Image Size and Resolution
iOS uses a coordinate system to place content onscreen. This coordinate system is based on measurements in points, which map to pixels in the display. On a standard-resolution screen, one point (1/72 of an inch) is equal to one pixel. High-resolution screens have a higher pixel density. Because there are more pixels in the same amount of physical space, there are more pixels per point. As a result, high-resolution displays require images with more pixels.
You must supply high-resolution images for all artwork in your app, for all devices your app supports. Depending on the device, you accomplish this by multiplying the number of pixels in each image by a specific scale factor.
|iPhone 6s Plus and iPhone 6 Plus||@3x|
|All other high-resolution iOS devices||@2x|
A standard resolution image has a scale factor of 1.0 and is referred to as an @1x image. High resolution images have a scale factor of 2.0 or 3.0 and are referred to as @2x and @3x images. Suppose you have a standard resolution @1x image that’s 100px × 100px, for example. The @2x version of this image would be 200px × 200px. The @3x version would be 300px × 300px.