Lead Developer, Stardock Entertainment
Published on November 13, 2009 By CariElf In Elemental Dev Journals

We actually made a fairly major change to the code this week so I thought that I'd actually write a dev journal about it: CodeCritter and I gutted the movement code and re-implemented it.  For the developers out there, that meant creating a branch which generally makes me a bit nervous because I always worry that merging the branch back into the trunk is going to go poorly. But making a branch was essential in case it took us longer than a week to get the code to the point where we could subject others to it.

I'd actually implemented code over a month ago that prevented a new turn from happening until all of the units (including AI units) were finished moving, but it took forever even for the very first turn when there weren't a lot of units.  I did some investigating, and eventually found out that it was just that the movement code was really inefficient.  It was moving all units pixel by pixel, even the ones that were offscreen or under the fog of war.  Besides taking longer, this meant that it was hard to catch targeted units because they were moving at the same time as their pursuers.  It also didn't check to see if a tile was blocked before a unit moved into it, so if you decided not to attack another unit, you had to be moved out of the tile.

Now, the developer who coded this is not a bad coder; he's actually one of our best developers. But this code was written in the VERY early stages of engine development, probably before we started working on Twilight of the Arnor, and certainly before we even had the concept of game turns in the engine.  Sometimes you have to get code roughed in and move on to other stuff before you can come back to it, particularly when your betas are more like alphas.  Also, it wasn't really bad code.  It mostly just wasn't scalable.

At the time, I couldn't start work on fixing the movement code because I had probably 5 other critical things to work on and I knew that it would take at least a week to fix the movement code.  So I had to comment out my code that made the new turn code wait for all the units to start moving, and put in code that just checked for the local player to be done moving.

So while I was working on other stuff, I kept the movement code on the backburner of my mind.  It was probably actually a good thing that I had other stuff to work on because you have to do a lot of thinking before you can start coding something this critical.  I'd done the moving code for all of the GalCiv titles (1 and 2) so this wasn't new for me, but frankly the movement code for the GalCiv games sucked.  Those of you who played any of the GalCiv games probably remember the stuck turn button bug.  The main problem with the movement code in the GalCiv games, especially by the end of Twilight of the Arnor, was that it was too complicated.  So I needed to come up with a design that was fast and simple and scaleable.  I actually came up with the design while I was getting ready for work one morning, proving yet again that the best design is done away from a desk. 

In GalCiv2, ships that were off-screen or under Fog of War were teleported from tile to tile until they ran out of moves or became visible.  This was definitely one thing that I wanted to do in Elemental, but it had always bothered me that there were 3 different movement functions in GalCiv2: Move, QuickMove, and Teleport.  (QuickMove actualy called Teleport), and there were 6 different collision detection modes: pathfinding, move check (before moving into a tile), move, (on moving into a tile), quickmove (when calling QuickMove, was kind of a combination of move check and move) and teleport (used by Teleport and was just a check to see if you could teleport or if you need to fall back into moving pixel by pixel).  That was way too complicated, and every time we made a change to the moving code, we had to change it in 3 locations. 

My idea was simple but elegant: What if all the units used the same movement code and then if they were on-screen, have the graphics animate them moving, otherwise just teleport them?  I bounced the idea off of CodeCritter (who is our graphics engine guru), and he thought that it would work and agreed to take care of the graphics code side of the problem.

The first thing that I did when I started working on the new movement code was re-enable my code to force the turns to wait for all the units to stop moving.    That way, I'd be able to tell if my code was doing what it was supposed to do: going faster and not making the turn button get stuck. 

Next, I created a static variable in the base mobile object class, g_bQuickMoveAlways.  If true, it would just move the units from tile to tile (rather than pixel to pixel) whether or not they were visible.  This will be a good option for multiplayer, and it made it easy for me to test. 

