WEBVTT 00:00.120 --> 00:00.660 Hello again! 00:01.020 --> 00:05.610 In this video, we are going to add interaction between the ball and the paddle to our game. 00:07.110 --> 00:13.290 To do this, we need to be able to detect whether the ball and the paddle have actually collided. 00:13.350 --> 00:17.070 And we also need some way to say, what will happen when that occurs. 00:18.710 --> 00:22.880 We are going to add a function, handle_collision(), which will make the ball bounce off the paddle. 00:23.540 --> 00:25.960 This will modify the ball, by changing its direction. 00:25.970 --> 00:27.650 So we take that by reference. 00:28.430 --> 00:36.950 The paddle is not modified, so we take that by const reference. And then later, when we add bricks to the game, 00:37.550 --> 00:40.940 we need a function which will make the brick disappear when the ball hits it. 00:41.570 --> 00:43.790 So we have an overload for the ball and the brick. 00:45.650 --> 00:50.930 We also need to detect whether the ball and the paddle are colliding. 00:51.500 --> 00:56.510 And in fact, this is true for any entity so we can write a polymorphic function. 00:56.840 --> 01:03.680 This takes the base class, entity, by reference, and then we can pass any subclass of entity, for either 01:03.680 --> 01:04.220 argument. 01:04.730 --> 01:08.420 And this will return true if the two entities overlap with each other. 01:09.620 --> 01:16.640 In C++ we only make functions a member the function if it is part of the class interface, and obviously 01:16.640 --> 01:17.560 that is not the case here. 01:17.570 --> 01:20.120 So we make these non-member functions. 01:21.830 --> 01:26.030 So that means that these non-member functions will need to change the direction of the ball. 01:26.420 --> 01:30.170 So we need to add some member functions which will do that. 01:30.650 --> 01:35.480 So we have functions to make the ball move up, and to make it move to the left, or to the right. 01:38.420 --> 01:45.650 So here is out entity header again. We have added the declarations of these member functions. 01:45.650 --> 01:47.390 We are going to make them pure virtual. 01:47.810 --> 01:50.030 So every subclass will have to implement them. 01:52.140 --> 01:56.600 Then in the ball class we implement these. To make the ball move up, 01:56.610 --> 02:04.740 we make the "y" member of the velocity negative. To make the ball move to the left, we make the "x" component negative. 02:05.370 --> 02:09.090 And to make it move to the right, we make the "x" component positive. 02:10.740 --> 02:13.260 And we also need to implement these for the ball. 02:13.830 --> 02:17.880 The paddle cannot move upwards, so we just leave that as an empty member function. 02:18.840 --> 02:24.090 And you could argue that these member functions really should belong in the ball class and not be in the entity. 02:24.810 --> 02:27.180 So again, that is an architectural decision. 02:27.720 --> 02:30.030 And in fact, I have made the exact opposite decision 02:30.030 --> 02:34.950 to the one I made in the last video, so I am afraid I am being rather inconsistent here! 02:37.540 --> 02:44.520 We have a new header where we declare these interaction functions. And then we have a source code file 02:44.540 --> 02:44.920 for them. 02:46.800 --> 02:54.030 To detect interactions, we can actually use it an SFML feature. So we get the bounding box for 02:54.030 --> 03:00.090 each of the entities. And then we can call this member function, intersects(). And this will return true 03:00.090 --> 03:03.690 if the two boxes overlap each other, otherwise false. 03:05.490 --> 03:11.610 And then in our function for the collision, we first of all check whether the paddle and the ball are 03:11.850 --> 03:14.820 overlapping. And if they are not, then there is nothing to do. 03:16.260 --> 03:18.630 If they are, then we make the ball bounce upwards. 03:18.630 --> 03:23.640 So, for the ball to hit the paddle, it must have been coming down, so we can now make it go up, to bounce 03:23.640 --> 03:24.000 off it. 03:24.720 --> 03:28.080 And then we decide whether the ball is going to bounce to the left or the right. 03:29.070 --> 03:34.200 So if the ball is on the left of the paddle, then we make it bounce to the left. 03:34.470 --> 03:36.180 Otherwise we bounce to the right. 03:38.280 --> 03:40.890 And then finally we need to add this code to the main function. 03:40.890 --> 03:42.540 So we include the header. 03:44.630 --> 03:50.240 And then when we do the update graphics, we add a call to this handle_collision(). 03:50.660 --> 03:52.780 So this will update the graphics. 03:52.790 --> 03:54.710 So the ball is now going in the right direction. 03:55.160 --> 03:59.120 If it has collided with the paddle. And there we are. 03:59.130 --> 04:01.790 So, yes, it does bounce off the paddle. 04:05.360 --> 04:11.450 So, if it bounces off the left, it goes to the left, and if it bounces off the right hand side of the 04:11.450 --> 04:11.900 paddle. 04:12.770 --> 04:14.960 (Sorry, it is very difficult to do this and talk at the same time!) 04:15.920 --> 04:18.590 So if it is... there is the left. 04:22.140 --> 04:24.840 There is the right. And the right again. 04:25.350 --> 04:27.060 Okay, so that is it for this video. 04:27.840 --> 04:28.650 I will see you next time. 04:28.860 --> 04:31.050 Until then, keep coding!