koobas.hobune.stream
How to playback on value changed data for replay?
I want to further optimize the replay stream by recording values of not frequently changing variables only when they change, instead of every keyframe. This will extremely reduce the replay filesize since I have alot of those variables.
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A good example might be the health value of characters as they will not taking every 1/n of a second damage (where n is the recording rate per second).
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When someone's taking damage, the binaryWriter writes the changed health value to the stream. But this isnt enough. When replaying, the system needs also to know at which time of the replay the characters had the right health in order to wait retrieving the next health-bytes.
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This is my approach:
public class HPKey {
public byte hp;
public ushort keyframe;
public HPKey(byte hp, int kf){
this.hp = hp;
this.keyframe = (ushort)kf;
}
}
private Dictionary<Health, HPKey> lastHP = new Dictionary<Health, HPKey>();
private void WriteHealth(Health health){
if(!lastHP.ContainsKey(health)){
lastHP.Add(health, new HPKey(0, 0));
}
if (health.hp != lastHP [health]) {
lastHP [health].hp = health.hp;
binaryWriter.Write (health.hp);
binaryWriter.Write (currentKeyFrameIndex);
}
}
private void ReadHealth(Health health){
if(!lastHP.ContainsKey(health)){
lastHP.Add(health, new HPKey(0, 0));
}
//currentKF should be compared to next healthchange kf, but how to retrieve on the bytestream?
if (currentKeyFrameIndex != lastHP [health].keyframe) {
health.hp = binaryReader.ReadByte ();
lastHP [health].keyframe = binaryReader.ReadUInt16 ();
}
}
Example: Player has at keyframe 0 a hp-value of 100. The dictionary adds the player, stores its current health and the binary writer writes the hp and the current keyframe to the stream. The next keyframe, WriteHealth gets called, but since the current health remains the same, nothing gets written to the stream. After 318 keyframes, the player gets hit by a bullet, now having 60 hp. Since the stored value in the dictionary is different (100) from the current (60), the code inside the condition gets called again.
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But this approach is not working when replaying, since the system doesnt knows when the next keyframe the hp gets changed.
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I would be very pleased if anyone ever implemented a replay system with this can help me out, since no implementation exists on the internet about this optimization, but only mentioned here (Step 5, Optimization).
Maybe this is not relevant regarding to your constraints, but you may keep a very basic implementation if processing bandwidth is not an issue, and data size is. Indeed, as your "per keyframe" data is very redundant (lot of times the same values), then it might be an excellent candidate for an arithmetic coding. You should try, IMHO, to store your raw data to a zip stream. That way, your code would remain extremely simple and maintainable. If you try, please let us know the compression ratio you achieved!
Hey thanks for your feedback. I already have done standard compression using GZipStream with Deflate Algorithm. I see this using as a last option, since it relies on what is being saved. As for your curiousity: I recorded the position (compressed to 6 bytes) and rotation (1(rarely) OR 4 bytes using smallest three method), health (1 byte), various states (weapon index, ability used 2bytes) 20 times per second for 5$$anonymous$$utes for ~50 objects (varies on my game). Also global values like teamsize (1byte), teamscore (Int32). That is per frame 13 bytes x 20keyframes/s x 300s x 50agents + (5bytes x 20kf/s x 300s): Without compression: 3,93 MB. With GZip it reduced the recording data by nearly 900 KB to 3MB. As you see, it does help, indeed, but the bottleneck is in scaling. Lets say I want in the future add more variables that would change not frequently. Using only zipstream ontop will not solve the problem and I am also targeting on Android where memoryspace is still relevant. And the replays will add up, for example if I could reduce it by half, I could store double the amount of replays with the same memory space used. Yes it might sound a bit irrelevant and not completely pertaining to the matter being talked about, but implementing this optimization is worth it and would the raw size reduce to a bare $$anonymous$$imum (= excluding position/rotation as they are recorded every keyframe -> 7-10b x 20kf/s x 300s x 50agents = 2.1-3MB + ~5kb rarely changed values).
Ha, indeed! Thanks for the details. Just as a side note: you may also enable delta-compression (store the difference with previous frame, so that you got a lot of zeroes, or near-zero values), and zip each value separately (one stream per value) so that a changing variable does not affect the overall compression by affecting the zip dictionary. But you're perfectly right: this might not be as scalable as needed.
Ok guys, Iam pretty sure there are better solutions to this, but since nothing on the internet covers the implementation I guess that doing it now will help someone in the future. If you have a better approach, please let us know!
public class ReplayData {
public byte hp;
public ushort skippedKeyframes_HP;
public long hpByteOffset;
public long lastHpByteOffset;
}
private void WriteHealth(Health health){
data.skippedKeyframes_HP++;
if (data.hp != health.hp) {
data.hp = health.hp;
binaryWriter.Write(health.hp);
binaryWriter.Write((ushort)0); //PlaceHolder
if(data.hpByteOffset != data.lastHpByteOffset){ //dont write on first change
binaryWriter.baseStream.Position = data.hpByteOffset; //move stream back to last PlaceHolder
binaryWriter.Write (data.skippedKeyframes_HP); //overwrite PlaceHolder with accumulated frames
binaryWriter.baseStream.Position = binaryWriter.baseStream.Length; //back to end of stream (new placeholder)
data.lastHpByteOffset = data.hpByteOffset;
}
data.hpByteOffset = binaryWriter.baseStream.Position; //points to placeholder
data.skippedKeyframes_HP = 0;
}
}
private void ReadHealth(Health health){
data.skippedKeyframes_HP += replaySpeed > 0 ? -1 : 1;
if(data.skippedKeyframes_HP <= 0){
health.hp = binaryReader.ReadByte ();
data.skippedKeyframes_HP = binaryReader.ReadInt16 ();
}
}
