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AllEightUp

Member Since 24 Apr 2011
Offline Last Active Today, 08:56 AM

Posts I've Made

In Topic: Throttling

28 July 2014 - 07:40 AM

So, I don't completely agree with hplus in regards to the sent data, given that I tend to send some redundant data in UDP, but I do agree that there is missing information in your blog post and above description.  How much data are you averaging per packet is the key missing item?  I will make some assumptions about the data but mostly just cover at a very high level (i.e. missing enough details to drive a truck through dry.png ) some of the problems involved with the naive solution you have.

 

First off, there is little reason to be sending packets at such a high rate.  Your reasoning for wanting to get things on the wire as fast as possible is relevant but when you look at the big picture, hardly viable.  The number you need to be considering here is latency, but of course latency consists of three specific pieces: delay till put on the wire, network transit time and actual receiver action time.  Assuming that your nic is completely ready to receive a packet and put it on the wire, your minimal latency is 5-10 ms because the nic/wifi/whatever needs to form the data into a packet, prepend the headers and then actually transmit the data at the appropriate rate over the wire.  Add on top of this the fact that you are sending packets every 33.33~ms you have a potential maximal latency of 40ish ms from the point you call the send API to when the data actually hits the wire.  If the network is busy, the wifi is congested or weak, you can easily be in the 50+ms range before a packet actually even hits the wire from the point you call the function to send the data.  In general, you need more intelligence in your system than simply sending packets at a high fixed rate if you want to reduce latency but still not "cause" errors and dropped packets.

 

The next thing to understand is that routers tend to drop UDP before TCP.  At a high level this is a technically incorrect description, it's more to the fact that the routers will see small high rate packets from your client, potentially even having two or three buffered for transit to the next hop, and then larger packets of TCP at a more reasonable rate and prefer to drop your little packets in favor of the larger packets from someone else.  Given there are easily 10+ hops between a client and a server, the packet lottery is pretty easy to loose under such conditions when the network is even minimally congested.  Add in reliable data getting dropped regularly and now your latencies are creeping up into the 200+ range depending on how you manage resend.

 

How to start fixing all these issues to deal with the random and unexplained nature of networking while maintaining low latency is all about intelligent networking systems.  Your "experiment" to reduce packet rates is headed in the correct direction, but unfortunately a simple on/off is not the best solution.  The direction you need to be headed is more TCP like in some ways, specifically you should be balancing latency and throughput as best you can while also (unlike what hplus suggested) using any extra bandwidth required to reduce the likelyhood of errors causing hickups.

 

I'll start by mentioning how to reduce the effect of errors on your networking first.  The common reliable case is the fire button or the do something button which must reach the other side.  In my networking systems I have a "always send" buffer which represents any critical data such as the fire button.  So, if I'm sending a position update several times a second, each packet also contains the information for the fire button until such time as the other side ack's that it received it.  So, baring massive network hickup, even through a couple packets may have been dropped the fire button message will likely get through as quickly as possible.  This is specifically for "critical" data, I wouldn't use this for chat or other things which are not twitch critical.  In general, this alone allows you to avoid the worst cases of having "just the wrong packet got dropped" which throws off the players game.  Yup, it uses more data than strictly necessary but for very good reason.

 

Moving towards the TCP like stuff, let me clarify a bit.  What you really want here is the bits which replace your "experiment" piece of code with something a bit more robust.  In general, you want three things: mtu detection (for games you just want, can I send my biggest packet safely), slow start/restart packet rates and a non-buffered variation of the sliding window algorithm.  So, the MTU (maximum transmission unit) is pretty simple and kinda like your current throttling detection, send bigger packets until they start consistently failing then back off till they get through.  Somewhere between where they were failing and where they are getting through is the MTU for the route you are transmitting on.  You don't need to actually detect the MTU for a game, you just want to know that if everything starts failing, MTU could be the reason and you should back off large packets till they get through.

 

The second bit, slow start/restart is actually a lot more important than many folks realize.  Network snarls happen regularly, either things are being rerouted, something has a hickup or potentially real hardware failures crop up.  In regards to UDP, the rerouting can be devastating because your previously detected "safe" values are now all invalid and you need to reset them and start over.  A sliding window deals with this normally and is generally going to take care of this, but I wanted to call it out separately because you need to plan for it.

 

The sliding window (see: http://en.wikipedia.org/wiki/Sliding_Window_Protocol) is modified from TCP for UDP requirements.  Instead of filling a buffer with future data to be sent, you simply maintain the packets per second and average size of the packets you "think" you will be sending.  The purpose of computing the sliding window though is so you can build heuristics for send rate and packet sizes in order to "play nice" with the routers between two points and still minimize the latencies involved.  Additionally, somewhat like the inverse of the nagle algorithm, you can introduce "early send" for those critical items in order to avoid the maximal latencies.  I.e. if you are sending at 10 a second and the last packet goes out just as a "fire" button is received, you can look at sending the next packet early to reduce the critical latency but still stay in the nice flow that the routers expect from you.  A little jitter in the timing of packets is completely expected and they don't get mad about that too much.  But, even if some router drops the packet, your next regularly scheduled packet with the duplicated data might get through.

 

I could go on for a while here but I figure this is already getting to be a novel sized post.  I'd suggest looking at the sliding window algorithm, why it exists, how it works etc and then consider how you can use that with UDP without the data stream portion.  I've implemented it a number of times and, while far from perfect, it is about the best you can get given the randomness of networking in general.


In Topic: Does anyone know of this game editor the video uploader is using?

24 July 2014 - 06:23 AM

If you are talking about the capture starting around 1:38, that's just 3DS Max with a custom plugin.  Otherwise, the only other stuff I saw was some in game UI work to help editing.


In Topic: Storing position of member variable

14 July 2014 - 06:29 AM

You might want to look at doing this in a more C++ like manner without the potentially difficult to deal with side effects of pointers into protected/private data.  The way I went recently was to implement a system like this using std::function.  Basically using accessors I could bind up access in nice little objects which worked with my serialization.  So, for instance, in your given example, I wouldn't expose x, y, z separately I'd simply bind the accessor to the vector as:

 

std::function< Vector& () > accessor( std::bind( &MyComponent::Position, std::placeholders::_1 ) );

 

With that, pop it in a map of named accessors and you can get/set the component data without breaking encapsulation of the class with evil memory hackery.  Obviously there is more work involved in cleaning this up to allow multiple types, separate get/set functions and a bunch of other things I wanted supported but it is considerably better behaved than direct memory access.  The primary reason I avoided the direct memory access was because I have a number of items which need to serialize but work within a lazy update system, if I bypass the accessor, the data in memory can be in completely invalid states.  With a bound member (or internal lambda), everything can be runtime type safe (unlikely to get compile time type safety), works with lazy update systems and generally a much more robust and less hacky solution.


In Topic: FBX and Skinned Animation

08 July 2014 - 06:24 AM


osted Yesterday, 11:33 PM
AllEightUp, on 07 Jul 2014 - 10:21 PM, said:
What this means is that basically unless Max has curves supported by FBX, it may be baking them and you won't have a way to find what the original two *poses* in your terms were. Generally you will iterate through time and sample the scene at a fixed rate. Yup, it kinda sucks and generally you'll want to simplify the data after sampling.
But that is what you want to do anyway.
There are several modes for interpolating between key frames and you want to reduce that down to all linear interpolations, because that is the only thing you should support at run-time.

 

Yes, this is generally what you will want to do and I didn't mean to suggest it was a useless thing.  On the other hand, I would really like FBX to retain original key location information in the animation data so I can mark those keys as more important when evaluating key reductions.  The added context can greatly help when trying to preserve C1 continuity so things like fingers at the end of long bone chains don't jitter around so much.  It generally works out best if you keep those keys in preference to others since very often they are the extreme's of the curves and removal can throw off other reductions and increase the positional/velocity errors to a very notable degree.


In Topic: FBX and Skinned Animation

07 July 2014 - 09:21 PM

I'm getting my bind pose data like this (pseudo):
 
bone=mesh->GetCluster->GetLink
bone->GetTransformMatrix() and bone->GetTransformLinkMatrix()
 
...that seems to be working alright.

The transform link is the way I do the bind pose matrix myself, so it looks correct to me.
 

I don't understand the concept of FbxAnimStack and FbxAnimLayer. I think a stack is a collection of layers, and a layer is what used to be called a 'take' (which I think is like a key). I also don't understand what the FbxAnimEvaluator does.

You can pretty much ignore the layers unless you intend to do some pretty advanced stuff, just collapse them at the start and you'll only have one layer per stack. (I don't remember the exact call, it's on the FbxAnimStack I believe.)

The FbxAnimStack is actually what used to be called a take as I remember it. For DCC tools which export multiple animations per FBX, you would have one of these stacks per animation. So, a file could have a walk stack, run stack, turn, jump etc. Maya seems to ignore this, I think Max will use them if you author the files in a specific manner.

Finally the evaluator is basically an animation playback system for the content of the FBX file. Basically if you set a stack as it's current context, it will allow you to sample the scene and get the transforms, etc from the scene at various times. This brings us to the rest:
 

I'm doing the modelling, rigging, and animating myself in 3ds Max. Let's say I want to do a simple two key idle animation for my soldier, I pose him at time=0 and at time=30. All I really need are the bone transforms for those two times (for my simple system.)
 
Can you put that in terms of FbxAnimStack, FbxAnimLayer, FbxAnimCurveNode, FbxAnimCurve, FbxAnimCurveKey ? I don't know where to look for the bone transforms at those two times.


The FBX SDK gives you a number of ways to get the data you might want. Unfortunately due to different DCC tools (Max/Maya/etc) you may not be able to get exactly the data you want. For instance, let's say you find the root bone and it has translation on it in the animation. You can access the transform in a number of ways. You can use the LclTransform property and ask for the FbxAMatrix at various times. Or you can call the evaluation functions with a time to get the matrix. Or you can use the evaluator's EvaluateNode function to evaluate the node at a time. And finally, the most complicated version is you can get the curve nodes from the properties and look at the curve's keys.

Given all those options, you might think getting the curves would be the way to go. Unfortunately Maya, for instance, bakes the animation data to a set of keys which have nothing to do with the keys actually setup in Maya. The reason for this is that the curves Maya uses are not the same as those FBX supports. So, even if you get the curves directly, they may have hundreds of keys in them since they might have been baked.

What this means is that basically unless Max has curves supported by FBX, it may be baking them and you won't have a way to find what the original two *poses* in your terms were. Generally you will iterate through time and sample the scene at a fixed rate. Yup, it kinda sucks and generally you'll want to simplify the data after sampling.

Hopefully this gets you past the understanding issues for the moment. FBX is fun stuff ain't it. biggrin.png


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