Physics Tutorial: Adding Crush Damage With Box2D part 1 of 2

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What my face sees seconds before it hits the monitor

There are two ways to really beat something up in a physical game: first, throw the object against the wall and, second, toss a heavy boulder on top of the object.  The former can be thought of as impact damage.  The velocity of the two objects colliding determines the magnitude of the impact.

The second is more challenging to get right.  This is crush damage.  It takes some processing to determine when something is crushed and how badly it is crushed.

In some games, you have a crushing ceiling that descends slowly and squeezes players into the ground.  Let’s take this simple example as a starting point and work from there:  the crushing ceiling descends on the player.  As the crushing ceiling pushes the player further into the floor, the player starts losing health and dying.

That’s great, but what happens in a more complicated simulation with rigid bodies that can roll out of the way?  Perhaps a heavy block falls on an object, but the object squeezes out to the left.  Sure, the big block penetrated your ball for a single frame, but this was quickly resolved by the physics solver by applying an impulse at to the proper point on the ball.  You can’t simply tag the ball as crushed and respond with death in your game code.

The rest of this tutorial looks at addressing this problem with Box2D.  Box2D is remarkably stable when dealing with significant mass ratio differences.

One way of solving this is to look at how long two fixtures (components of rigid bodies) stay interpenetrated and to apply damage when a time threshold has been reached.  The assumption is that the solver does a great job of applying impulses to separate the two bodies, so when they fail, it’s a sign that there is not enough space for them to coexist.  This is encroachment, and if it endures for a period of time, one of the encroaching objects is crushing the other one.

Let’s recap what we want out of production quality crush detection, and then we’ll get into an implementation you can build from.

  • Detect impact damage separately from crush damage.   Crush damage is the continuous pressure from forces from one object to another, causing encroachment.  Impact damage happens in a single frame. These fire different gameplay events.
  • Determine which object is crushing and which one is doing the crushing.  A heavy object crushing another because of gravity is one thing, but an object with propulsion can push another object against the wall, causing it to be crushed.  Horizontal pressures causing encroachment are still potentially a crush scenario!
  • Compute crush magnitude independent of framerate.  Keep in mind that force-based solvers tunnel in most situations.  Box2D is no exception — if you have a long frame, you’ll tunnel further.  We need to pass predictable and stable crush values to the game code regardless of framerate variability.
  • Handle light objects bumping without crushing.  If a paperclip lands on your head, you don’t want to die.  Most players would object and promptly post an embarrassing yet funny video to Youtube.
  • Scale the simulation to a large number of objects.  Spot all of the opportunities to exit early in your crush detection routines and take advantage of them!

So, that’s what we want.  In the second part, we’ll get our hands dirty with some source code.

Scripting Languages: They Aren’t ALL Bad

Fireworks

Scripting languages have really fallen out of favor in modern game
development. It’s easy to rile a working programmer up to a lather
about poor debuggers, lack of threading or slow compile times related
to a scripting language they were saddled with on a given project.

That said, the pendulum has swung too far in the anti-scripting
direction.

The beginning of a console cycle is epitomized by a need for
compatibility: “how can we ship on multiple prototype hardware
platforms?” and tool efficiency: “how can we reduce the spiralling
cost of development inherent in expanding horizons?” These questions
led developers to make scripting plays in their engines.

In contrast, we are near the traditional end of a console cycle. We
understand how to get a lot out of the hardware so the focus is free
to move towards code execution efficiency in order to exceed the
experiences players have had on the fixed hardware to date.

Looking past the consoles, memory latency is forcing us towards
cache-friendly approaches to implementation which avoids indirection
inherent in runtime bytecode interpretation and vtable lookups. Even
if we push the CPU Ghz up, we aren’t going back to peppering our
runtimes with vtables anytime soon thanks to memory latency overhead.

In this environment, it’s fully understandable that we would deign to
avoid developing in languages like UnrealScript where only the rote
engine-level loops are not bytecode interpreted. None of this means
that scripting languages should be cut out of game development.

I see two places where scripting still beats out compiled code by
providing new experiences for players:

First, scripting can be for one-off level scripting events. Scripts
will always have a place as glue code. If you are nesting your loops
in a level script, you have probably gone too far. Because you are
not attempting to implement large parts of the game world in script
and beacuse they do not run on tick, the performance impact is
minimal.

A few lines of script is a natural alternative to abstract event
firing systems. Visual scripting languages, ironically, are rarely
informative at a glance. It’s much easier to see all of the code in
one place than to click around a 3D blueprint looking for wires
between entities and still need to guess at execution order.

