Better collision detection in part6-breakout

part6-breakout/README.md

@@ -85,8 +85,33 @@ i=8 : (i % zoom) = 0 -> advance the pointer
 ...
 ```
 
-Perhaps you can see why we changed our loop to count from 1 rather than 0?
+Perhaps you can see why we changed our outer loop to count from 1 rather than 0?
 
 If you want, you can now make these changes to _fb.c_ in your part5-framebuffer code and exercise them properly in _kernel.c_ to check that they work. Don't forget to update the function definition in _fb.h_ to include the `zoom` parameter too.
 
 It is also now trivial to modify `drawString` to take a `zoom` parameter and pass it through, so I won't document the changes here.
+
+Object tracking
+---------------
+
+As we can now draw text (e.g. a score/lives counter), rectangles (paddle & bricks) and circles (ball), we can recreate the Breakout game screen. Check out `initBall()`, `initPaddle()`, `initBricks()` and `drawScoreboard(score, lives)` in our new _kernel.c_.
+
+In addition to the graphics code, you'll see that we're keeping a record of each game object we create in the global `objects` array:
+
+ * its (x, y) coordinates
+ * its width & height
+ * what type of object it is (ball, paddle or brick)
+ * whether it's "alive"
+
+We also store a global pointer to the ball and paddle object, so they're easy to track down!
+
+As we'll need to knock out our bricks during gameplay, we create `removeObject(object)`, which simply draws a filled black rectangle over the object we pass, and sets its `alive` parameter to 0 to signal that it's now out of play.
+
+To know that our ball is about to hit a brick (or indeed the paddle), we'll need to detect **collisions**. We simply conduct a search of the alive objects and return the first object we find whose coordinates overlap. If no object is found, we return 0. `detectCollision(object, xoff, yoff)` implements this. Note that `xoff` and `yoff` can be negative since the ball could be travelling in any direction.
+
+Keyboard input
+--------------
+
+We'll be using the UART to take input, just like we did in part4-miniuart.
+
+`getUart()` simply checks if a key has been pressed and, if so, it returns the character, otherwise 0. We don't want this function to wait for a key, because gameplay needs to continue regardless.

part6-breakout/kernel.c

@@ -53,42 +53,23 @@ void moveObject(struct Object *object, int xoff, int yoff)
     object->y = object->y + yoff;
 }
 
-struct Object *objectAt(unsigned int x, unsigned int y)
+struct Object *detectCollision(struct Object *with, int xoff, int yoff)
 {
     for (int i=0; i<numobjs;i++) {
-        if (x >= objects[i].x && x <= objects[i].x + objects[i].width) {
-           if (y >= objects[i].y && y <= objects[i].y + objects[i].height) { 
-	      if (&objects[i] != ball && objects[i].alive == 1) {
-		 return &objects[i];
-              }
+	if (&objects[i] != with && objects[i].alive == 1) {
+	   if (with->x + xoff > objects[i].x + objects[i].width || objects[i].x > with->x + xoff + with->width) {
+              // with is too far left or right to ocllide
+	   } else if (with->y + yoff > objects[i].y + objects[i].height || objects[i].y > with->y + yoff + with->height) {
+              // with is too far up or down to ocllide
+	   } else {
+	      // Collision!
+	      return &objects[i];
 	   }
-	}
+        }
     }
     return 0;
 }
 
-struct Object *detectCollision(int xoff, int yoff)
-{
-    struct Object *collision;
-
-    unsigned int x = ball->x + xoff;
-    unsigned int y = ball->y + yoff;
-
-    collision = objectAt(x,y);
-    if (collision) return collision;
-
-    collision = objectAt(x + ball->width, y);
-    if (collision) return collision;
-
-    collision = objectAt(x, y + ball->height);
-    if (collision) return collision;
-
-    collision = objectAt(x + ball->width, y + ball->height);
-    if (collision) return collision;
-
-    return 0;
-}
-
 // KEY HANDLER
 
 unsigned char getUart()
@@ -207,7 +188,7 @@ void main()
        uart_loadOutputFifo();
 
        // Are we going to hit anything?
-       foundObject = detectCollision(velocity_x, velocity_y);
+       foundObject = detectCollision(ball, velocity_x, velocity_y);
 
        if (foundObject) {
           if (foundObject == paddle) {