sokol_app: dpi_scale seems to be off on iPhone 13 mini

[jspohr]

Hi, this is more of a question than an issue, thanks in advance! I noticed that I was getting a weird horizontal resolution for an iPhone 13 mini from sapp_width() - it returns 2338 while the screen is actually 2340 pixels as far as I know. As a side effect of this, there's an ugly vertical line of noise on the side of the screen because those pixels aren't cleared.

I traced this back to UIScreen.mainScreen.nativeScale returning a float value of 2.88000011. UIScreen.mainScreen.bounds returns 812x375. When I type 812 * 2.88000011 into my trusty calculator, the result is 2338.56008932. So I thought, maybe I have a configuration issue in Xcode somewhere as usual, maybe the Storyboard? That would mean this is out of sokol_app's control and I have to fix my project. However, UIScreen.mainScreen.nativeBounds returns a CGRect with the correct resolution: 2340x1080. Aren't those values supposed to be linked / derived from each other? Or is there maybe a rounding error involved, because the correct scaling value of 2.881773399014778 is unusually ~~baloney~~ odd?

At least, if I hardcode the framebuffer resolution to 2340x1080 then the noisy line disappears. But I would be really interested in the root cause of this. Thanks again!

[jspohr]

I just noticed this commit, in which rounding was added to the computation of the framebuffer size. This results in a framebuffer resolution of 2339x1080 for the iPhone 13 mini in landscape mode, a 50% reduction in terms of relative error :) However, it's still off by one pixel...

To fix my use case, I now changed _sapp_ios_update_dimensions to calculate framebuffer size like this:

_SOKOL_PRIVATE void _sapp_ios_update_dimensions(void) {
    CGRect screen_rect = UIScreen.mainScreen.bounds;
    // mainScreen.bounds respects device orientation, while nativeBounds is always portrait
    const bool is_portrait = screen_rect.size.width < screen_rect.size.height;
    CGRect native_bounds = UIScreen.mainScreen.nativeBounds;
    _sapp.framebuffer_width = is_portrait ? (int)native_bounds.size.width : (int)native_bounds.size.height;
    _sapp.framebuffer_height = is_portrait ? (int)native_bounds.size.height : (int)native_bounds.size.width;
    _sapp.window_width = (int)roundf(screen_rect.size.width);
    _sapp.window_height = (int)roundf(screen_rect.size.height);
    ...

This gives the correct resolution without any rounding necessary. I left the roundf() for window_width/height, although I'm not sure if it's needed. However, there's a caveat, this branch will no longer be entered: https://github.com/floooh/sokol/blob/efc3a2b3d89d1d6371e130dd2d1046c976a4a9cc/sokol_app.h#L4128-L4141

The dimensions are now always exactly the drawable size. I'm not sure if this has side effects in case the orientation changes, since my app is landscape only.

[floooh]

Thanks for the investigation! I wasn't aware that there are iOS devices with 'odd' scaling factors like that.

I need to investigate why there's this precision problem when multiplying mainScreen.bounds with mainScreen.nativeScale, because IIRC I've been through quite a few iterations in that area (for instance computing dpi_scale from .bounds and .nativeBounds), and all those iterations had one problem or another, so I'd prefer keeping the current code but fixing the precision problem.

[jspohr]

It's definitely weird that the nativeScale isn't exact, but apparently rounded down to two decimal places. There could be a reason they're doing that, but I couldn't find anything useful via google. And I agree, especially since the dpi_scale is used for converting input positions to pixel positions, it should definitely be based on what iOS uses as the internal scaling value.

[jspohr]

Can be reproduced in the Simulator:

1. sokol-samples % ./fips set config sapp-metal-ios-xcode-debug; ./fips gen
2. Open sapp-metal-ios-xcode-debug/sokol-samples.xcodeproj
3. Right-click sokol-samples project -> New File... -> iOS -> Launch Screen -> Next -> Select target triangle-sapp -> Create
4. Left-click sokol-samples project, left-click target triangle-sapp -> General -> Launch Screen File -> enter "Launch Screen"
5. Open triangle-sapp.c, add .high_dpi = true, in sokol_main
6. Debug triangle-sapp on iOS Simulator iPhone 13 mini
7. Set a breakpoint in sg_begin_default_pass: width is 2339, but should be 2340

You need to create the launch screen storyboard to get actual fullscreen on iOS 15 (?). At least that's the only way I know that works.