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How to Make a Side Scroller with RUBE and Felgo - Box2D Level Editor Game Tutorial


Can you image a game without different levels? Unless we're talking about a really simple mini-game, different levels contribute a lot to create an awesome gaming experience and keep the player interested over a long period of time.

It doesn't matter if you have few large levels, like in open-world games, or many small ones, like in typical platformers. One thing is clear: The level creation always consumes a lot of time. In many cases, you will create your levels manually because your game is not suited for an automatic level generation. But level creation and level design are not easy tasks. You can spend a lot of time to set up and polish each and every level. For that matter, good level editors help you save time and provide visual support to make it easier to set up your levels.

About RUBE

R.U.B.E. is short for Really Useful Box2D Editor and is made by Chris Campbell / iforce2d. As the name suggests, it is mainly meant to be a graphical editor for Box2D objects. But you can also use it to create complete levels!

As Box2D is also the physics engine we use at Felgo, why don't we use RUBE to create our physics-driven levels? With that in mind, we created some components that let you import your custom levels from RUBE and use it in your game! They are especially useful for physics-based games like sidescrollers or platformers. RUBE is available for Windows, MacOS and Linux.

Let me give you some directions:

To give you a small overview of what's possible, these are just a few features of RUBE:

  • You can translate, rotate, scale, mirror and duplicate objects.
  • You can edit bodies, fixtures, vertices, joints, images, objects (= sub-scenes) and samplers.
  • You can nest scenes, for example create an obstacle once and use it many times in the finished level.
  • The editor can create complicated polygon fixtures around your obstacles, based on their alpha edges.

Scope of the Tutorial

After you complete this tutorial, you should be able o create your own levels with RUBE and use them for your Felgo game. This tutorial covers:

  • Setting up a common project structure to be used by both RUBE and Felgo.
  • Working with RUBE to create different Box2D objects and combine them to form a level.
  • Linking your RUBE objects to Felgo entities with custom properties.
  • Using the RubeParser, RubeBody and RubeImage components to load a RUBE level and add custom game code.

As this is an advanced tutorial, you should already know the basic concepts of working with Felgo, Qt/QML and QtCreator. If you are just getting started, please consider taking a look at these tutorials first:

In case you prefer to directly look at the source code of this demo game - you can find it along with the other demos in the Felgo SDK. See here where to find the demo in your SDK directory.

For an overview of the game features, see R.U.B.E. Editor - Side Scroller.


All the resources of the game should be placed within the assets/img directory of the project. You can download them here.

The images used in this project are based on these sources:

Pingi and the Icy Cave

Before we start to create our game, let me explain our game idea! As you might know penguins love speed. The penguin in this little game, let's call him Pingi, has built a bottle rocket glider. He wants to use it to fly out of the icy cave he's trapped in. Unfortunately, some gigantic sharp edged snowflakes are spinning in his way. To reach the end of the cave, he needs to avoid them - otherwise they will cut him into pieces.

Now that we decided on a game idea, we can start to create the game. In order for RUBE and Felgo to work together, three steps are required:

  1. We use RUBE to create and configure our physics-driven level with all the objects, fixtures, images, ...
  2. Then, we export the whole level to a JSON-file.
  3. Some special Felgo components now allow us to load the JSON-level and bring everything to life with custom game code.

One thing you have to keep in mind, is that the RUBE editor knows nothing about Felgo.

  • After you set up your level, it is important to add some additional properties to your objects. These properties then allow us to link the objects to our custom game entities.
  • When using images within your level, only the path to the images is stored in the JSON file. But we want to be able to directly read and use the JSON file in Felgo with correct relative paths. For this purpose, we set up a certain project structure that allows us to use images in both RUBE and Felgo with the same path.

So let's start with creating our project!

Setting Up the Project

First, create a new empty Felgo project in a directory of your choice. Then, create a new folder, e.g. "rube", within your project directory. This folder will contain your RUBE scenes. By default, this additional directory is not included in the deployment - the RUBE files won't be part of the final game. However, by using this subfolder of the project for RUBE, we automatically get correct file paths when we use images of our assets folder.

