Should all .png files be in power of two on the Iphone?

Question

Hi All,

When creating an UIImage file from a .png to be displayed on a button, view/cell background, etc. for a standard Iphone application, should all of them be in powers of 2 for optimization reasons?

Answer 1

Apple uses odd and arbitrary dimensions for all the images it adds to the interface on your behalf, such as system toolbar items. The best optimization you can do is anything that reduces compositing, which basically means setting the opaque property of views and layers whenever possible.

If you have the choice between a transparent png that will be composited over a static background and an opaque png with the background already included, you have a chance to optimize. When the images will be sliding around or the background will change, you have to composite, otherwise choose opaque.

Answer 2

No, this will have little or no benefit, I usually suffice at doing my own optimization using photoshop "Save for web or devices" option.

Please see http://iphonedevelopment.blogspot.com/2008/10/iphone-optimized-pngs.html for a detailed explanation about the iPhones pre-optimization of pngs.

Answer 3

Here is an article on optimization of iPhone images -- basically tells you why to use PNG files. The size shouldn't matter unless you are using OpenGLES.

Answer 4

As others have said, no - but you should generally use images with even dimensions. This is because when views are positioned with the center property, it'll position an odd-dimensioned image at some half-pixel position. This will cause the image to appear blurry.

As long as you're aware of this it shouldn't really cause you any problems, but it's still a good idea to use even sizes just to be on the safe side.

(This applies for UIKit, not necessarily OpenGL)