Unity's ability to deploy to mobile devices has contributed greatly to its popularity among hobbyist, small, and mid-size development teams. As such, it would be prudent to cover some approaches that are more beneficial for mobile platforms than for desktop and other devices.
Note that any, and all, of the following approaches may become obsolete soon, if they aren't already. The mobile device market is moving blazingly fast, and the following techniques as they apply to mobile devices merely reflect conventional wisdom from the last half decade. We should occasionally test the assumptions behind these approaches from time-to-time to see whether the limitations of mobile devices still fit the mobile marketplace.
Mobile applications are more often bottlenecked on Draw Calls than on fill rate. Not that fill rate concerns should be ignored (nothing should, ever!), but this makes it almost necessary for any mobile application of reasonable quality to implement Mesh Combining, Batching, and Atlasing techniques from the very beginning. Deferred Rendering is also the preferred technique as it fits well with other mobile-specific concerns, such as avoiding transparency and having too many animated characters.
This concern goes hand in hand with the concepts of Batching and Atlasing. The fewer Materials we use, the fewer Draw Calls will be necessary. This strategy will also help with concerns relating to VRAM and memory bandwidth, which tend to be very limited on mobile devices.
Most mobile devices feature a very small Texture Cache relative to desktop GPUs. For instance, the iPhone 3G can only support a total texture size of 1024x1024 due to running OpenGLES1.1 with simple vertex rendering techniques. Meanwhile the iPhone 3GS, iPhone 4, and iPad generation run OpenGLES 2.0, which only supports textures up to 2048x2048. Later generations can support textures up to 4096x4096. Double check the device hardware we are targeting to be sure it supports the texture file sizes we wish to use (there are too many Android devices to list here). However, later-generation devices are never the most common devices in the mobile marketplace. If we wish our game to reach a wide audience (increasing its chances of success), then we must be willing to support weaker hardware.
Note that textures that are too large for the GPU will be downscaled by the CPU during initialization, wasting valuable loading time, and leaving us with unintended graphical fidelity. This makes texture reuse of paramount importance for mobile devices due to the limited VRAM and Texture Cache sizes available.
We have already covered this topic in Chapter 4, Kickstart Your Art, but it is worth revisiting the subject of GPU-level Texture Compression. The GPU will find it difficult, or simply be unable to compress the texture if it is not in a square format, so make sure you stick to the common development convention and keep things square and sized to a power of 2.