Grapple Hook Revisited

There was a request to go into some more detail for the grapple hook. Original Post

So what I've done is taken a fresh copy of the Platformer 4.0 solution and added only the Grapple Hook stuff. The ZIP of the solution is attached below.

Basically, there are 3 new classes and some changes to Player.cs

The three new classes are in the GrappleHook Folder:

Collision.cs - This contains the core functions used "under the hood".
DrawPrimitives.cs - This class contains some functions used to draw various shapes. The main method used is DrawLine2.
Grapple.cs - This class contains the guts of the grapple hook logic outlined in the initial blog post

Changes to Player.cs:
There are some new member variables and a few methods. I tried to comment everything that was new with "GRAPPLE HOOK"

I also added WhitePixel.bmp to the Content/Sprites/Test folder. This is used to draw the line. (Also removed the music to cut down the size of the zip)

A few changes to the logic and style of the Grapple Hook:
1. The grapple hook "rope" will now shoot out 500 pixels at a rate of 2000, which is pretty much instant.

2. The grapple hook will stay extended as long as you hold the button (instead of collapsing back to 0). This makes it easier to attach.

3. You can't shoot the grapple hook if isOnGround = true. This makes use a little more natural and prevents the odd bug where you would attach and then the grapple curve would force you under the world or the tile below you.

4. The swing itself is slightly slower (using 2000 as the gravity constant).

Places for Improvement:
1. Some games have the grapple hook continually "shrink" while it is attached (Hook Champ, etc). This gives the player the feel that the grapple is "elastic". You could implement this by moving the player slighly "up" the rope every tick, and then re-calculating the Swing Curve. The hard part is getting the velocities right so the swing is smooth, since we're not using actual angular velocities / momentum, but the path of the pendulum curve.

2. Prevent the Grapple hook from attaching to horizontal walls. This would require a change to the GrappleCollision function (and the getLineCollision function), to only attach if the Grapple "hook" is colliding through the lower X plane, instead of the Y plane.

3. Better formulas for angular velocities entering and exiting the swing. Right now I'm using some cheap hacks (delta position -> velocity) to determine the initial and launch velocities when starting or exiting the grapple swing. Some real physics might make it feel a little smoother.

Download: GrappleExample.zip

Playtest

A new version of Zombie Bash is in playtest.

I was hoping to get it to market sooner, but I figured you might as well do the thing right the first time, even if it takes a long time (The Blizzard philosophy). And its amazing how time consuming it is programming High Score screens to work just right...

Zombie Bash Playtest

It's a Trap!

Traps are essential for 2D games. They present a challenge for a player that can only be defeated by timing and patience, rather than reliance on gunpower.

I decided to build 3 different types of traps for Platform Hack. Moving Blocks, Spinning Blades, and Flame Turrets. Each of these uses slightly different technology to work, but since they were similar enough I crammed all the functionality into a single class, called Trap.



The Moving Block is the simplest type. Its basically just a moving tile that hurts the player on collision. Some games have blocks that "crush" the player, and can also provide solid collision from the other directions (top, sides). I decided against this type of falling block, since it would require implement a "crushing" system of player damage. Instead, if you touch the block from any direction, the player will be damaged. Good level design (encasing the falling block inside Impassible tiles) can be used in conjunction with the trap type to good effect.

The Collision is pretty simple:



The Update function contains the only interesting piece of logic, using Linear Interpolation to smoothly move up and then slam down as a funtion of the entire TrapTimer



The Spinning Blade is the next type of trap.

This uses slightly more advanced collision and update logic. Given the same TrapTimer, we do a full rotation of both the texture, and the Rotation Matrix thats use for pixel-based collision detection:

Update:



Collision:





Draw:



One of the "gotchas" of Translation matrices in XNA (or I guess any graphics framework), is being sure to multiple by a translation of the negative origin. Matrix.CreateTranslation(new Vector3(-p.Origin, 0.0f)) This is needed whenever the texture is rendered using an origin. You need to adjust the Translation Matrix in the same way, so that the collision detection is "synched" with the drawn texture.

The collision detection with rotation is right out of the XNA tutorial found here

Lastly, the most complex type of trap is the Flame Turret.

The Flame Turret is using both Beam collision and the particle system for effect.

Beam Collision:

The benefit of using beam collision is if you have something like bullets that travel quickly. If the bullet moves 30 pixels in a single update, is has the chance to pass right through an enemy without causing collision. a Beam object can be used to represent a "line segment" in order to do collision. Of course, you can always Draw the single dot as the bullet, it just uses the Beam object to do the collision.

