Discussion for Tutorial 10 - Game Structure

[jonny]

You will be pleased to know that the first part of tutorial 10 is now available for viewing. It’s taken me much longer than I would have liked to get this ready, but I’m glad that at least part 1 is out.

This part of the tutorial covers the changes to the project structure I have made along with how I am managing different game scenes such as the main menu and core game scene. It also covers some of the changes I’ve made to the Image class etc.

[rookieStudio]

Good job on this forum. Keep the tutorial coming. Thanks!

[Elphaba]

Mike.

I have a question about your code... I've been looking at this all day and can't for the life of me work out what you are doing and why it works..

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// Macro which returns a random value between -1 and 1
#define RANDOM_MINUS_1_TO_1() ((random() / (GLfloat)0x3fffffff )-1.0f)

// MAcro which returns a random number between 0 and 1
#define RANDOM_0_TO_1() ((random() / (GLfloat)0x7fffffff ))


I mean, why the hex? Why those values? Why does this work?

Surely this makes more sense and would work much better:

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#define RANDOM_0_TO_1() ((arc4random() % 100 ) / 0.01f)


No?

[mike]

Hi Elphaba

That's not code I've looked at in a while and if I remember rightly it was code borrowed from Cocos2d when I was working on my particle emitter.

Using the hex is just a cleaner way of holding a big number. 0x7fffffff is in decimal 2147483647 which is 2^31, so the macro is just taking the value from random() and dividing it by a big number. I don't see anything wrong with your approach either, other than more operations would be necessary to work it out. That is why I think they are just dividing random by 0x7fffffff. This macro ends up getting run a lot in the game loop by things like the particle emitter so having it take as few cycles as possible is a good thing.

That's about all I can say on it really, it was a case of it worked and so I didn't look at it again, but good question

Mike

[Elphaba]

Thank you again Mike for taking the time to answer.

[Tony]

I was looking at this last week after getting the same 'random' numbers over and over. After some googling, I changed everything to arc4random. It gave much better results.

Here are a couple of posts on the subject:

http://discussions.apple.com/thread.jsp ... ID=1590314

http://iphonedevelopment.blogspot.com/2 ... andom.html.

Tony

[Elphaba]

Yes.

Random() will ALWAYS give repeatable results, unless you seed 'srand' with time.

However, even so, the 'randomness' of the 'Random()' statement is much poorer than the 'arc4random()' statement.

I can't speak if 'my' line of code is more efficient / quicker that the ones in the Tutorial, but you don't need to keep re-seeding and also get a much better 'random' number.

Maybe that's worth the 'possible' slight loss of efficiency.

[Tony]

It's definitely worth the slight loss.

[mike]

I am also going to try out arc4Random() and see what happens in terms of performance. Even though it may need a few more ticks, having more randomness than random() would be a real advantage.

If I see any performance problems I'll report back but I am expecting things to be fine given the load I'm seeing in my current game project

Thanks for the info chaps, very useful.

Mike

[Elphaba]

And I'm an idiot.

My code had an obvious typo.

It should have been / 100.0f rather than / 0.1f

D'oh!

So it all should look something like this...

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#define RANDOM_MINUS_1_TO_1() ( (( arc4random() % 100 ) / 100.0f ) - 1.0f )

#define RANDOM_0_TO_1() ( ( arc4random() % 100 ) / 100.0f )

#define DEGREES_TO_RADIANS(__ANGLE__) ((__ANGLE__) / 180.0 * M_PI )


HOWEVER, I would suggest replacing all DIVIDES with the appropriate MULTIPLYS because usually (and esp. on Intel chips - not so sure about ARM but it's very likely) MULTIPLYS are less operationally expensive than divides. So always use a * instead of a / where you can.

So the 'optimal' code would be the following

Code: Select all

#define RANDOM_MINUS_1_TO_1() ( (( arc4random() % 100 ) * 0.01f ) - 1.0f )

#define RANDOM_0_TO_1() ( ( arc4random() % 100 ) * 0.01f )

#define DEGREES_TO_RADIANS(__ANGLE__) ((__ANGLE__) / 180.0 * M_PI )