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GameMaker tutorial: high speed collisions

If you have a small object, such as a bullet, collisions can be problematic for GameMaker if the bullet’s speed is faster than the length of the bullet.  Any time an object moves faster than its width, it is possible for the object to skip over object in its way that it should have collided with.

The illustration below shows why:

high speed collision skipping

Here’s a simple, elegant solution for your high speed object to avoid this problem, regardless of its top speed.

Make the bullet sprite’s length as long as the bullet’s speed.  If your bullet moves 16px per step, then its sprite should be 16px long.

If the bullet’s speed is variable, then the thing to do is vary the length of the bullet as the speed of the bullet varies.  You can use this using the image_xscale property to scale the bullet.

Say the base bullet speed is 16px.  If the bullet can somehow accelerate to 32px per step, then it should lengthen to 32px, by setting the image_xscale to 2.  We can see from this that a general expression that solves the problem for any sprite width and speed would be:

image_xscale = speed/sprite_width

But it’s probably a good idea to use image_xscale = max(1, speed/sprite_width), to keep the bullet from shrinking if it is moving slowly.

A nice thing about this effect is that it makes high speed bullets look like elongated streaks, very similar to a blurry action photo where the subject of a photo is moving too fast for the shutter speed. In effect, that’s exactly what’s happening in GameMaker, if we consider the game speed to be like the shutter speed of a camera. So this creates a very natural looking visual effect, and solves a problem with high speed collision skipping.

This technique may be applicable to other objects besides bullets, but it works best with smaller objects because the stretching isn’t as noticeable.  As the instance is moving at a high speed while being stretched, it can trick the eye into thinking it’s just a motion blur effect.  With larger objects, this illusion is not as convincing, and the apparent distortion is more obvious.

Pinpointing the point of impact

Let’s say you need to know the exact point of impact from your high speed bullet. Since the bullet is stretched out, the x, y coordinates are no longer approximate enough.

high speed bullet point of impact

We an use point_direction to find the angle at which the impact occurred.

point_of_impact = point_direction(other.x, other.y, bullet.x, bullet.y);

If the target object is a circle, then calculating the point of impact is simple from whatever angle:
impact_x = other.x + lengthdir_x(other.radius, collision_direction);
impact_y = other.y + lengthdir_y(other.radius, collision_direction);

The above will work as long as the high-speed bullet isn’t positioned such that the x,y origin of the sprite is on the far side of the target object. If it is, the point of impact will be on the opposite side of the target object. If this is a problem, you can do additional calculations using the direction of the bullet’s travel as a clue to determining where the true point of impact was. You can also reduce the problem by making the bullet sprite’s x-origin 0, so the bullet’s [x,y] position will always be at the rear of the bullet.

If the target object is non-circular, or non-centered, you’ll need to do a bit more work to determine the point of impact, and exactly how to do that is outside the scope of this article. But using built-in variables image_xoffset, image_yoffset, bbox_top, bbox_bottom, bbox_left, bbox_right, you should be able to figure it out, at least approximately enough to be useful.

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