from the fret, which may be confusing. But ultimately it's your choice!)
Cons: Leaning into the whole accidental/natural thing, rather than doing something more interesting with color. I can't directly see the scale I want.* (- unless I edit the image!)

Fret Zealot

Pros: I can directly see the scale I want, and I can change it without reinstalling the thing!
Potentially the be-all end-all of guitar fretting learning products, because it could be programmable.
Cons: Presumably the most expensive. Requires power, but that's no biggie. Requires an app?
Notes: This thing had better be programmable. It could also totally function as an extremely low-DPI display for text, which could let you select scale types by name (i.e. Locrian, Dorian, Double Harmonic etc.)... I'm almost certainly gonna want to do more with it than the app allows.

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*When I say "I can't immediately see the scale I want" I mean I can't choose a scale to learn and easily see what notes fall in that scale. However, you might be able to see scales as sets of colors, that might be a reasonable way to learn scales.

**Even if they're cheap, free is better, especially if you don't have a credit card.

(Imgur album with the above images: https://imgur.com/a/tSiHm)

[1j01]

Possible App Features

I might make it rather customizable, but it should still have good defaults!

Useful generally:

• Arbitrary scales, in arbitrary keys
• Arbitrary tuning
• Left handed / right handed modes (Do lefties string their guitars the opposite way? Normally the lowest string is physically higher up and the highest is physically lower)
• Note circles either colorful (either one for every pitch class, or with accidentals having half-colors as in Necknotes Guitar Trainer), or two colors (accidental/natural), or single color
• Note names with sharp or flat (or both? which I find ugly and cluttered; it's easy to do the mental translation)
• Note names either rotated so they match your perspective, or straight up and down
• Note names either on, subtle, or off (I would go with subtle, because mainly I just care about "is this note in the scale I'm trying to play in?", but having a chart of note names directly on the guitar could be useful at times); maybe separate options for the names on the notes in the scales and the names which aren't

Useful for printing:

• Could do two modes, one where it's all one piece (and you can slide it generally into place at once, but still have to tape each strip individually, and one where it's separate strips)
• Allow breaking up into multiple "rows", but include enough to be taped together (could probably be done with CSS page break rules and for the part that connects things, pseudo elements)
• Show fret numbers, wrapped around the edge of the neck, great for reading tablature
• Specify widths of the span of strings at the zeroth and last fret
• Specify widths of the neck (or the frets on the neck, rather, which generally span the width of the neck) at the zeroth and last fret
• Specify widths of fret spans (should have you measure the distance from zero to the last fret and count the number of frets (excluding zero / open / the nut), but could have you measure one fret span (the first and largest) and show you how the single-fret measure lines up with the total measure)
• Specify widths of frets? Or in the other mode, the widths of the strips, which should be smaller than the fret span (minus the fret width)

For total creative customization:

• Export PSD with smart objects for the notes, probably with the note labels all on a separate layer, and other labels on another layer or multiple layers
• Export SVG with <use> copies (do SVG editors actually let you use <use> usefully?)