The structure of your project should look like this:

Folder Description
assets/ Put exported levels from RUBE (.json) here.
assets/img/ Put sd versions of your images here. Use these for adding images in RUBE.
assets/img/+hd/ Put hd versions of your images here.
assets/img/+hd2/ Put hd2 versions of your images here.
qml/ Put your QML files here.
qml/entities/ Put QML entities, that should be linked with RUBE objects, here.
rube/ Put .rube files here.

When we've finished our rube level, we will export it as a .json file directly to the assets folder. You are not allowed to place these JSON files anywhere else, as the parser component expects them to be within the assets directory. In addition, the JSON files will automatically be deployed along with the assets folder when we build the final game.

Before we go on, be sure to download all necessary game resources here. Just place the images within your assets/img folder.

The folder also contains the directories +hd and +hd2. Felgo automatically uses the images in these directories to support different screen resolutions (see MultiResolutionImage). The tutorial Supporting Multiple Screen Sizes & Screen Densities with Felgo also includes more details about this topic.

The QML components in the qml/entities/ folder will be used by Felgo to automatically create game entities for certain RUBE objects. With this it is possible to add custom game code.

Creating the Level in RUBE

We will need four rube scenes (objects) for our game:

  • The snowflakes obstacle that the penguin has to avoid. We call it saw in our game.
  • The penguin, that we use as the player character.
  • The cave, that consists of ground and ceiling walls. We call it level.
  • After creating these scenes, we put the level, penguin and multiple saws together to create our sidescroller game.

Let's start with our first RUBE scene, the saw.

Saw Obstacle

Fortunately, the saw is a rather simple object. It just contains a body, a circle fixture and an image in RUBE.

If you already have experience with the 3D graphics and animation software Blender, you are lucky. The controls in RUBE are quite similar to blender. For example:

  • You can move the camera by dragging the scene with the right mouse button.
  • You can select and deselect items with the left mouse button.
  • Which items you can select is determined by the mode your are in.
  • You can switch the mode by pressing the corresponding key on the keyboard: B for bodies, F for fixtures, V for vertices, J for joints, I for images, P for samplers and O for objects. Note that the item selection panel on the left is independent from these modes.
  • The world mode has no key assigned.
  • Press space for a menu in which you can create items and use special RUBE features, e.g. create a polygon fixture from an image.

When you have an item selected you can edit it with the buttons shown in the context help on the left. For example, if you selected an image and you are in the image mode you can press T to start translating the image. Move your mouse and press the left mouse button to finish the translation. You can also rotate (R), scale (S), copy (Ctrl+C), paste (Ctrl+V) and delete (Del) the selected item. When you're in vertex or sampler mode, you can move and insert the vertices by pressing E. On the right is the properties panel where you can edit e.g. the density of fixtures or the body an image should belong to.

Now, you know how to get along with RUBE. Don't worry I will keep it easy in the beginning, e.g. remember you to switch the modes. Follow these steps to create a sub-scene for the saw:

  • Open RUBE and create a new empty scene via the menu File->New Sceneor Ctrl+N.
  • Press space and click Add body with circle fixture.
  • Drag'n'drop the saw image from assets/img/ into RUBE. Or press space, add an image and assign the file property.
  • Switch to image mode by pressing I. Click on the saw image and set its center to (0, 0).
  • Press S to start scaling the image. Make the image fit into the circle fixture and then press the left mouse button.
  • If the image's center does not match the saw's center, translate the image with T.
  • Click on to the body property of the image and then select the circle fixture body. The image is now linked to the body and will move with it.
  • Save the scene as saw.rube in the rube folder.

Nice, our snowflake obstacles are already finished!


The saw was pretty easy, wasn't it? Now we create the penguin, but we use a hand-crafted polygon fixture this time.

