Starting the Game - iPhone Game Blueprints - page 18

format is native for the PowerVR GPU used in iOS devices (and some other plat-
forms,sotheformatisprettyuniversal),soitisaccuratelyoptimizedforgraphichard-
ware and uses advanced methods of data compression (up to 8:1).
PVRTC does not required software-based decompression, images take less volume
of memory, there are lesser amounts of data to be transferred, and is managed by
the hardware; therefore, the files are rendered much faster than traditional PNG. The
performance and thrifty usage of memory resources are the main advantages of the
format. But there some disadvantages; first of all, the compression algorithm is lossy,
so some portion of graphical information is lost and there can be some visual arti-
facts. This can be critical for elements that require some pixel accuracy, for instance,
UI elements. In this connection, it is wise to strike a compromise and use PNG for
static elements with a lot of small fragments but PVRTC for texturing fast objects
where the ideal quality is not an issue.
Another problem with PVRTC is some lack of support from popular graphics soft-
ware; there is no native support for it by default; special plug-ins are needed. Never-
theless, the issue can be solved by compressing tools with the official cross-platform
software suite PVRTexTool ( http://www.imgtec.com/powervr/insider/powervr-pvrtex-
tool.asp ) . There are also some specific requirements for images: they should be
square and their dimensions must be in the power of two, but in most cases that is
not onerous at all. The following screenshot shows the tile sheet from a puzzle game
Rail Maze developed by Spooky House Studios UG ( haftungsbeschränkt ):
The sprites and tiles are not usually stored as single images because there are
dozens of sprites in games and the content folder would be crowded with image files.
It is more convenient to collate them in special graphic sheets stored in large image
files. The sprite sheet (alternatively, texture atlas ) is divided into small fragments in
which each sprite fits. The size of the fragment depends on the game ; usually, sprite