9-slice scaling

Top: Traditional scaling, corners are distorted. Bottom: 9-slice scaling, corners aren't distorted.

9-slice scaling (also known as Scale 9 grid, 9-slicing or 9-patch) is a 2D image resizing technique to proportionally scale an image by splitting it in a grid of nine parts.

The key idea is to prevent image scaling distortion by protecting the pixels defined in 4 parts (corners) of the image and scaling or repeating the pixels in the other 5 parts.

A variation of the concept, the 3-slice scaling, consists in a grid of 3 parts in which only the pixels in 2 parts (the edges) are protected and the pixels on the middle part are repeated.

History and use

The concept was first introduced in a consumer application by Macromedia in Flash 8 (2005). and it was available as feature to scale symbols. Later on, in 2007 Adobe introduced it as a feature in Adobe Fireworks CS3, Adobe Illustrator, and Adobe Flash. Today it is also present as a feature of game development software like Unreal Engine, Bevy, Urho3D, and Unity 3D.

The technique can be used to manipulate both bitmap/raster graphics and vector graphics. A current implementation of the 9-slice technique is present on the CSS 3 Backgrounds and Borders spec by using the border-image property.

27-slice scaling

The concept used in 9-slice scaling can be extended to 3D, which enables a more natural way of scaling certain 3D meshes. The number 27 is derived by splitting a cuboid into 3 sections on all 3 axes.

The general method of scaling remains the same, whereby the 8 corners of the cuboid do not scale, similar to the 4 corners present in 9-slice scaling. The central core scales in 3 dimensions. The 6 faces of the cuboid scale in 2 dimensions. The remaining 12 edge pieces scale in 1 dimension. All of which are relative to the actual scaling applied.

A functional implementation is done in Unity as a downloadable package.