Here's my Beam struct:



There are two vectors, A and B. There is also a list of coordinates between the beams, which can be calculated with the following method. Note: this may not be an optimal method, comments welcome.



A beam can represent either a bullet moving quickly across the screen, or a burst of flame shooting out of a trap turret.

We can determine if the beam hits a player (or enemy) using the following collision detection method (again, possibly not optimal):



The last piece of the puzzle is a method to get a bounding rectangle around the beam. You *always* need to do bounding rectangle collision before pixel-based collision.



We can check for collision between the player and the beam in the following way:



Finally, we want our Flame Turret to look cool. This can be done using Particle Effects.

The particle engine I've used is heavily modified from the Particle Sample. The details are a topic for another post.

But briefly, we spawn fire particles along the length of the beam:



That's pretty much it for simple traps. With good art and creative level design, you can turn these three features into some pretty dastardly challenges for the player.

Web 2.0 Stuff

My game - Zombie Bash - is currently in Playtest for XBox Live Indie Games: http://forums.create.msdn.com/forums/t/78185.aspx

Also, I caved and created a twitter account.  I figure its really the only way to spread the news about the games I'm building outside the small group of people I know in RL who would care ;)

Follow me at @daydalus!

Our Hero!

We have the first test sprites of our hero in action!

Optimization!

I finally got around to purchasing the Creators Club Membership.

The main benefit is I can deploy the games I make to XBox 360, and after Peer Review, have them show up on the XBox Live Indie Games channel.

The set-up and deployment process was pretty painless, but once I loaded up Zombie Bash, I noticed something bad.

The game was incredibly slow! I'd be surprised if I was getting 1 frame per second, especially when the level first started and a bunch of enemies spawned.

The game ran fine on the computer, but the XBox has much weaker hardware. Also, the way XBox environment works under the hood (like disk IO) is different as well.

After doing some research and reading the forums, I came away with some suggestions to try:

1. Run the Xbox Remote Performance Monitor when the game is running on Xbox.
This is provided with the XNA Game Studio under Tools. I was able to get it up and running and look at some of the results in real-time. Unfortunately, the specifics that are profiled were a bit too low level to be of much help. I could tell that the Garbage Collection on the XBox wasn't keeping up with all the objects I was spawning, but why?

2. Run a .NET CLR profiler as you run the game on the computer.
I was able to download the Open Source tool SlimTune and configure it to profile my game. This turned out to be a lot of help. SlimTune actually attached to the CLR and uses Reflection to determine which methods are using the most processing time. This quickly points out the functions and calls that are slowing down the game. (Note: This program can often crash when testing XNA games, however it will still save the profiling to a file).

Which leads us to point number 3 -

3. Look for instances in the code where expensive operations are being done in real-time.
The number one difference between programming games and regular business apps - the same game code runs dozens (hopefully 60+) times a second. If there are any expensive operation in any of the loops, even if they occur once or twice (aka - loading assets), they can slow the entire thing down and lead to a bad gaming experience.

This was the culprit. Every time a new Enemy spawned, I was loading all its sprite textures from disk. This had been fine in the sample code, because the enemies were spawned once when the level was loaded. But Zombie Bash has Enemy Spawners, and dozens of enemies can pop out every few seconds. While the computer can easily handle that kind of disk IO, the XBox was choking.

There were a few other objects (weapons, particles) where the same thing was being done - realtime loading of texture assets into the newly instantiated object.

What I did was create a Loader class that is run once when the game first starts. This goes through and reads all the textures, and also calculates the required bounding boxes and Color arrays needed for collision. It then holds on to them in memory for whatever object needs them down the line (Enemies, Weapons, etc). It remains to be seen if this will lead to issues if the game gets sufficiently large (hundreds of enemies, etc)

Load them (textures and animations) once and save them for later!



A similar approach was taken for the particle system. Each individual particle was calculating its own Sin and Cos curves when initialized in order to follow curved paths. By moving those calculations to the parent class, and simply passing in a pre-initialized Curve object to each individual particle, the particle system was greatly improved!

This is incredibly expensive to do 50 times at once!



To sum up:

-The XBox is much weaker than a computer
-Make sure you don't load or calculate anything expensive during the game loop. Do it before!

For the Love Of...

Great write-up on the appeal of Rogue-likes: http://www.gearfuse.com/unevenly-distributed-how-i-learned-to-stop-worrying-and-love-the/