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.