I'm writing a project in Xcode 7 / Swift 2 that it is optimized for iPhone 6/6s (i.e. the project has a launch screen file and launch screen images for iPhone 6/6s).
Fortunately or unfortunately, iPhone 6 users have the ability to turn on the ‘Display Zoom’ setting on device which enlarges elements of the interface. When turned on, this setting effectively enlarges a standard iPhone 5 screen size to fit in the iPhone 6 screen space, upsampling to x1.171875. This upsampling causes elements that are raster based such as images, icons, or views that contain UIBezierPath() drawings to display blurred (mildly but noticable).
A few questions:
Appreciate any experienced responses on this conundrum. Thanks.
1 - How can I instruct elements (e.g. a UIView) on the Storyboard in code to disregard the Display Zoom setting when the user has turned it on?
2 - What techniques are there to ensure pixel perfect accuracy remains when Display Zoom is on? (e.g. Is it possible to render graphics using OpenGL rendering, if so, how?)
3 - Is it possible to replace a x2 image with a x4 image to reduce any blurring when Display Zoom is on? (i.e. will iOS downsample a x4 image to x2 image on iPhone 6?)
4 - How can UIBezierPath() drawings maintain pixel perfect accuracy when Display Zoom is on?
There's nothing you can do about this. A user who chooses zoomed mode is deliberately throwing away pixel accuracy. The points in the drawing no longer match the pixels on the screen one-to-one (or one-to-two or one-to-three or any integral ratio). This choice therefore blurs the screen for everything the user does, not just your app.
Nor can you detect what is happening, because in effect zoomed iPhone 6 is presented to your app as an iPhone 5 (and a zoomed 6 Plus is presented to your app as a 6).
As @matt says, there's nothing you can do about this for normal UIKit content
However, for Open GL ES or Metal content, you are able to opt-out of the sampling that the device does, and render straight into the device's physical coordinates - allowing for pixel perfect drawing.
In a graphics app that uses Metal or OpenGL ES, content can be easily rendered at the precise dimensions of the display without requiring an additional sampling stage. This is critical in high-performance 3D apps that perform many calculations for each rendered pixel. Instead, create buffers to render into that are the exact resolution of the display.
Open GL ES
Set the contentsScale of the CAEAGLayer to the [UIScreen mainScreen].bounds.nativeScale, or use a GLKView which will automatically do this.
You will then want to create your framebuffer with the size of the device's physical coordinates.
Metal
Set the contentsScale of your CAMetalLayer to the [UIScreen mainScreen].bounds.nativeScale, or use an MTKView which will automatically do this.
You will also want to adjust the drawable size to account for the scale (lifted from the docs):
CGSize drawableSize = self.bounds.size;
drawableSize.width *= self.contentScaleFactor;
drawableSize.height *= self.contentScaleFactor;
metalLayer.drawableSize = drawableSize;
See also this interesting blog post on how the iPhone 6 Plus renders content, plus the follow-up post specifically about Display Zoom.