Then I just had to look at the existing moving code.  I copied the existing function, renamed it, and started stripping out anything that had to do with graphics or animation.  I also broke the code that actually handled moving into a tile into its own function, to make it really clean.  All I had left to do was make sure that the unit actually did a collision detection before moving into the tile.

This was actually the most time-intensive part of the operation for me.  I had to go through all the different kinds of objects (units, goodie huts, improvements, etc) and make their hit detection functions handle two modes, MoveCheck and Move instead of just Move.  For most of the objects, this was fairly simple, but the units have to check to see if they're going to attack.

At this point, I realized that I was going to have to work on more than just the movement code.  We'd been planning on making it so that you have to be at war with the owner of another unit before attacking it, but that required interface code we didn't have, and player relations code that we didn't have, so units could attack at will.  I had two choices: hack something in so that it would work and change it later, or start laying the groundwork for the real code.  I decided that it wouldn't take me that much longer to write code that wasn't a hack, and it would save me the trouble of having to rip out the quick hack later.

There was rudimentary player relations code in that was based on GalCiv2's relations code, but it was missing some key concepts.  It didn't have checks to prevent your relations from improving from being at war to being merely hostile.  It didn't have checks to prevent your relations from just dropping into war, instead of requiring the player to declare it.  It didn't have a concept of being permanently at war, which we used for the pirates and Dread Lords in GalCiv2, and space monsters in GalCiv1.  I added all the necessary checks, made some wrapper functions for checking to see if you were at war with another player or if you were allied with them, and went to work on the existing DeclareWarOnPlayer function.

The DeclareWarOnPlayer function was just setting your relations to being at war, and it was only doing it on one side.  So you could declare war on the AI, but they wouldn't be at war with you.  It also wasn't moving all enemy units out of the territory.  So I made the war declaration mutual (easy) and started working on the problem of moving all enemy units out.  The same code in GalCiv2 was very simple, it just looked for the first tile out of enemy territory, which might be 1 tile away.  This was pretty cheesy and ineffective at preventing sneak attacks.  So in Elemental, I send the units to the nearest city.  However, what if you have no cities?  While I figured that this probably wouldn't happen very often, I had to account for it. 

Luckily, each player keeps a list of all objects that they know about, including forests and mountain ranges.  So all I had to do was go through the list and move them to one that wasn't on a tile owned by the enemy player (or someone you're not at war with). 

My next problem was that I had no interface for declaring war.  So I made it so that if you right click on a unit, it would pop up the prompt that asks you if you want to declare war on that unit's player. It's kinda lame, but it gets the job done and even after we get the interface in, it might save you some clicks.

So now that you had to be at war before attacking another player, that made it much easier to finish the unit movement code.  If a unit tried to move into a tile with another player's unit that was not its destination, it would be blocked and have to re-calculate its path.  If the tile was its destination, it would bring up the declare war prompt. 

If the unit passes its collision detection check for moving, then it moves into the tile and calls the collision detection with the mode set to Move and performs any necessary code like merging armies, collecting the goodie from the goodie hut, etc.

I was now ready to test.  I loaded up the game and started moving my sovereign, and building units. I had to tweak the code a bit as I caught bugs that made units get stuck, and the attack code needed to be tweaked a bit since attacks were now initiated before the attacker moved into the title.  Once it seemed to be working as it should be, I committed my changes to the branch and let CodeCritter start working on the graphics part of it.

While CodeCritter was changing the code to only move the unit model smoothly (as opposed to teleporting it) if it was visible, I worked on a few movement bugs that were now easier to fix after having rehauled the movement and collision detection code.  The first bug was that if you built a city and your sovereign had no moves left, he would neither move off the city tile nor be stationed within the city.  This was a quick fix, as he just wasn't being added back into the list of moveable units after being given another move to get him off the city. The second was that units leaving a city didn't do any collision detection on the tile that they landed on, so they wouldn't automatically form an army with another unit, or get the goodie from the goodie hut, etc.  Since I had my new handy MoveToTile function, I just made it use that instead of setting its position on the tile directly.