The second use for scripts is more interesting. Right now, a big
trend in games is towards online.

If you send state updates to your clients, the only way to add new
behavior to your game is to patch your client. If you send script
code for interpretation, you are only limited by the assets in the
local cache and the hardcoded simulation to what your players
experience.

The first version of Capture the Flag for Quake was written by Zoid
before QuakeWorld. He took advantage of the fact that, before
QuakeWorld, the Quake client was mostly a dumb terminal sending a
subset of QuakeC over the pipe. Players could connect to his unique
server without any mods and play CTF. This low barrier helped make
Threewave Capture the Flag the first really popular Internet teamplay
game.

If you are sending script over the pipe in 2011, please remember to
fuzz your interpreter… and don’t drop in an off-the-shelf language
that wasn’t designed with unsafe code in mind. Thanks.

Procedural Content Generation Wisdom

Some wisdom on procedural content generation.

  • Don’t build a procedural content gameplay generator until you have built two fully realized gameplay segments yourself. If you don’t know what “it” is, don’t waste resources by investing in automation of “it”.
  • Building a PCG because you have insufficient level design time to populate a universe is not the same as building a PCG because you want users to have access to unlimited content. The former justifies an offline PCG and the latter argues for an online one. The offline one can take bigger technical risks, incorporate more processing time and output the level for a level designer to further tweak. Know why you need a PCG before coding one up.
  • 3D PCG is largely unexplored outside of terrain heightmaps. When I wrote a random indoor level generator for Doom 3 years ago (unreleased), filling the room space with prefabs took an hour of CPU time even though my implementation was in good, clean, common sense C++ running on a reasonable processor from the middle of the decade.

These videos are some of my favorite “outer space” PCG generators. I really like the zoom out on the third video down. (Fast forward to about halfway through.)

http://www.infinity-universe.com/Infinity/index.php?option=com_content&task=blogcategory&id=17&Itemid=93

Gauging a Game’s Potential

At Frogtoss, we are looking at a lot of different design directions and making decisions about what we are working on next.

We tend to avoid market calculations when choosing directions, preferring to look towards qualitative guidelines instead. Specifically, we aim to answer “Can this be really great?” rather than “What does it look like if we can capture 1% of the iOS market at $1.99?”. The former question pushes us to make a game that is worthy of customers, thereby shaping our plan. We feel that the second one simply counts the money before we’ve earned it and does not allow for meaningful course correction.

The axiom “good is the enemy of great” comes into play here; it is easy enough to come up with a fun idea that is cool and plays well. It is also easy to weed out the truly bad ideas.

What is a much tougher challenge is separating the good ideas from the great ones. In our process, it is inextricably linked to gauging a game’s potential to be enjoyed by a large number of customers.

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Whenever an idea is ready for evaluation, I run it against a set of questions. These questions are steeped in a personal philosophy of what makes a game have the potential to be great.

The Questions

Is there one person anywhere who would consider this game to be their favorite? If so, why?

If there is one person in the world who considers your game to be their favorite it means that it has bubbled to the top of their consciousness and that they cherish it. If even one person can say this about your work, your game is truly special and should hold at least a strong niche appeal.

Is there an obviously new experience for players that is core to the game?

This question implies a philosophy: new experiences resonate the most. Without anything new, you are forced to do what someone else has done, but better. Make this concession consciously because if you do neither, your game probably does not have a lot of potential.

Picture the ideal player. What is being done to give this player a sense that they would be crazy to pass up the game because there is so much value in it for them.

My partner has spent over 300 hours in Left 4 Dead. She bought the game for $50. When it goes on sale for $10, she is completely amazed at how much gaming goodness opens up to you for such a small amount of money.

You need to be able to convince yourself that your ideal players need to find your game an extremely easy value proposition to agree with. This is important because everyone else is a step down from that.

Why does the game have the potential to create news stories after its launch window that prompt people to look into playing it?

They say that marketing is a tax you pay for being unremarkable. While not even close to the whole picture, having your game nurture conversation about itself is a huge contributor to the success of many games.

Are other developers going to be influenced by the advances you have made in the product?

Advancing the state of the art of game development — whether through design or technology — is a sign that your game is made of a unique fabric. This holds promise that you final product stands out and provides a unique experience.

In five years, is the game going to be remembered as a variant of a theme (Eradicator) or a gold standard in its genre (Duke Nukem 3D)?

Doing an honest gut check on whether your game is memorable helps you speculate what mindshare you can expect to have.

This question is saved for last because it is the macro question that summarizes your take on the product.

