This idea simply aims to provide users with a way of practising guitar chords whenever they please, even if they do not have a guitar with them.
I originally abandoned this idea because I found that most smartphones are much wider than standard guitar necks, which is also why I think something like this has not yet been developed. Apps like Fret Trainer show you where the chords are found, similar to an inventory or dictionary, but does not require users to make the actual shapes and put their fingers in the right positions. There are multiple apps available that show the different chords, but none that allow you to actually "play" them. Upon further reflection, I came to the conclusion that even if your phone's screen would not be able to accurately simulate a guitar neck, simply making the correct shapes would still be beneficial, especially for beginners
I imagine such an app could be gamified to resemble something like Guitar Hero, focussing only on the chords and not on the strumming. That said, if your right-hand rhythm also needs practising the app would also be able to allow you to strum along without having to worry about the chords. You would then either be able to play the chords of a song, as it is playing, or strum along, to practise rhythm.
Such an app would serve as an aid for beginners, it would not be able to substitute practising on a guitar, but it might help.
I checked the Fret Trainer you linked and it seems that they have thought about the width of the guitar neck (check the picture below). It doesn't cover the whole screen, but yes, it depends on the screen size of your phone (thus, hard to resemble the exact guitar neck width). But that seems to be the problem with any app trying to resemble something on the screen. Therefore, I suggest adding an ability to change the guitar neck width in the app to perfectly match the width of your guitar of choice.
Figure 1. Screenshot taken from https://www.frettrainer.com/.
Automatic rhythmical strumming in the background
You mentioned also that in the existing apps, you don't have the ability to actually play the chord. I think that's because you don't have an option to scan the movements of the right hand (strumming). There could be a feature to add rhythmical strumming while you "train" your chords.
3-D touch determining the finger pressure needed to play the chord clearly
Depending on the guitar (classic or acoustic), wires are made of different materials and consequently, different to play. Three plastic wires on classic guitar needs to be gently pressed, while the metal ones on the acoustic require a stronger pressure with fingers. Therefore, an additional feature could be to adjust the finger pressure needed for the chords to play by adding the 3D touch feature into the app. I know the iPhone has this ability integrated, so iOS could be the starting point for this feature development.
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Guitar-neck-imitating attachment for your smartphone
J. NikolaDec 16, 2021
With the app you mentioned, I suggest creating a smartphone accessory that imitates the guitar neck and body (Figure 1). It would attach in a similar way to the "selfie stick" and solve the below-mentioned problems:
practice strumming patterns - you would be able to put your finger on the phone screen in the form of a chord, while simultaneously playing the strumming pattern of your choice; the attachment could have a small screen representing wires or have the actual wires for the real feel
practice using both arms at the same time, like on a real guitar
makes the handling of the smartphone easier, because while your left arm fingers are busy forming chords, you can hold the phone by holding the guitar neck attachment
The gadget could be created in a cheap, nonprecise, and bad-sounding version as well as in an expensive, professional, guitar-like version that could easily substitute guitar on random parties.
Figure 1. Smartphone picture taken from https://www.frettrainer.com/.
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Creating hardware to accompany the app
Spook LouwDec 16, 2021
One solution to the problem of the average size of a phone in relation to the average size of a guitar neck would be to introduce separate hardware that is compatible with the app. The hardware would pretty much look like a section of a guitar neck. This could have sensors that send information to the app which would illustrate where your fingers are and where they should be. The same could be done for the right hand.
The issue I have with this is as soon as you have to start carrying things around with you the app loses its appeal, it would be better to just carry a normal guitar with you.