Game Design, Programming and running a one-man games business…

Category: programming

Random heap corruption.

Posted on October 24, 2011

My heap is corrupt. No I’m not talking about my car, or my house, but it’s a boring programming thing. Essentially, it means somewhere, I’ve been silly and have written code that writes to memory that it shouldn’t be. This is something that is very very hard to track down.

The trouble with C++, is it lets you do silly things. Not just write to memory that isn’t yours, you could do crazy stuff like define + as meaning subtract, and vice versa. C++ is the programming practical jokers language of choice. There are even competitions to see who can write the most unintelligble crap with it.*

Of course, I try to write the most sensible, intelligible code that I can. After all, I’m the poor schmuck who has to support it, and fend off bug reports from angry customers. The BIG problem with C++ is that when you write some code that does crazy stuff, even when it goes wrong, you might not know about it until much later, maybe seconds or even minutes later, which means hundreds of thousands, if not millions of lines of code have been processed since the actual error.

In other words, you can’t look at a call stack and see where you screwed up, which you can do, in 99% of bugs. Even multithreaded bugs can be at least reasonably interrogated with the call stack and watch window.

With a heap screwup like mine, you are back to pre-debugger, stone age thinking such as.
“When did this start happening”

“What had I done before it crashed? and what had I *not* done?”

The last one is especially helpful because it rules out certain causes. My crash is NOT a result of the level editor, the unit design screen, units exiting the map, nightvision, nighttime, order-giving or path-assigning. It happens in debug, and its only been happening the last week, maximum.

So that means I’ve probabyl screwed up the new pathfinding code or the new AI code. These are two recent areas of much rewriting. I’m combing through them right now. I’ve tried various tools and debug flags to help, but sadly they either don’t run, or slow the game to unplayable speeds, where the bug will likely never trigger.

Fun fun fun!

*C++ is FAST, which is why I use it. If you think 2D games don’t need speed, try cloning GSB or GTB to run at 1920 1200 res and run smoothly on a 4 year old video card. You NEED a lot of speed for all this stuff.

 

Programming AI in a tower defense game

Posted on October 12, 2011

This is not an easy task. It is for most Tower Defense games, because they are simply scripted. The level has a set number of enemies of certain types and times, and they appear as attackers. And of course, most TD games don’t let you play as the attacker, so programming the defense AI is not needed. I need both of these to be non-scripted for Gratuitous Tank Battles.

I need attacking AI that ideally adapts mid battle to your decisions, and this is harder than it sounds. The first approach I took was to have a system with 3 types of placement timing mode (steady placement, or hoard points for a big attack or spam the current unit) and two types of unit selection (random, and select the one that has been most effective so far…with some variation).

The AI would switch randomly between these combinations and it looked pretty convincing. Sudden waves of identical units followed up by who-knows what. The problem was, it obviously did so with little care as to what it was up against.The ‘most effective unit’ code was good, because it meant totally useless designs which charged into a hail of bullets would not see much repeat business, but something major was lacking. That was the anticipating of future events.

If the defender has placed 6 machine-gun turrets (rubbish, but devastating to infantry) you shouldn’t place down infantry, regardless how well they did earlier in the battle. If they have lots of laser turrets, place down shielded units to get past them…etc.

So today, at the end of a long day of video rendering and editing, and tweaking, and bug fixing, I’m starting work on a more generic system for opponent army composition analysis, that can take snapshots of the enemy forces and realise that its 63.2% anti-shield units, and thus we should de-prioritise shielded unit selection etc.

This is annoying, fiddly, long winded code that nobody will ever see, as such, but will make Gratuitous Tank Battles a convincing challenge, which is well worth spending time on.

Debug Tools and defensive AI.

Posted on October 4, 2011

Life is so much simpler when you bunch up all your hotkeys and toggles for debugging the game into a single menu. I feel like I have a rough idea what’s going on now. I also feel more confident about coding decent defensive AI when you play as the attacker:

The code for defending basically needs to manage three things. The play style (Is it spamming units, hoarding points, or steadily placing), The choice of units (Does the enemy have units weak Vs lasers? Are there lots of infantry needing flamethrowerising?) And the choice of unit placement. I’ve been concentrating on the unit placement debug UI today, so I can see what the AI is thinking when it chooses to place a turret at X rather than Y. More work to do, but currently it evaluates proximity to enemies, their size, victory-point value, closeness to the exit and proximity to the placement, plus it also has bonuses for being able to cover multiple paths from a single location.

