I’ve updated r20 of the Android SDK and started the 9 Patch tool.
There seems to be a new feature – Holding Control and clicking adds layout bounds which are referred to as red pixels.
I’ve taken a look at the documentation but it seems out of date.
My question is, what are layout bounds used for and what is the best way to use them?
Original: It’s not documented because it’s not part of the OS yet, only the tools. The red pixels for setting layout bounds are not used by the Android framework (as of Jellybean 4.1.1), so don’t set them at this time. You may speculate about what it means to declare where on your image the layout bounds of the control should be. And watch http://android-developers.blogspot.com/ for announcements.
It’s for “Optical bounds layout”. More details can be found in the Android 4.3 API description. Excerpt:
For views that contain nine-patch background images, you can now specify that they should be aligned with neighboring views based on the “optical” bounds of the background image rather than the “clip” bounds of the view.
For example, figures 1 and 2 each show the same layout, but the version in figure 1 is using clip bounds (the default behavior), while figure 2 is using optical bounds. Because the nine-patch images used for the button and the photo frame include padding around the edges, they don’t appear to align with each other or the text when using clip bounds.