The Exercise

As an exercise, try running through the questions with your favorite classic game from the vantage point of the year that the game was released. You may find, as I did, how easy it is to positively answer the questions with truly great games.

Now pull up a good game on your Steam list that you tried out and put an hour into… perhaps a game that did alright but was not exactly a cultural shockwave. Notice how this good game does not make the grade.

With this contrast in mind, take the idea you’re working on and answer away. How does it stack up?

Good is the enemy of great and it helps to have tools to identify the enemy as early as possible!

Nice Writing, But Why Do I Care?

The average video game has writing that is indistinguishable from filler. Writing makes its way into games through dialog and story. One approach to improving the quality of writing is to employ inspired writers. Unfortunately, this is as useful as a VHS repairman unless you design in reasons for the player to care about your writing in the first place.

It is not just the writing that needs to improve. We need to refine the techniques to bring narrative to the player, making the experience personal through integration into the core game. In the order of most to least effective, here are some examples of methods that have been used:


  • Scrolling text intros (Crystalis, NES)

  • Cutscenes in between levels (Ninja Gaiden, NES)

  • Interactive story segments (Half-Life 2)

  • Dialog selection (Mass Effect series)

  • Playing the narrative (Heavy Rain)

Giving the player a reason to care about the story is as important as the story itself. The humor in Portal 2 is a lot more personal because you are unfolding it by moving forward. Imagine the game popping away to cutscenes to crack the same jokes. It just wouldn’t be about you anymore.

Interactive story progression requires a set of techniques that, when employed, ensure the player is paying attention when the story unfolds. It also helps the story progress at a pace they can comprehend.

First person shooters have done a pretty good job of making people care about their stories. But what about platformers or turn based strategy games? There is a lot of work to be done before these genres have empowered their writers.

The Biggest Challenge

The biggest challenge the game industry faces right now is not the invention of new games and IPs but the discovery of new models to supplement the existing, more traditional ones.  Model experimentation is to the game industry in 2011 what cellular regeneration is to an injured human.

One example of a model innovation is the Free to Play model.  In Free to Play, an effective game designer teaches players about a value system in their game.  Once this is underway, they upsell the player on in-game features that have value within that system.  In this model, the developer ultimately convinces people to buy the game.  Usually this job belongs to the marketing team.  (Yes, Free to Play should be called Pay to Play).

In traditional boxed retail, the marketing team teaches the customers the value of the product through screenshots and trailers.  If the proposition resonates with the customer, they’ll probably buy the game.

In some situations, Free to Play teams have, effectively, put marketers game designer’s chair.  These games are often thin veneers overtop of a spreadsheet that optimizes customer conversion.

A lot of dedicated gamers love escapist multiplayer experiences.  If Free to Play is to truly succeed amongst dedicated gamers, it is the game designers and developers who must stretch to understand how to account for player behaviour and incorporate it into their tool belt.

Free to Play is definitely coming to your favourite types of games.  Game development incorporates such a wide range of professions and skill sets.  I am excited that it now incorporates one more.

If you find Free to Play ominous, you are always free to prove out a different model. :)

Signed, a guy who plays Free to Play Team Fortress a whole heck of a lot.

There is No Mother Lode of Eyeballs

There is no mother lode of eyeballs waiting just on the other side of your game’s launch.

Frogtoss released Zombie Minesweeper a few weeks ago.  Let me tell you this: if you’re planning on making a living releasing games, make sure you can answer the question: Why will people check out your game?

That’s a very different question than Why should people check out your game.  If you answered “because it’s really good and focus tests have gone SO well” then you are answering the second question, not the first one.

If you ask people who have been successful in launching games on the Internet in recent years, they will frequently tell you that there was no one action that they took in order to build up an audience and anticipation.  Instead, a lot of smaller actions built up their presence over time.

Maybe you saw how it worked in the early days of game promotion where all you had to do was release a product and people would check it out and it could rise up.  Picture the Monolith from 2001: A Space Odyssey landing and monkeys jump up and down around in a primal world.  Try planting a black, shiny block in the middle of Times Square.  People would just walk on by and probably just drop litter at its base.  We’ve seen shiny and black.  You don’t deserve the people’s attention just because you showed up.

Your Game

One of the most exciting press stories is by the most hardcore indie developer in the western hemisphere, Brian Provinciano.  He is the main man behind Retro City Rampage.  Take a look at his Twitter and Facebook pages to see how he has been incrementally building buzz for years.  Stop looking for the next big leap forward and take a cue from Brian.  Brian is uniquely impressive in that he can write NES assembly but also capture an audience of thousands with his words.  He is doing interesting work and he deserves your attention.  People will check his game out because he’s out there.

