TL;DR: Apple doesn't allow this yet.
I absolutely agree with you — I wish I could make it paid up front (it would save me a lot of time), but Apple wouldn't approve the app.
The situation is like this: a developer can restrict an app for devices that support certain features (i.e. a developer can make an app available only for devices that support ARKit). My original intent was to restrict the app only for devices with the TrueDepth camera (currently iPhone X, XR, XS, 11, 11 Pro and their Max variants) and without the useless camera part, but it turns out Apple won't let you do this (I've filled bug reports together with other developer and suggested that Apple let us, developers, restrict apps for TrueDepth camera-only devices).
So if you submit an app that is fully working only on the iPhone X and not on other devices, your app gets rejected. This is the reason why I made the app free with the useless camera part that's available on all devices and included the 3D Scanner part as an IAP that can be bought only on supported devices (i.e. on devices with the TrueDepth camera).
Because the app can be dowloaded on non-supported devices (again, no way to restrict it to supported devices), I thought it'd be silly to ask money for an app that just takes photos on devices other than iPhone X — i.e. I didn't want to disappoint non-iPhone X users.
Then you need to scan (i.e. move the phone) more slowly, especially at the very high Precisions, such as 0.5mm, 0.8mm or 1mm. Try playing with the speed of scanning to see how it influences scanning quality. For example when scanning with 0.5mm Precision try moving your iPhone very slowly (e.g. 5mm per second) and then compare it with faster movement — you should see improved quality of scan.
There're 4 ways you can download your models and photos;
Infinite Scanning is experimental feature that lets you scan (theoretically) infinitely large spaces — with this feature enabled, you're no longer limited to ~800MB scan sizes — the only limitation is your free storage on device!
To enable this feature, go to the iOS Settings app > Heges and turn on Infinite Scanning
Please note that this feature requires significant computational power, therefore it is advised that you scan slowly and if possible use the newer devices with Apple's A12 chip and newer (iPhone XS, XR, 11, 11 Pro iPad Pro 2018 with FaceID). This feature works perfectly OK on older devices though (i.e. on iPhone X).
How to share screen:
To disable Screen Sharing, double-tap the live video preview again and stop Screen Sharing (do it on both devices). You can also control the video-quality of Screen Sharing: go to the iOS Settigns app > Heges > [adjust the slider]. Screenshot here.
[new in version 1.2] You can start/stop scanning session from the device you are sharing screen to. Example: you have an iPhone XS that you want to scan with and an iPad Air that you want to Share Screen with. After you enable Screen Sharing, you can tap the round toggle button on the iPad to start/stop scanning session on the iPhone.
It is possible to hold a small mirror in front of the TrueDepth camera to make scans, so that you're able to see what you're scanning. 3D Pete made this mirror holder for iPhone XS which can be used for this purpose.
When you're scanning using a mirror, the models are going to be horizontally flipped. To prevent this, please enable the Mirror-scanning toggle button. As models scanned using mirror are horizontally flipped, enabling this button will horizontally flip models that are being exported into STL/PLY. You don't need to have this option enabled when scanning with mirror — just when exporting scanned models.
I'd recommed first exporting your scanned model into PLY (contains colors) or STL (doesn't contain colors) and then using the non-AR viewer.
Use the Internal format AR viewer only when the exported PLY model is too big to be displayed.
The Heges app's 3D model viewers are meant just for a quick preview of the model — any editing of the scanned models should be done on PC or in another app.
In version 1.2 of Heges there was added a new underlying scanning technique (and it was made default) that enables you to more easily "rescan" parts of model that have already been scanned and that may contain geometry glitches; to "rescan" a part of model that appears glitchy, just point your iPhone/iPad to the place you want to rescan and stay still for a moment (you should see that the model readjusts). Generally, try experimenting to see for yourself.
If you don't like this new way of scanning (or if it causes scanning session abortions for you), you can switch to the old scanning technique in the iOS Settings app -> Heges -> turn off "Use New Scanning Method".
The black edges are present due to the new Color Improvement feature. If there would be no Color Improvement, you would see color glitches more frequently (try for yourself how Color Improvement level changes color quality and size of black edges).
To get rid of the black edges, you can turn Color Improvement off in the iOS Settings app -> Heges -> Color Improvement -> Off, or you can change its intensity to either Low, Medium, High, Highest, OP or Über. With Low being the minimal Color Improvement and Über the maximal.
You can go into the "iOS Settings app -> Heges -> Exported Model Units" and then select whether the models should be exported in either meters, centimeters, millimeters, inches or feet.
You can mirror the scanning preview in the iOS Settings app: navigate to: the iOS Settings app > Heges and enable Mirroring preview around vertical axis. Force quit the app if it's open and now the preview is mirrored! Take a look at this screenshot.