Question

The restriction of 1024×1024 as the largest image for an iPhone is a problem with iPhone 4. However if an @2x image is used with maximum dimensions of 2048×2048 everything looks equally good on the 4 as it does on a 3 – tried and tested in simulator and device. Question is, does the image dimension restriction relate to the UIImage or the resource that it contains? I can’t imagine resources of more than 1024 pixels are discouraged with the 960 pixel height of the screen.

The right answer is really to use tiles so that things look even better, but the deadline for for this deliverable is too close – it’s a future thing.

Answer 1

From UIImage class reference:

You should avoid creating UIImage
objects that are greater than 1024 x
1024 in size. Besides the large amount
of memory such an image would consume,
you may run into problems when using
the image as a texture in OpenGL ES or
when drawing the image to a view or
layer. This size restriction does not
apply if you are performing code-based
manipulations, such as resizing an
image larger than 1024 x 1024 pixels
by drawing it to a bitmap-backed
graphics context. In fact, you may
need to resize an image in this manner
(or break it into several smaller
images) in order to draw it to one of
your views.

That is, views are rendered and composited with the iPhone’s GPU. If your view, for example, overrides drawRect and tries to render a very big UIImage you might run into problems. Newer generation iDevices, such as iPad and iPhone 4 support bigger textures than 1024×1024 (2048×2048 I think).