iPhone Game Optimizations. Ultimate Guide

Interesting reading.

I have accumulated a vast stockpile of iOS and OpenGL ES 2.0 optimizations over several years and even I learned a few things here.


L. Spiro

Thanks! I plan to write few articles, that will dive deeper into optimizations described here. Probably one article per weekend or two...

A few slides were a bit ambiguous. You want to align to 4 bytes manually, or let GPU do it for you?

You have to align them manually, otherwise, driver will do it for you.

Thanks for sharing!

Thanks for sharing this. Really great read.

Thanks for the info Stix, it's quite a gold mine of information.
One tip that sounds a bit funny:
"Driver will pad your NPOT textures to next biggest POT value"
I never heard of anything like that. Just wondering it that's really true.
I did hear of a different bug regarding NPOT. If you have a NPOT width that isn't multiple of 4 it will allocate too much memory.

I meant, that your NPOT texture, that is of 480x480x32, that will eat 700kb of your client memory will be padded to 512x512x32 by the driver and will eat 800kb of "video" memory...

I meant, that your NPOT texture, that is of 480x480x32, that will eat 700kb of your client memory will be padded to 512x512x32 by the driver and will eat 800kb of "video" memory...

That's crazy. I thought the whole purpose of NPOT textures was to save memory.

By "still need to sort opaque geometry" you mean to minimise statechange and not to avoid overdraw, right?

That is correct. Sort only by shader and textures. If you add depth to that test your FPS will go down.

A few slides were a bit ambiguous. You want to align to 4 bytes manually, or let GPU do it for you?

StiX already answered but I wanted to give some more detail and an idea of how critical this is.
When you call glDrawElements() or glDrawArrays() there are a few things that can cause it to take a slower path which causes it to copy your entire vertex buffer to a new location in “GPU” RAM (of course there is no such thing in a unified memory model but it is easier to think of memory managed by the driver as GPU RAM).
One way is to simply not use VBO’s. Another way is to pass misaligned data (attributes not aligned to 4 bytes).
These copies obviously involve a lot of extra cycles, even though it uses an optimized memcpy() when possible (it can’t when realignment is necessary), and to give you an overhead of just how much that is, on an average game it means the difference between 20 and 45 FPS.

In going into extreme detail, if you benchmark with Time Profiler and you see a function called glDraw[Arrays|Elements]_ACC_ES2Exec() taking a large amount of time, check your vertex alignments.
If you see glDraw[Arrays|Elements]_IMM_ES2Exec() taking a lot of time then your problem is likely the lack of a VBO.


L. Spiro