Pygame Platformer Part 3: Gravity and Platforms

Key Takeaways

In this episode of game development with Pygame, we're going to continue with part three of our platformer game. In this video, we will learn how to add gravity and also some platforms that our player can land on. In the last video, we added our player sprite. Um, worked on the movement so we have a kind of a realistic movement with a little bit of inertia, friction slowing us down uh when we let go of the key. Um, and we only did side to side movement because obviously for this we're going to want to have downward movement be caused by gravity. Now, gravity itself is going to be really easy to do. Uh, we just want an acceleration downwards. Now, of course, as soon as I add gravity, we are going to fall off the bottom of the screen. So, we're going to need to add some platforms pretty quick. But, let's go ahead and talk about how we do gravity. So over in our player sprite, we have in our update section, so the part of the sprite that runs every frame of the animation, um we're setting the acceleration in the x direction and in the y direction to zero. So for gravity, we want an acceleration in the y direction, uh positive y direction. So we could put say 0.5 here. So every frame we will be accelerating downwards, right? So you want that to be an acceleration, not a velocity because we don't fall at a steady speed. We accelerate. That's what will look realistic. Okay. So let's see what that looks like. All right. Now, you might notice I'll run it one more time. I don't look like I'm accelerating. I'm falling at a steady speed. Now, can you guess why that is? has to do with what we did last time. Remember when we added the friction? So, we actually have friction slowing us down, basically putting a a maximum on our speed in the y direction. But we don't want that. We want this friction to only apply in the x direction. So, we're going to have just the acceleration in the x direction um affected by friction. And that will make gravity work a little more like we expect it to. There. See how I fell off the bottom of the screen. Okay. So, before we can do even begin to talk about jumping, uh we're going to need to have uh some platforms to land on, right? And eventually, we're going to want to have multiple platforms, but to start with, uh we need to put something that we can land on uh at the bottom of the screen, sort of like the ground of the of our world. So, I'm going to hide my player class for the moment, and we're going to jump down here to the bottom, and we're going to add a class platform. Now, platforms are going to be pretty simple, uh, at least to start with, uh, because all we really want them to do, underscores there, all we really need is when we spawn a new platform, we're going to want to specify where on the screen it is. So, we're going to give it an X and a Y, and we're going to give it a a width and a height for how big we want that platform to be. So, every time we make a new platform, we give it an X and Y location and a width and a height. So um so what we're going to do is first we're going to create the image and that's going to be a surface uh using the width and height. I'm going to just use green for my color. that I'll fill the image with. And then the rect. And we just want to place the rectangle at the at the x and y we specified. Okay. And that is our entire platform class. That's all we need to make the platform work. So, if we come over to our main uh and we go to our new, we're going to be able to spawn a platform. But I'm also while I'm here going to add a platforms group um to hold all the platforms so that we can do collisions just like we did in the schmup where we had all of our mobs in one uh group to do the collision very easily. So So we're going to spawn a platform. We're going to add it to the sprites and to the platforms. Now to start with I'm just going to manually spawn one. Okay. Um, and this is going to be a platform. And remember, we need to give it the X, Y, width, and height. So, I want this to be at the bottom. So, I want the X to be zero. I want the Y to be height minus 40. So, a little bit up from the bottom. I want the width to be the same as the width of the screen. And I want the height to be 40 pixels. Okay. And then I'm going to do uh go ahead and add it to the groups. So it'll be ready to work with our collisions. So that should give us a platform down here at the bottom. 40 pixels tall, width of the width of the window wide. So now we're ready to start talking about collisions. So if we go over here and look at our player sprite, um remember we have our acceleration, our velocity, and our position, right? And those are being calculated using those equations of motion. And at the end of all that calculation, we have whatever our position of our sprite should be. And we're using that position variable to set the center of our player's rectangle. So it puts the sprite wherever the position has been calculated that it should be. And that's okay and it works fine. But it is does mean that if we look at this diagram that I drew that we are going to have to do a few other things. So, if here's our yellow player sprite, and when the player sprite falls and hits the platform, we want to put the bottom of the player against the top of the platform, right? But that means that we want to put the players y position, we want to put this dot this far above the top of the of the platform. So, we're going to have to use some sort of formula like this where the y position of the player is equal to the top of the platform minus half the height of the player. And that's kind of cumbersome and it really doesn't make a whole lot of sense to track the center since most of the time when we're standing on things, we're talking about the feet of the player. So, we might as well do it like this. So instead of using that center position, we're going to use this position of the rectangle, which is called the mid bottom. So all we have to do when we collided with the platform is set the player's Y position equal to the top of the rectangle uh of the rectangle of the platform. And that is a lot clearer and simpler and we don't have to deal with all those height over two problems. So all we have to do to change that is just go right here and set this to mid bottom. So now our position calculated position will always be the mid bottom of the player. And then we're ready to start trying the collisions. So if we go over here to our update section of our game loop, we're just going to do a collision check. We're going to do the collision check between the player and the platforms. And we're going to put false. Remember that tells you whether or not to delete the item you ran into. And we don't want to delete the platforms when we run into them. So, if we hit something, then we want to take this the player's Y position and set it equal to whatever we hit its top. And you might think that that would be enough, but something weird is going to happen. I'm going to fall. And you see how I fall through the platform? I'll run it one more time so you can see it. I sort of sink through there. Now, why does that happen? Well, if you think about what's happening, remember we have our our velocity and our acceleration are continuing to be calculated, right? And we because we keep accelerating, we have this acceleration. So, when we hit that when we hit that platform, our velocity is not zero. Our velocity is some number greater than zero because we're falling. And even though we stopped the rectangle and put it at the top, we didn't set our velocity to zero. So our velocity is going to continue to grow because of the acceleration. So our velocity keeps getting bigger and bigger. So we keep moving even though we said to stop. So we need to make sure we also tell set the players y velocity equal to zero. That way we can fall and stop moving and we can run along the platform, but we don't fall through it. So, I know the last couple of videos have been kind of long and we're always I always try to keep them around 10 minutes and we've just crossed the 10-minute mark now. So, um, just one other thing that I wanted to do was, um, I'm going to add one more platform just so that we have something else to look at. Okay. And so, this platform is going to be I'm going to put it, uh, width over two, uh, say minus 50, just so it's not right at the center. Um, I'm going to put the y at height time 3 over 4, 3/4 of the way. Um, oops. And I'm going to put the width to 100 and the height to say 20. Okay. Now, you can see I have to add it to the groups. And in an upcoming video, we'll talk about a cleaner way to do this. So, we don't have to keep doing all the repeating all of this. added to the group, added to the group, and and we're not going to manually create a whole bunch of platforms, but just for now, just have a second one. It's okay to do it uh temporarily. So, I just wanted to show you that if we had if we had add here, if we had two platforms on the screen, then see, we'll land on the one we hit. And if I walk off the side, I'll fall through it. Just so you see how it works with the platforms. Okay. And I will stop it there. So you can experiment with your gravity. You could make your gravity lower or higher. Right? Go in and change this uh 0.5 right here. Um if you make that number bigger, um you're going to fall faster, right? You have a stronger gravity. And if you make it lower, you can pretend like you're on the moon uh and fall slower. Um, so in the next video we'll actually get started having jumping. All right, thanks for watching.