Being a small developer means stretching in all directions.  When there is no one place to go to get attention for your work, be prepared to go in a hundred small directions.

Hardware is the basis of understanding rendering

Hardware is the basis of understanding rendering. Not numerical problems, not geometric problems and certainly not memorizing OpenGL or Direct3D APIs.

One of the most first questions that needs to be asked when deciding to implement new graphical features is what needs to be implemented on the general purpose CPU and what can be computed on a GPU (or even a set of SPEs). This question is impossible to answer without a fundamental understanding of the hardware you are programming for. The abstraction of your favorite API does less to mask this as more programmatic options become available. Consider:

  • What are the latencies and bandwidth of ports and buses in the graphics pipeline?
  • What caching and swizzling operations exist that offset those latencies?
  • What can or must be batched in order to reach expected performance targets?
  • What is the performance and capacity of the target hardware framebuffer memory?

Once you can competently speak on these points, you can start to devise a hypothesis about how to best divide your hardware resources to render a typical scene for your game.

At this point, you have a shot at guessing what data needs to be where in your pipeline and when it needs to be there. This is the point when numerical and geometric issues move into focus.

Learning OpenGL to learn graphics now considered harmful

OpenGL (with the exception of OpenGL ES) is a pool of functions, many of which superficially achieve the same results but with different approaches to dealing with bus latency. As memory latency becomes more of an acute problem in paring down hardware implemented pipeline stalls, literature promoting immediate mode and display lists continues to be the most prominent information on OpenGL in spite of the strong need for sizable data batches.

I’m always going to be a fan of getting stuff up on the screen quickly and programming some sample apps in OpenGL does give you something to mentally referenece when learning theory, but you aren’t programming anything really interesting until you’ve understood the graphics pipeline and at least the timeline of how hardware acceleration has crept backwards through the graphics pipeline over the past ten years.

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Jim Blinn’s Corner: A Trip Down the Graphics Pipeline – An old one that covers software rendering, but gives you a basis of understanding of the graphical pipeline.

Real-Time Rendering – Currently in 3rd edition. The modern replacement for Foley and VanDam. The chapters on performance and hardware combine with the fundamental understanding of the graphics pipeline to help you really understand what’s going on.

Technical briefs for hardware prepared by NVidia and ATI for target hardware. This information is made available on their respective sites.

The Secret Sauce is Ketchup

The secret sauce is ketchup. Really good ketchup.

What makes a game great? These days, it needs to do a lot of things
very well, or the user will be annoyed. But, we don’t choose our
entertainment such that we avoid annoyance. Some of the greatest
games of all time have had very annoying experiences associated with
them. As core gamers, we overcome annoyances in order to access our
preferred form of entertainment. (Ever tried to get a modem game of
Doom working back in the 90s?)

The Quake games are great games. Id and Carmack have been well praised
for the renderers and tech that underpin these classics. As outdated
as the assumptions underlying the Quake technology may be in 2009, the
games feel much more responsive and enjoyable to play than, well, a
handful of the top titles from this console generation.

The secret sauce to making a game great is making the game respond
well to its users. It’s obvious and simple. And, like the ketchup on
you dining room table, the ingredients are right out there for you to
inspect. Where, you ask? Some of the greatest movement and control
code in the history of games has been GPL’d and released by Id in
their three Quake releases.

For all of the praise Id gets for their tech, their movement code is a
scant few hundred lines, overlooked by most. In response to mouse and
keyboard input, it glides the user through the level seamlessly,
giving the user the expected return for every input. That’s the
secret sauce.

Behind making a responsive game lies a handful of techniques and
principles. There are too many for one blog entry, but I’ll address
some of the technical considerations here, moving on to design in a
future post.

Aim for 60 fps.

60 frames per second just makes your game feel more responsive. If
you are making a current gen console game, it is technically
achievable, though it requires team wide discipline. In response for
that discipline, the user receives a viscerally responsive feedback
loop.

  • On high quality displays, the difference between 30 fps and 60 fps
    is very perceptible.
  • There is a sense of quality and production values associated with a
    game that has a high framerate.

I understand that there are many development scenarios where 60 is not
plausible. Maybe your platform doesn’t refresh well (I had this
experience while developing a Flash 9 arcade game). Maybe your
publisher won’t allow you to commit to a game design and art direction
that lets you target sixty.

