libGDX- Exact collision detection - Polygon creation?

Question

I've got a question about libGDX collision detection. Because it's a rather specific question I have not found any good solution on the internet yet.

So, I already created "humans" that consist of different body parts, each with rectangle-shaped collision detection.

Now I want to implement weapons and skills, which for example look like this:

Skill example image

Problem

Working with rectangles in collision detections would be really frustrating for players when there are skills like this: They would dodge a skill successfully but the collision detector would still damage them.

Approach 1:

Before I started working with Libgdx I have created an Android game with a custom engine and similar skills. There I solved the problem following way:

1. Detect rectangle collision
2. Calculate overlapping rectangle section
3. Check every single pixel of the overlapping part of the skill for transparency
4. If there is any non-transparent pixel found -> Collision

That's a kind of heavy way, but as only overlapping pixels are checked and the rest of the game is really light, it works completely fine.

At the moment my skill images are loaded as "TextureRegion", where it is not possible to access single pixels. I have found out that libGDX has a Pixmap class, which would allow such pixel checks. Problem is: having them loaded as Pixmaps additionally would 1. be even more heavy and 2. defeat the whole purpose of the Texture system.

An alternative could be to load all skills as Pixmap only. What do you think: Would this be a good way? Is it possible to draw many Pixmaps on the screen without any issues and lag?

Approach 2:

An other way would be to create Polygons with the shape of the skills and use them for the collision detection.

a) But how would I define a Polygon shape for every single skill (there are over 150 of them)? Well after browsing a while, I found this useful tool: http://www.aurelienribon.com/blog/projects/physics-body-editor/ it allows to create Polygon shapes by hand and then save them as JSON files, readable by the libGDX application. Now here come the difficulties:

• The Physics Body Editor is connected to Box2d (which I am not using). I would either have to add the whole Box2d physics engine (which I do not need at all) just because of one tiny collision detection OR I would have to write a custom BodyEditorLoader which would be a tough, complicated and time-intensive task
• Some images of the same skill sprite have a big difference in their shapes (like the second skill sprite example). When working with the BodyEditor tool, I would have to not only define the shape of every single skill, but I would have to define the shape of several images (up to 12) of every single skill. That would be extremely time-intensive and a huge mess when implementing these dozens of polygon shapes

b) If there is any smooth way to automatically generate Polygons out of images, that could be the solution. I could simply connect every sprite section to a generated polygon and check for collisions that way. There are a few problems though:

• Is there any smooth tool which can generate Polygon shapes out of an image (and does not need too much time therefor)?
• I don't think that a tool like this (if one exists) can directly work with Textures. It would probably need Pixmaps. It would not be needed to keep te Pixmaps loaded after the Polygon creation though. Still an extremely heavy task!

My current thoughts

I'm stuck at this point because there are several possible approaches but all of them have their difficulties. Before I choose one path and continue coding, it would be great if you could leave some of your ideas and knowledge.

There might be helpful classes and code included in libGDX that solve my problems within seconds - as I am really new at libGDX I just don't know a lot about it yet.

Currently I think I would go with approach 1: Work with pixel detection. That way I made exact collision detections possible in my previous Android game.

What do you think?

Greetings Felix

Answer 1

I, personally, would feel like pixel-to-pixel collision would be overkill on performance and provide some instances where I would still feel cheated - (I got hit by the handle of the axe?)

If it were me, I would add a "Hitbox" to each skill. StreetFighter is a popular game which uses this technique. (newer versions are in 3D, but hitbox collision is still 2D) Hitboxes can change frame-by-frame along with the animation.

_{Empty spot here to add example images - google "Streetfighter hitbox" in the meantime}

For your axe, there could be a defined rectangle hitbox along the edge of one or both ends - or even over the entire metal head of the axe.

This keeps it fairly simple, without having to mess with exact polygons, but also isn't overly performance heavy like having every single pixel being its own hitbox.

Answer 2

I've used that exact body editor you referenced and it has the ability to generate polygons and/or circles for you. I also made a loader for the generated JSON with the Jackson library. This may not be the answer for you since you'd have to implement box2d. But here's how how I did it anyway.

/**
* Adds all the fixtures defined in jsonPath with the name'lookupName', and 
* attach them to the 'body' with the properties defined in 'fixtureDef'. 
* Then converts to the proper scale with 'width'.
*
* @param body the body to attach fixtures to
* @param fixtureDef the fixture's properties
* @param jsonPath the path to the collision shapes definition file
* @param lookupName the name to find in jsonPath json file
* @param width the width of the sprite, used to scale fixtures and find origin.
* @param height the height of the sprite, used to find origin.
*/
public void addFixtures(Body body, FixtureDef fixtureDef, String jsonPath, String lookupName, float width, float height) {
  JsonNode collisionShapes = null;
  try {
    collisionShapes = json.readTree(Gdx.files.internal(jsonPath).readString());
  } catch (JsonProcessingException e) {
    e.printStackTrace();
  } catch (IOException e) {
    e.printStackTrace();
  }
  for (JsonNode node : collisionShapes.findPath("rigidBodies")) {
    if (node.path("name").asText().equals(lookupName)) {
      Array<PolygonShape> polyShapes = new Array<PolygonShape>();
      Array<CircleShape> circleShapes = new Array<CircleShape>();

      for (JsonNode polygon : node.findPath("polygons")) {
        Array<Vector2> vertices = new Array<Vector2>(Vector2.class);
        for (JsonNode vector : polygon) {
          vertices.add(new Vector2(
            (float)vector.path("x").asDouble() * width,
            (float)vector.path("y").asDouble() * width)
            .sub(width/2, height/2));
        }
        polyShapes.add(new PolygonShape());
        polyShapes.peek().set(vertices.toArray());
      }

      for (final JsonNode circle : node.findPath("circles")) {
        circleShapes.add(new CircleShape());
        circleShapes.peek().setPosition(new Vector2(
          (float)circle.path("cx").asDouble() * width,
          (float)circle.path("cy").asDouble() * width)
          .sub(width/2, height/2));

        circleShapes.peek().setRadius((float)circle.path("r").asDouble() * width);
      }

      for (PolygonShape shape : polyShapes) {
        Vector2 vectors[] = new Vector2[shape.getVertexCount()];
        for (int i = 0; i < shape.getVertexCount(); i++) {
          vectors[i] = new Vector2();
          shape.getVertex(i, vectors[i]);
        }
        shape.set(vectors);
        fixtureDef.shape = shape;
        body.createFixture(fixtureDef);
      }

      for (CircleShape shape : circleShapes) {
        fixtureDef.shape = shape;
        body.createFixture(fixtureDef);
      }
    }
  }
}

And I would call it like this:

physics.addFixtures(body, fixtureDef, "ship/collision_shapes.json", shipType, width, height);

Then for collision detection:

public ContactListener shipsExplode() {
  ContactListener listener = new ContactListener() {

    @Override
    public void beginContact(Contact contact) {
      Body bodyA = contact.getFixtureA().getBody();
      Body bodyB = contact.getFixtureB().getBody();

      for (Ship ship : ships) {
        if (ship.body == bodyA) {
          ship.setExplode();
        }
        if (ship.body == bodyB) {
          ship.setExplode();
        }
      }
    }
  };
  return listener;
}

then you would add the listener to the world:

world.setContactListener(physics.shipsExplode());

my sprites' width and height were small since you're dealing in meters not pixels once you start using box2d. One sprite height was 0.8f and width was 1.2f for example. If you made the sprites width and height in pixels the physics engine hits speed limits that are built in http://www.iforce2d.net/b2dtut/gotchas