It's not a phone app, but I made a web app[1] that can get pretty close[2]. From looking at the images I think the reason why other other apps don't look the same is that they try to emulate it use an ordered dither, while it looks like the Gameboy camera is either adding some randomness or noise as well. I have a Bayer (R) algorithm that adds randomness and it seems to get closer to the Gameboy camera output.
The mGBA PC emulator supports running the Game Boy Camera ROM and passing video from a webcam or from a still image you select. I think some GB emulators on Android also support this, but I haven't tried them.
I think it's very hard to achieve for the same reason why almost no black and white filter can reproduce a photo taken with an actual black and white film - you can get really close but something is always missing. It's not enough to apply a GB-like filter to a modern smartphone photo - you'd need to take into account the unique limitations of the GB camera and somehow reproduce them.
I don’t think it’s very hard, I just think not enough people care enough about getting an equivalent result. Basically 80/20, but no one will pay for that 20 (yet)
On the topic of emulating old hardware, this reminds me of the Nintendo 64 VRU accessory, which is a mic + voice recognition that was used in games such as Hey You Pikachu. Nobody has emulated the hardware which performed the actual voice recognition, instead opting for modern VR libraries. So while you can use it in an emulator, the performed VR won't match what you'd get with the limited hardware implementation.
>This is an HLE implementation. I did not emulate the manner in which the VRU does voice recognition
I suspect it is because of hardware limitation. The silver base one actually have multiple layers, not just one thin silicon light receptor layer. That depth and the randomness of the silver particles make the result very difficult to emulate. Perhaps if one can use those staggering focus???
I have retroboy[1] installed. I got it from fdroid. I like it. It also has several other filters, for C64, Apple classic Mac, Amstrad CPC464 among others. Lots of settings to play with.