It all needs lots of tweaking and balancing obviously, but it’s getting there. Attacking AI is much simpler.

After getting this stuff done, I’m thinking of experimenting with a unit ‘cooldown timer’ beyond a certain level to prevent spamming the same unit everywhere. Essentially there are supply delays if you plonk down 4 heavy tanks all very quickly, as new ones need to be dispatched from your offscreen HQ. I think it might be a good mechanic that encourages variety.

Shadow maps make me happy

Posted on October 2, 2011

I’ve been watching some WW2 footage, some war movies, some video game footage, etc, in a bid to up the visuals in GTB. A lot of it can’t be seen in a screenshot, it’s moving-picture stuff really. UI’ve also been re-visiting how I make the shadow maps, and massively boosting their resolution. What PC’s can’t handle a 2048 square texture these days? None, I suspect.

Anyway here is a very sparse GTB screenshot:

And here is the same frame, but without the shadowmap.

I think the shadowmap wins by a huge margin. (It’s more apparent full screen, when moving and zooming etc. Combining a grayscale shadowmap with tileable textures not only makes it look less 2D, but it also means you can detect the repeating tile patterns a lot less. I am using a tile-based system to afford easy level design for players. I’m also toying with the idea of automatically generating the shadow maps from the player-designed paths. I think it’s quite doable, although non trivial, and better than expecting players to use photoshop etc.

Comments?

Anatomy of a gratuitous bug

Posted on September 28, 2011

I think I’ve fixed a bug in the gratuitous space battles campaign game. I’ll know ‘officially’ soon, but it fixed it on my machine :D Here’s what was involved.

A player complained of a random crash bug at the end of some campaign battles. I could not ever reproduce it, and back-and-forth emails began. Eventually, this awesome customer provided me with exact steps to reproduce, and all their save game data to let me replicate it. First run through and….. nothing. It was fine. Roughly every third run-through, in release mode it crashed…

Step 1! Hurrah! it actually crashes for me. This is 50% of the battle because then I *KNOW* that it is the games fault, and not the gamers system, or software. This is good, although frustrating news.

Step 2! It crashes in debug mode. This is another 25% of the battle because I can actually see what data is corrupt. As it turned out, the ‘firstfleet’ pointer in the code that assigns captured ships to the players winning fleet is clearly trashed. How did this happen?

Step 3… debugging. It transpires that the firstfleet pointer is accessed multiple times before this point, and after being initialised, confirming that it *must* have been valid, and becomes invalid between initialisation and access when adding captured ships to the fleet. This means we  can step through and watch what happens, if I break on initialisation..

Step 4 discovery! Stepping through the code confirms my suspicions. Once the battle ends, the code updates all the players fleets and removes ships that died in battle. Then, other code innocently picks the first of the players fleets in the battle, and initialises a dialog box listing the enemy captured ships that will be assigned to the fleet. Later…. *drum roll* it deletes any fleets that are now empty. Can you see the bug yet?

Step 5: fix! Changing the code that naievely picked the ‘first fleet’ to pick the first player fleet that still has some intact ships ensures that the later deletion of an empty fleet, and invalidation of the pointer is harmless, because the captured ships are now getting added to a surviving fleet. Bug probably fixed, pending the player confirming that a new exe fixes it.

Why did I not spot this bug six months ago? Well here is what has to happen.

  1. The player has to fight a battle with multiple fleets at once (common).
  2. He has to win (fairly common)
  3. The ai has to lose by the right margin to leave some captured ships (fairly rare).
  4. The winning player has to have enough ships removed from the *first* fleet in the list to have that fleet entirely deleted, despite winning overall. (pretty darned rare).

Simply put, This didn’t happen to me once in testing. It hasn’t happened to many players either, as I understand it. And if it has happened to you… I may have good news :D