A Gradle Primer for Libgdx Users

Every time I set up a new project I learn something about how Gradle works and the default project setup for Libgdx. I’ve decided to write this down to save myself time the next time I start a project. I’m using Intellij IDE from what I read things are different in Eclipse. If using Intellij it is important to note that a Gradle run configurations targeting a sub-project run task  can be used in lieu of an application run configuration. The advantage of doing this is that asset location and jvm arguments are all handled in Gradle files making project set up easier for multiple users.

Most start with Gradle sites say you should start by installing gradle. Good news is Libgdx uses Gradle Wrapper, which provides scripts for both Linux and Windows allowing Gradle to be run without installing. Yay!

The next thing most starting with Gradle tutorials will say is ‘create a hello world task’ and this is where we’re going to diverge a bit since files and tasks were created during project set up. So I’m going to discuss the various Gradle files the Libgdx project setup program created.

The Files


gradle.properties

  • Not totally sure what this does, but it appears to be arguments for when Gradle gets launched by the wrapper.

settings.gradle

  • This is a short file indicating the structure of the project. It indicates how the various sub-projects are organized. The basic setup just lists ‘core’ + all of the target platforms.

build.gradle

  • There are a number of build.gradle files; one in the main folder and one in each of the platform folders. Having multiple build.gradle files the standard format for Gradle projects with subprojects.
  • The build.gradle file in the main folder contains dependencies for both external libraries and the internal subprojects. This is where the platform subprojects are related to the core subproject and the libgdx and other library dependencies are managed.
  • The build.gradle files in the subproject folder contain build tasks associated with each of the projects. For example the desktop subproject has a run task that can be used to launch the desktop application.

Now let’s look at modifying things.

If we want to add a runtime argument to indicate the location of a log4j2 configuration file we would add the line

“jvmArgs = [‘-Dlog4j.configurationFile=log4j2.xml’]”

To the top of the run task.

 

If a client-server project is desired where the platform and core projects become sub projects of the client project this can be done by prepending ‘client:’ to the existing project names in the settings.gradle file and the main build.gradle file. This assumes that both the platform and core sub-projects are contained in the client folder as shown below.

Bot Escape Demo released

Hey Look A Thing!

You can download the demo release here.

About the Game

What do you in Bot Escape? Drive around a robot and look for a door. How do you do it? glad you asked.

  1. Use a controller! One of the purposes of this project was to learn how to get controller input and that works.
  2. Keyboard Controls (keyboard controls can be changed under options)

‘a’ left

‘d’ right

‘w’ jump

‘e’ shoot missile

Here’s some screen shots

Even More About The Game

Generally don’t expect too much. This project originated as a way to learn about box2D, controller input, Wang tiles, and maze generating algorithms. I think there’s some potential to be a decent game if some good levels are designed. I tried my hand at designing two levels and have come to the conclusion that level design is hard and I’m not currently in the mood to do it. So who knows when we’ll get real levels for Bot Escape. In the mean time try out the maze and let me know what you think of the controls.

Known issues

Sometimes the robot catches on horizontal surfaces in the generated mazes. This is an artifact of box2D, jump to unstick the robot.

Platformer Update 9 – Bot Escape

Well I’m back from vacation, but I’m apparently still running on vacation time. The good news is I did some coding over vacation, which means we finally have some screen shots. Woot!

I’ve been using the working title BotEscape for this project for a few months since one of the original thoughts for this project was that you’re a robot escaping from your alien overlords, a crumbling factory or some combination of the two. As you can tell from that detailed description, I haven’t developed a storyline yet, so the current “levels” are just piling all the pieces together to make sure the interactions are working correctly. I haven’t checked to see if this name overlaps with any existing properties so I’m just going to run with it for now. It might get changed later if it looks like it poses a conflict with existing works.

There are three main tasks that need to be completed before I put up a demo of sorts:

  • Menu screens
  • A health system (read: a way to leave you a twisted pile of scrap metal)
  • A couple of levels that have some thought in them and aren’t just piles of clutter.

So over the next few weeks I’ll be working on these and any thing else that comes up to get a demo ready.

Platformer Update 8

And another slow week although for different reasons. Last week I discovered Wang Tiles and remembered that aperodic tiling is a thing. So instead of working on adding features. I spent time figuring out how to implement Wang Tiles.

I’ll discuss aperiodic tiling and Wang Tiles next week, since this is going to be another slow week due not to me being distracted by a new shiny but because of life.

Continuing Fun Times With Box2D

I’ve spent most of the week working on making destructible boxes for the platformer. The actual mechanics of this are fairly simple, and psedocode for the process is below.

if condition is met then

destroy box

The conditional statement can be checked during the regular update cycle (otherwise known as polling) or whenever a different condition is met (otherwise known as a listener).

For the destructible boxes the condition would be met if the impulse on the box would exceed a threshold value. I decided to use the listener method of checking the destruction condition; since, I already had a Box2D contact listener for the projectiles.

And now the fun starts so let’s add some equations. first impulse courtesy of Wikipedia.

\mathbf {J} =\int _{t_{1}}^{t_{2}}\mathbf {F} \,dt=\Delta \mathbf {p} =m\mathbf {v_{2}} -m\mathbf {v_{1}}

According to our good friends at Wikipedia impulse is the sum of the forces applied over a time period (t1 to t2) which can be expressed as a change in momentum and as mass times change in velocity.   hyper physics

It is not surprising then that the initial implementation measured the change in velocity between when the Box2D contact listener called beginContact() and endContact() on the destructible box. (there are legitimate issues with this implementation I know about them. If you’re having trouble imagining them think of someone pushing a sled gradually up to speed and then the pusher stops abruptly.) This was done because velocity is relatively easy to visualize and calculate. for example if the box were to start falling and impact the ground the some time later. The impact velocity could be calculated using either the fall time or the distance.  Assuming the velocity prior to impact >0 and immediately after impact is <=0. This should make it easy to tune the destruction velocity threshold depending on how many stories an object is expected to fall and remain intact.

After doing some mathematical manipulation and assuming initial velocity is zero. we get v² = 2ax. And we’re already to pick out some velocity thresholds.  So let’s move into the real err platformer world; where I took the liberty of having destructible box impact velocities logged for the record as well as start heights.

Experiment acceleration distance expected velocity actual velocity
Experiment 1 12 m/s/s 7.4 m 9.4 m/s 2.4 m/s
Experiment 2 12 m/s/s 7.3 m 9.4 m/s 1.99 m/s
Experiment 3 12 m/s/s 20.5 m 15.7 m/s 3.4 m/s

Something weird is definitely going on here. I’m currently assuming this is an artifact of how I was getting the velocity from Box2D. But I didn’t really dig into it too far since I realized there were other problems with this method, and switched to getting a ‘PostSolve’ impulse reading from Box2D which is closer but still different from the values I’m calculating. I suspect that I’m getting readings at the wrong time or that Box2D is taking additional things into consideration that were left out of the simple motion equations. (e.g. friction).

My conclusion for the week is, since a 10 cm difference in height resulted in a massive shift in impact velocity. This is annoying because it means that objects will require manual tuning of impact forces.