CodeCritter and I merged the branch back into the trunk Wednesday night, which mostly went smoothly.  We both nearly had heart attacks when CVS told us that we needed to update before commiting our changes, which meant that someone had checked in code after we'd started the merge.  I started shouting death threats and CodeCritter put his head in his hands, but we only had 3 minor merge errors to deal with so I didn't actually have to kill anyone. 

We're still tweaking the movement code so that the units move smoothly while on screen at all zoom levels, but we've made huge progress. Since the code is now much simpler than our movement code has ever been in any game, it should be less prone to cause bugs like the stuck turn button.

Anyway, I hope that I haven't bored all of you to tears.  I've been very excited about this new code, so I just had to share.  

 

 

 

 


Comments (Page 3)
on Nov 17, 2009

Read the first half and the last part about deaththreats

 

Scoutdog and Denryu,

 

Gameplay is king and takes precedence over everything else. Don't you agree with that statement ?

on Nov 18, 2009

I would rather a unit encountering resistance to "stop moving" as opposed to losing their turn or auto-directing to someplace I may not want them to go "like right into a dragon", or "on the other side of a mountain range, going more backwards than forwards, in relation to the destination.

to me, penalties for running "into a brick wall" only encourage movement micromanagement. Kind-a seems prone to mis-clicks ... at least devastating in multi. At most game-breaking.

on Nov 18, 2009

Since the code is now much simpler...

I love simple code.  It's SOOO much easier to write and maintain.  A lot of what I've been doing at my current job is to refactor and simplify code.  Doing the same process faster and in less lines of code? Priceless! 

on Nov 18, 2009

Scoutdog and Denryu,



Gameplay is king and takes precedence over everything else. Don't you agree with that statement
Of course. What makes you think otherwise?

on Nov 18, 2009

Scoutdog

Of course. What makes you think otherwise?

Scoutdog - That's refreshing. Of course, the auto-teleport-out system is a bit unralistic even for a fantasy game, but we'll pick this up at a later date. Nice to see that something more inventive is in the works!

 

Denryu - I'm with Scoutdog, I really do not like the "declare war get teleported out of their territory and they get teleported out of yours" option. I would much prefer something along the lines of taling a severe diplomacy hit from all factions as they note that you are a no-good sneaky bastard who attacks someone who they had right of passage. I realize they put that kind of teleport code in for a reason, but I really think there would be better ways to discourage sneak attacks on allies.

 

It may be that you want games to be as realistic as possible but one could guess that you two are ready to sacrifice some gameplay for realism.

on Nov 18, 2009

I am a big realism nut, true, but I firmly draw the line at ANYTHING that might even slightly hurt gameplay. There are very few ways to cover up the teleport-out, but something else like the conflict idea works just as well from my angle.

on Nov 20, 2009

Recently I've been playing Master of Magic and noticing the problems with its pathing system.  It was a fine game for its time, but discovering enemy units in the middle of the enchanted road (railroad, 0 MP cost) will put a spot on your shorts when you aren't expecting it. You may want to implement enchanted road system just because train wrecks are fun to watch.

If you decide you have too much time on your hands, consider implementing supply lines as a constraint on movement.  It makes Zones of Control matter and light cavalry valuable.  Everybody loves heavy cavalry, until they have to start providing wagonloads of grain to feed them.  Wizardry can, and should, counter this, even if the graphics for bottomless feedbags aren't interesting when they aren't exploding.  (Although the graphics of a burning bottomless bag corn could be amazing!  You could call it the Reddenbacher effect...) Most strategy games get logistics wrong, and that's a shame since real strategy is more about moving beans than dispensing bullets.

