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Rotating Hitboxes, images and more


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#1 Kelsys   Members   -  Reputation: 102

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Posted 22 September 2012 - 12:11 AM

Okay, a few things to sort out. I am talking about a game called MapleStory, and an attacking skill in that game called Spectral Light.

I was wondering how the game handles the skill named Spectral light. It just boggles my mind at every turn, it seems like it'd be impossible to make.


This is the skill in question:
Posted Image

The skill works as follows: depending on the arrow key pressed, you can swivel the laser in nearly a 180 degree range about the player (who acts as the axis). The laser (and its corresponding hitbox) can then hit different monsters depending on its current position.

This is a video of how it works: http://www.youtube.com/watch?v=5x7eGzIQ0IY

What I am curious about is how the hitboxes, rotation and animation loop code work.

For a better illustration of what I am referring to, here is a graphical representation of what I mean by 'points' at which the laser can be fired from. I will be defining this 'points' term within the context of this topic for easier discussion purposes.
Posted Image
The animation in question is rather smooth and as such I suspect a large number of points (perhaps around 15-20).

1) Hitboxes (obviously a normal rectangle isn't going to work for the various angles; is it more efficient for gameplay for the coder to plot a new polygonal hitbox that is associated with every 'point' of rotation, or are there other methods? If there are, what are they and how do they work?

2) Rotation- The actual 'loop' of the laser is 12 frames long. Is it possible that there are 12 individual pictures in the data for every single 'point' from which Spectral Light can be fired? If this were true, there would be a staggering number of images within the data just for this one skill. Won't the method simply refuse to run/crash/suck up immense amounts of memory and then cause a crash? Does anyone have information how this could be done in an efficient manner (fast loading, no memory leak, no crashing the program)?

3) Animation loop code- I understand that in Flash you can have an animated image that can be rotated while it loops its animation- however MapleStory is made in C++ and not Flash, so I was wondering how they accomplished this.
Say frame 1 plays at point x, when you swivel to point y, how did the coder do it such that the game knows it must play frame 2 of point y and must not start again from frame 1 of point y? From the videos I've watched, the frames are looping as they should as you swivel the laser, and the laser's animation does not re-start from frame 1 when it is swiveled to a new 'point' of firing. How could this have been accomplished?

I find this entire skill very puzzling. I've been trying to figure out how they did this.

Edited by Kelsys, 22 September 2012 - 12:15 AM.


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#2 clb   Members   -  Reputation: 1796

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Posted 22 September 2012 - 12:28 AM

It is a standard technique/feature that you can render your 2D sprite rotated to an arbitrary angle in realtime. They just use a rotation matrix that they apply to the sprite rectangle vertices when rendering, and the GPU deals with the rotation and filtering in realtime, no problem. Rendering a sprite axis-aligned versus rendering it in an arbitrary angle does not even carry a performance penalty, it's the same performance for the GPU. As for the assets, you only need a single animation sequence for the effect in the axis-aligned position. Googling for "2d rotating a sprite" finds some good hits, e.g. this is an example of how to do it in SDL.


For hitboxes, the case is the same. They probably have defined a rectangle, or a polygon that marks the hit area of the effect. This vector shape is rotated to the appropriate angle from its default axis-aligned orientation before testing for collision against the vector shapes of the other objects. Since there's only a very few points needed to represent such a shape (perhaps 10 at most?), it's very cheap and could be done in realtime without performance implications even on a mobile device.
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