Game Design Game Development Linux Game Development

My May #1GAM’s Progress

I’ve been trying to use my participation in One Game a Month to take each game project and try to learn one new aspect of game development in its development.

One thing I’ve never done is make a platformer. I figured that it would take some time to figure out the jumping physics and timings and collision detection with the ground versus a wall versus a floating platform, and I had no interest in pursuing it during a Ludum Dare compo.

That said, my project for May was meant to be an attempt at capturing the feel of a game I used to play a lot as a child. Frogs and Flies for the Atari 2600 used to keep my sister and me occupied for many an afternoon. Looking at Google Image Search, I can see that I’m not the only one who has ever wanted to make a game like it.

Oh, well. The important thing is that this is an opportunity to limit the scope of the game while I try to code up some jumping physics. The ground is static. There are no platforms to leap onto.

Unfortunately, due to day job crunch and various other responsibilities, I’ve only been able to dedicate a few hours to this project all month.

You can see the current status in this May #1GAM demo video, as it would be more interesting to see a box jumping around than a still screen:

I had some trouble with the jumping being inconsistent. Since I was using fixed-time steps, it made no sense to me why one jump would reach a different height than another.

Then I found that I wasn’t handling the jump states very well.

Jumping has three states: NOT_JUMPING, JUMPING, and FALLING.

In my code, if you hit and hold the spacebar, the state would be JUMPING and you could control how high you jump by how long you hold the key down.I f you let go of the spacebar, the state would change to FALLING. Once you hit the ground, the state changed to NOT_JUMPING.

For some reason, holding the spacebar down for the entire jump resulted in inconsistent heights reached. Imagine playing Super Mario Bros and getting frustrated that taking the same action results in randomly not being able to jump high enough to get to the next platform. What gives?

The video shows a white and gray box on the left side to indicate the max jump height and the last jump height respectively as I tried to figure out if it was a real or merely perceived problem.

I realized what the problem was eventually. Gravity was always being applied to the vertical velocity of the frog, even when it was on the ground. So in one update, the frog’s position hasn’t changed, but it’s velocity has. The next update, the position would change due to the velocity, and so when it fell below the ground, I’d reset the velocity and the position.

So while the player wouldn’t see any change in position, as it is a static box, there’s a 50% chance that the spacebar would be pressed while the player’s velocity is lower than 0, and the velocity of a jump was added to the player’s velocity instead of being assigned to it, which would have masked the issue (or solved it in the first place, if you prefer).

I changed the code so that gravity is only applied if the player is not in the NOT_JUMPING state, and everything worked much more consistently.

Then I added a direction and provided horizontal impulse as well, so each jump not only moves the frog up but also left or right. I want the player to use small jumps to move about, and bigger jumps to try to catch flies, similar to the advanced mode on the Atari 2600 game.

I’m pleased with how the jumping feels, although I find it awkward to figure out how to manipulate the relevant variables to get the jump how I want it. Gravity, initial jump impulse, additional jump impulse if you continue holding the spacebar during a jump, and time all conspire to say how fast and how high you jump, and I’ve been under time constraints to do the probably simple parabolic math that I’m sure it involves.

Like my past few months, I’ll probably have to limit the scope of this project significantly if I want to get it completed before the end of the month. I still need to add flies and way to catch them. I probably won’t have time to implement falling off the lily pad or day/night cycles. Even though I have a week left, it’s a rough week.

How’s your #1GAM project going?

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