Correct Image and Font Scale on different Devices?

Question

I have a simple view with a text and an Image. I ran this app on iPhone6Plus and iPhone5. Then I made a screenshot of both and enlarged the iPhone5 screenshot such that it match

Answer 1

Images

I am the creator and one of the developers of the Swift Weather app. The app doesn't use three versions of images because I didn't make those images and it was a Pull Request from another developer. I don't have the origin images.

As @Daniil Korotin mentioned, iOS uses points to calculate image and font sizes. iOS uses let screenScale = UIScreen.mainScreen().scale to retrieve the screen scale and pick up the proper size (1x, 2x or 3x) of the image. If we don't provide the proper size of the image, for example, in SwiftWeather app, we have only 1x version of the image (as the screenshot below), iOS will upscale the image to render on retina devices. On iPhone 6 Plus, it actually does downsampling for 3x assets. Please have a look at iPhone 6 Screens Demystified. In some case, if you don't provide 2x or 3x images, on retina devices, the image upscaled from 1x may looks blurry. We should always provide 1x, 2x and 3x images if possible.

Fonts

iOS renders fonts according to the specified points. It will automatically convert the points to certain pixel based on the devices' screen scale (as mentioned above).

How can I achieve that text and image have identical proportions on different screen sizes? How does the Weather App do it? The answer is Auto Layout

You can see we set constraints for the image view (used for the weather icon) as below. The width and height are always 150 points, please notice it is points nor pixels. It will render the same size (for look and feel, not for exactly pixels) for different devices. For your first image (iPhone 6 Plus vs. iPhone 5), it looks different because maybe your simulators have different scale. A better way to check how auto layout elements lay on the screen is to use Preview in Interface Builder.

Open the main storyboard, and click on Assistant Editor. On the right hand side, select Preview (on the top left). And click the plus sign ( on the bottom left) to add different devices. You can see they are identical proportions on different screen sizes.

If you have any questions, please let me know.

Something maybe off topic

If I design the images/assets, I would like to use some vector base tool like Sketch to design the assets and export them to three different sizes. Please have a look my another project iOSAnimationSample. It has a Sketch file for the design.

Sketch design

Export assets to different sizes

In that case, iOS can pick up the proper assets for different devices.