For Kobold Kit (the Sprite Kit game engine) we’re working on Super Stick Spy, a 2D platformer game. Like so many others, we started out using the (Box2D) physics engine that’s so neatly integrated in Sprite Kit to get everything up and running quickly.
But we knew full well that for the final product, we’ll have to scrap the physics engine altogether and follow best practices when it comes to platformer-programming.
Now with the demo version nearing completion (see video) I can tell you in full detail why you don’t want to use a physics engine for a 2D platformer!
Moving Platform Hell
A moving platform with physics needs to be a dynamic body. Don’t even try moving static bodies, at least in Box2D that will end up in jumpy movement of the body. Though kinematic bodies work better (if available).
The player or other game characters standing on a moving physics body will have the platform slide underneath their feet. The characters won’t magically move along with the platform! And there is no feature in the physics engine that lets you set this up. You have to program it to synchronize character movement with the platform they’re currently standing on, and end the synchronization as soon as a character lifts its feet up from the platform.
Which can be a problem for downward-moving platforms as the player loses contact with the platform every other frame, starts falling, and lands right back on the platform. To put it in Homer’s words: “Doh, doh, doh, doh, doh, doh, doh …”. So you need to make the character stick to the platform, yet allow him to fall off of the ledges and jump, and possibly also allow him to be forced off the ground by normal collision events (projectile impact, platform moving through a tiny crevice). Continue reading »
Cocos2D is a rendering engine. Note the emphasis. 90% of what it does is draw stuff onto the screen and animate it. It adds some input processing and scheduling and the rest is up to you.
A game engine is to cocos2d what cocos2d is to OpenGL. The list of things I want in an actual game engine is long.
The iOS mobile platform has advanced far enough that a pure rendering engine just isn’t that much of a help anymore. We’re effectively moving back towards where we were back in 2008 if we don’t start pushing the boundaries, hard.
Here are some ideas I have for and would like to see in a 2D game engine, in no particular order. It is not a feature list for Kobold Kit, but it does reflect what I want to make possible with / encourage for Kobold Kit. Continue reading »
You have a tilemap and you want physics collisions on it? The solution seems obvious: create a rectangle shape for every blocking tile.
But ouch! This solution is not just hugely wasteful and unnecessarily slows down the physics collision code, it also introduces the well known problem of characters getting stuck even on flat surfaces.
This is in particular a problem for Box2D because its collision mechanic doesn’t work well with flat surfaces subdivided into smaller segments (rectangle shapes in this case).
A workable but still very awkward solution to work around this behavior is to create characters with bevelled edges at the character shape’s bottom at the risk of bopping characters as they walk about the map.
Lupines in the Moore Neighborhood
A good solution to generate physics collisions is to implement the Moore Neighborhood algorithm to generate chain shapes which are more suitable for tilemap collisions. The downside is that adding or removing individual blocking tiles at runtime requires updating the shapes - this is not implemented in this project.
Every flat surface, no matter how many tiles form the surface, will then consist of only one straight collision segment. Here’s a quick demo video of the project discussed in this post that shows the algorithm at work and the resulting “game”:
If you ever wanted to build your own 2D top-down view train driving game, here’s … well, the things you need to consider plus a rudimentary source code example. Because a train following tracks is not as simple as it might seem, unless you restrict curves and switches to 90° angles and allow only very short cars and locomotives.
Here’s a video of my example project. Any stuttering is due to the screen recording software taking a toll on my system combined with the video playback framerate not rendering 60 fps (I assume it’s the standard 24 or 30 fps for Youtube videos). The video shows a sequence of three runs with a medium curve radius, a large curve radius and a ridiculously small curve radius. The yellow line indicates the track being followed by the axles, the purple line indicates the car chassis position (center point between axles). Continue reading »
Here’s a crazy thought: with commercial game kits (game source code products) being popular and financially rewarding – why not crowd-fund an iOS game by selling it’s source code, resources and development insights while you’re creating it?
Marcus and I will give this idea a spin. Marcus is a game designer I worked with at Electronic Arts Phenomic for 6 years. I’m sure you know me. Together we’re going to create a tilemap-based physics game using cocos2d and KoboldScript (Lua scripting for cocos2d). And we are going to sell everything we’ll create practically from day one.
If that sounds even slightly intruiging to you, we’d love to get your feedback!
Visit the launch page and take our survey which has already helped us tremendously to focus on what’s important for you. For example I’ve converted the entire KoboldScript library to use ARC seeing how important ARC is to you.
But do keep on reading for more details … Continue reading »
Scheduled for release on November 7th, 2011. Continue reading »
I recorded a screencast to give you a first impression of the current alpha version of Kobold2D. I’ll show you what is is, what it includes (so far) and how it improves the cocos2d development process.
Since recording the screencast I’ve added the Doodle Drop project from the book as an iOS game template project. I also added the Box2D & Chipmunk physics projects from the book and improved them to work on both iOS and Mac OS X.
Update
Here’s a screenshot showing the current list of template projects (Hello Kobold2D, Hello Cocos3D, Hello Cocos2D-X, Physics Box2D, Physics Chipmunk, Physics Chipmunk SpaceManager, Doodle Drop) and the Doodle Drop template running:
