How to make a PhysicsBody based on Alpha Values

Question

Suppose there there is a scene as follows: There is a scene with the same size as the frame of the device. The scene has a red ball, which is able to move throughout the 'world'. This world is defined by black and white areas, where the ball is ONLY able to move in the area that is white. Here is a picture to help explain:

Parts of the black area can be erased, as if the user is drawing with white color over the scene. This would mean that the area in which the ball can be moved is constantly changing. Now, how would one go about implementing a physicsBody for the an edge between the white and black areas?

I tried redefining the physicsBody every time it is changed, but once the shape becomes complex enough, this isn't a viable solution at all. I tried creating a two-dimensional array of 'boxes' that are invisible and specify whether most of the area within each box is white or black, and if the ball touched a box that was black, it would be pushed back. However, this required heavy rendering and iterating over the array too much. Since my original array contained boxes a little bigger than a pixel, I tried making these boxes bigger to smooth the motion a little, but this eventually caused part of the ball to be stopped by white areas and appear to be inside the black area. This was undesired, since the user could feel invisible barriers that they seemed to be hitting.

I tried searching for other methods to implement this 'destructible terrain' type scene, but the solutions that I found and tried were using other game engines. To further clarify, I am using Objective-C and Apple's SpriteKit framework; and I am not looking for a detailed class full of code, but rather some pseudo-code or implementation ideas that would lead me to a solution.

Thank you.

Answer 1

If your deployment target is iOS 8, this may be what you're looking for...

+ bodyWithTexture:alphaThreshold:size:

Here's a description from Apple's documentation

Creates a physics body from the contents of a texture. Only texels that exceed a certain transparency value are included in the physics body.

where a texel is a texture element. You will need to convert an image to the texture before creating the SKPhysicsBody.

I'm not sure if it will allow for a hole in the middle like your drawing. If not, I suspect you can connect two physics bodies, a left half and a right half, to form the hole.