It looks like you have a great graphics base, and seeing what you could do with light horse archers, or even Cataphracts, would be fascinating.  The thing is that light horse archers are all about raiding caravans and other logistics chains.  When you don't have logistics, light horse is just annoying.  When you have logistics, light horse is terrifying.

on Nov 23, 2009

Rather than teleport out of suddently enemy territory, couldn't you just set an auto-move destination out?  That way, if the player wants to push the issue, their troops are still in the old player's territory.  It makes for more interesting diplomacy, too: "I see your troops are within our, ahem, new borders.  You see, our people need lebensraum..."

on Nov 23, 2009

WALL OF TEXT, oh yeah!

Glad you succeeded in merging the code .

P.S. Why CVS and not SVN, if I may ask?

on Nov 24, 2009

I rather like the "teleportation out" .... because it grants those sneaky mechanics which allow you to stay within borders while attacking.

Each game has different sneaky mechanics. Sometimes its just a One-time power of a certain faction, sometimes its with following a certain guild, sometimes its with following a certain religion/mafia type.

For instance, the Svartalphar can cast "Veil of Knight" which allows an army within friendly borders to start attacking things. This works well with assasins because you can take out critical targets in one fell swoop if your ally doesn't pay attention. AKA, walk in, assasinate their hero, and leave. It also helps if you have a good number of units with you as well, to prevent them from killings you.

on Nov 24, 2009

Rather than teleport out of suddently enemy territory, couldn't you just set an auto-move destination out? That way, if the player wants to push the issue, their troops are still in the old player's territory. It makes for more interesting diplomacy, too: "I see your troops are within our, ahem, new borders. You see, our people need lebensraum..."

We could, but then we'd have to implement code to make them not able to divert their destination and then that complicates the code that would happen if they were attacked. 

Teleporting the affected units out of enemy territory when war is declared keeps the code simple.  Maybe we can look at changing it later if a lot of people are against it, but for now there's still a lot of core functionality that I'd like to be worrying about instead.

P.S. Why CVS and not SVN, if I may ask?

We've been using CVS since I started working here over 9 years ago and we've never really needed to change.  We've been talking about switching to SVN but we haven't set a hard date yet.

on Nov 24, 2009

Cari - What if you just set an auto-destination, but allowed the player to override it?  Doing so would harm relations (perhaps declare war?), but it would simultaneously remove the micro-management of making the player do it, and open up options for "what happens if I ignore this turn of events".

I guess I just don't like the game mechanic of moving other players' pieces around just by expanding my territory.  I'd rather the territory push result in a diplomatic push, rather than a physical push.

Does that make sense?

on Nov 24, 2009

CariElf


Teleporting the affected units out of enemy territory when war is declared keeps the code simple.  Maybe we can look at changing it later if a lot of people are against it, but for now there's still a lot of core functionality that I'd like to be worrying about instead.

I am against it.   I assume a lot of people in this forum against it, but should I make a poll out of this??

The idea of "Conflict" feels way better, as I've described in reply#20.

on Nov 25, 2009

Excelent post Cari. Very informative. Keep up the Great work.

on Nov 25, 2009

Climber

Quoting CariElf, reply 41

Teleporting the affected units out of enemy territory when war is declared keeps the code simple.  Maybe we can look at changing it later if a lot of people are against it, but for now there's still a lot of core functionality that I'd like to be worrying about instead.

I am against it.   I assume a lot of people in this forum against it, but should I make a poll out of this??

The idea of "Conflict" feels way better, as I've described in reply#20.

Well, for what it's worth, I'm strongly against your Conflict proposal that makes blitzkriegs more or less impossible. Teleporting out of the way is unrealistic and unfun, but largely preferable to being unable to conquer a city for any amount of time in my opinion.

Teleportation out addresses an AI issue:  Don't trust people who send troops into your territory. If someone has a right of passage and he wants to stab you in the back, he should be able to in my opinion. The only reason it's there in most games it's in is to avoid the ai to be alpha striked to death by a previously trusted (human) ally. Make the AI clever and it's going to say "We agree to a right of passage, but don't send your mighty army nearer our capital or we're going to be really angry". Then you don't need conflicts or teleportations.