Avoiding Common Physics And Collision Detection Pitfalls In Game Loops

Integrating Physics Engines

Physics engines provide simulated physics for games, allowing realistic interactions between game objects. However, integrating a physics engine into a game’s update loop can introduce challenges. Care must be taken to properly interleave the engine with existing game logic and render cycles.

When integrating a physics engine like Bullet or PhysX, first determine the level of physics fidelity needed. Complex simulations require more processing power. Often a basic rigid body engine offers sufficient realism.

Next, decide where physics simulation should occur in the update cycle. Performing physics before game logic allows logic to respond to collisions. But doing physics after allows logic to configure physics objects.

Furthermore, structure code to compartmentalize physics into a separate module. Abstracting physics into a separate layer keeps complexity contained and interfacing clean. Utilize engine callbacks rather than polling status. Cache transforms for improved performance.

Common Pitfalls When Implementing Collision Detection

Forgetting to Check for Collisions Every Frame

A common mistake when coding collision detection is failing to check for collisions every frame. Game state can change dramatically between frames. Objects in motion cover large distances rapidly. Checking only occasionally all but guarantees missed collisions.

Solving this requires recognizing that collision detection is integral to properly simulating physics. A separate physics update phase that interleaves with rendering and game logic is highly advisable. Every pass should poll for new collisions by intersecting bounds or directly comparing object positions.

Not Accounting for Fast-Moving Objects

Fast kinetic motion combined with low frame rates is a recipe for problematic collisions. With objects moving rapidly across large bounding boxes, collisions get missed as objects apparently jump over each other between frames.

Solutions include smaller time steps in physics simulation to minimize per-frame motion, interpolating movement rather than teleporting objects during position updates, or checking swept volumes along the path of motion rather than static bounds.

Ignoring Trigonometric Errors

Implementing collision detection using naive trigonometric functions results in subtle errors due to floating point precision limitations. Small discrepancies escape notice until problems manifest.

Robust code relies on swept and expanded volumes with tolerances, not raw float comparisons. Precise intersection tests give way to paranoid boundary inflation. origins shift toward centroids to minimize trig effects. Graceful handling of degeneracies and parallel edges also helps produce reliable results.

Allowing Collisions Between All Objects

Carelessly permitting every game object to collide with every other quickly creates performance issues. Incremental all-to-all collision testing scales O(n^2) in complexity. 100 objects requires 10,000 checks!

Optimization mandates early filtering by layers and coarse broadphase passes. Fine resolution intersection only occurs between object subsets likely to collide. Categorize objects by type into organized layers. Perform broadphase across layers before engaging narrowphase intra-layer tests.

Strategies to Optimize Collision Detection

Layered Approaches

As noted above, effective optimization divides objects into layers filtering unlikely collisions. Player characters collide against level geometry but not other players or distant decorations. Projectiles strike physical bodies but ignore fluids and field effects.

Layers partition collision domains into fast rough checks between layers, avoiding exponential all-combinations explosion. Important collisions get dedicated layers.Infrequent intersecting layers check only boundary boxes as accelerator filters. Expected collisions then flow into focused narrowphase collision queries between their members..

Spatial Partitioning

Spatial partitioning further accelerates collision detection through object localization. Subdividing worlds into cells allows rapidly rejecting distant objects as candidates. Cells checklist neighboring cells rather than all objects.

Common structures include octrees extending bounding boxes, BSP binary space partitioning planes, and regular n-dimensional grid subdivision. Dynamic indexes require updating moving objects. Tradeoffs exist balancing build/refresh costs versus culling efficiency.

Bounding Volumes and Boxes

Tight-fitting bounding volumes attached to objects facilitate fast broadphase rejection through intersecting proxies. Testing bounds proves cheaper than detailed mesh intersections. Volumes include spheres for rotation invariance or boxes aligned to axes.

Boxes shine for dense clustered objects. Containment within common regions simplifies inside/outside checks. Applications subdivide large worlds into small cells, each cell managing local contents. Some games integrate multiple strategiescombining boxes, grids, partitions adapted to each area’s demands..

Examples in Code

Basic Collision Detection Function

function checkCollisions() {

  // Loop through all objects
  for(let a = 0; a < objects.length; a++) {

    // Compare against others 
    for(let b = a + 1; b < objects.length; b++) {
    
      // Check boundaries for overlap
      if(objects[a].bounds.overlaps(objects[b].bounds)) {  

        // Precise intersection test
        if(objects[a].shape.intersects(objects[b].shape)) {
          onCollision(objects[a], objects[b]); 
        }
      }
    }
  }
}  

Optimizations with Quadtrees

function checkCollisionsWithQuadtree() {

  // Quadtree cells with objects
  let cells = quadtree.allCells(); 
  
  // Loop over each cell  
  for(let i = 0; i < cells.length; i++) {
  
    // Intersections only within cell
    let objects = cells[i].objects();
    
    for(let j = 0; j < objects.length; j++) {
      for(let k = j + 1; k < objects.length; k++) {
        if(objects[j].intersects(objects[k])) {
          onCollision(objects[j], objects[k]);
        }
      }
    }  
  }
}

Bounding Spheres

function checkBounds(a, b) {

  // Bounding sphere centers and radii
  let ac = a.center(); 
  let ar = a.radius();
  let bc = b.center();
  let br = b.radius();

  // Distance between spheres
  let dist = ac.distanceTo(bc);  

  // Check if spheres overlap
  return dist < (ar + br);
}

function checkCollisions() {

  // Loop over objects
  for(let i = 0; i < objects.length; i++) {   
    for(let j = i + 1; j < objects.length; j++) {

      // Broadphase sphere check    
      if(checkBounds(objects[i], objects[j] {

        // Narrowphase intersection test
        if(objects[i].intersects(objects[j])) {
          onCollision(objects[i], objects[j]);
        }
      }
    }
  }
}