If that’s the case, you need to vsync lock at 30 without wavering. If Gears
of War can look that good and hit a steady 30, why can’t you?

And, if you’re making a PC game, just wait a year or two and your game
will fly on a $500 machine. Just don’t do the dumb thing and lock
your renderer at 30 so it can’t take advantage of it, eh?

Commit to locomotion based movement at your own peril.

Use velocity-based movement unless you have strong animation
talent. If your character moves through the world by calculating the
model space displacement of its animations, your animation team is in
charge of character movement and your programming team is not.

If you’re doing an involved interaction system with doors to open,
cover to hide behind and detailed reload animations, you need to
iteratively work with your animation team to ensure the animations are
interruptible, blend very quickly and can be updated quickly to help
your team.

Alternatively, a velocity-based movement scheme allows you to assign
acceleration impulses to an entity. On each tick, you extrapolate
the current position using the velocity, executing collision and
response. This is a procedural approach to character movement and it
can feel very organic. With this approach, your animations do not
change the rate of your player’s movement.

I’m not going to say locomotion based movement is bad. It can look
better than velocity based movement. You just need to seat your
animator and your programmer next to each other. Remain analytical
and vigilant and you will come out on top.

As a bottom line, you need to make sure the player gets the movement
response he wants and expects from all input at any time.

Sub-sample your digital input to allow for light taps on buttons.

This is a technique that isn’t immediately obvious, but is difficult
to argue against once you’ve considered the ramifications.

In a given tick, you can have up to two samples for a single button
press whether it’s a keyboard button, an Xbox gamepad button or a
mouse button. Test for button down and button up states — cut your
output velocity by half if you get both.

There are two cases where this is useful:

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First, consider that the user wants to move forward lightly. If a key
is tested for down and not up state, you multiply the velocity by 1.0.
However, if a key was tapped down and up in a tick, you multiply the
velocity by 0.5. You can use this to allow half-height jumps in
response to a light tap, for example.

The second point is more important. When a user’s framerate dips to,
say, 20 fps, he is likely to overshoot his goal by walking off a ledge
or firing for too long. By cutting his velocity in half, you assist
in reeling in the negative effects of the framerate dip. Your user
would thank you for doing this, but he’ll never know you did it.
He’ll just inexplicably like your game better than the competition’s.

Interpolate analog input, but only if your framerate is good.

Different mice and gamepads send analog updates at different speeds.
How do you make sure you have a stick or mouse movement update ready
for processing when it’s time to queue up a new frame? Ya can’t.

Even if your first person shooter is vertically synced and running at 120 FPS on
a beautiful 24 inch widescreen CRT, you can pan a crappy Radioshack
serial mouse around your scene and watch the camera chunk at 20
updates per second. The rest of your in-game animations may seem
smooth, but you won’t notice because the panning is rate is as crappy
as your mouse.

A common approach to dealing with this problem is interpolating the
mouse from the last known input and the one before it. This means you
are always a percentage of the way between 2 known mouse samples.

If you are running at 60 fps, this is acceptable for 99.5% of the
population. Even if you’re 20 years old and hyperperceptive, you’re
not going to be able to determine that you are a quarter of a mouse delta away
from the truth.

If you’re running at 25 fps, a mouse or gamepad starts to feel laggy.
25 fps is a new frame every 42 milliseconds and you are partially
between the mouse delta received 42 milliseconds ago and the one
received 84 milliseconds ago. That sucks, and perceptibly so.

Unfortunately, unless your framerate is a steady 60 fps, you do not
want to filter your inputs. If you are making a PC game, make input
filtering an advanced user choice. And, please, think about it – it
does not make sense to automatically disable filtering on a framerate
threshold or the user will perceive a small mouse leap when you toggle
the filtering.

Oh, and if you’re a gamer, buy mice that refresh quickly. This is
basically resolved by buying a quality, modern gamer mouse. In the
old days, we had to seek out specific mice and run custom programs to
bump up the refresh rate. Good riddance.

Avoid uneven framerates by avoiding an uneven distribution of work.

When developing a game, there are lots of temptations to halve the
processor demand for a calculation by performing it every other frame.

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“It’s taking our AI 5 milliseconds to assess all of the threats in the
world. Let’s run that on even frames only!” This is a naive but
common suggestion heard in game programming.

Unfortunately, if your AI runs its threat test on every other frame,
your framerate fluxuates by 5 milliseconds. Do that enough and the
game will have a difficult to describe hitchy feel.

Some modern game perf tools and profilers work on a per-frame basis
and this is an excellent way to have your code evade performance
analysis tests.

Find another way.