Let's give Pingi a physics shape:

  • Create a new empty scene.
  • Press space and add a body with square fixture.
  • Drag'n'drop the penguin image into RUBE. Set the image's (I for image mode) center to (0, 0) and scale to 0.5. Assign the body to its body property.
  • Switch to vertex mode with V. Select one vertex of the square fixture. Press T to move it to a corner of the penguin within the image. Press E twice to get into the insert vertex mode. Insert vertices with the left mouse button and move them until you your polygon fixture is accurate enough to match the penguin.
  • Save the scene as penguin.rube in the rube folder.

Now that we have our penguin and saw, the only sub-scene left is our level, the cave.


The cave consists of a ground and a ceiling, with many hills. The graphics for the cave are split up in several images. If we place the parts next to each other, we get the complete cave.

For the fixture of the cave ground and ceiling, we again need a polygon shape. Creating the polygons to follow the complex cave path seems a bit complicated, don't you think? That's why we let RUBE create them for us.

As you already placed all the images in your assets folder, you can directly start to put the cave level together in RUBE:

  • Create a new empty scene.
  • Drag'n'drop all the level's images into RUBE. Better do this one by one or you will have a hard time getting them in the correct order again. Keep the top and bottom parts separated for now.
  • Enable grid snapping by clicking the right-most button below the menu bar. Now, you can hold down Ctrl while translating the items. They will snap to the grid.
  • Press space and create a sampler. Move it and set its output size to cover all the top pieces completely and nothing from the bottom pieces.
  • Select the sampler, press E (a few times) and set way points around the the top images. Also go around the corners and over the top of the ceiling images.
  • Refresh the edges in the sampler's property window.
  • Configure the max points and min angle of the sampler to let the polygon follow the ceiling as accurate as you want.
  • Create an empty body. Select the sampler, the body and the upper part images. Don't forget to switch the modes.
  • Press space and click on Sampler->Create fixture from sampler. Now, RUBE creates a polygon fixture from the alpha edges of the selected images and assigns this new fixture to the selected body. This step would fail if we had both the top and bottom parts together, because Box2D can only handle single convex fixtures.
  • Repeat the process for the bottom parts.
  • Move the upper and lower parts closer together. Move both the images and the fixtures. Depending on your desired gameplay and difficulty, you can choose a distance that works best for you.
  • Save the scene as level.rube.

We've now finished all the parts that make up our game: the saw as an obstacle, Pingi as the player's avatar and the level.

Putting Everything Together

Let's now combine them to create the game:

  • Create a new empty scene.
  • Press space and add an object. Select the level.rube in the file dialog. Select the level object in RUBE (O for objects). Place it at (0, 0) and set its scale to 10.
  • Also create objects for Pingi and one saw.
  • Select the saw and duplicate it with Shift+D as often as you want.
  • Scale and place the saws along the cave in a way that's fun to play. This is finally the nice part for level designers ;). However, RUBE seems to have a bug here, at least on Windows 10. If you re-load the scene, it occurs that some objects can't be loaded. RUBE somehow messed up the paths to the .rube files of the objects. Therefore, you may have to correct the paths manually by editing this scene's .rube file.
  • Save the scene as sidescroller.rube.

All the fixtures and graphics for our game are set up now. You could already try to export everything as sidescroller.json to the assets folder and load it with Felgo. But you won't be happy yet. To your surprise, the whole level will just fall down together with the saws and Pingi. Obviously, this can't be correct. What's missing are some physics settings for the fixtures and bodies in RUBE.

Refined Settings

Let us fix that:

  • Open the saw.rube and set its body's type property to kinematic. Also set its angular velocity to 360. With these changes, the saw always stays at its initial position and rotates once per second for the rest of its life.
  • Next, open penguin.rube, set the linear damping to 1.2 and set the angular damping to 15. The linear damping prevents Pingi from getting too fast. The angular damping is really strong, which causes him to stop turning almost immediately when you don't apply any torque. Once you can play the game later, you may try different settings for your game.
  • Open level.rube and set the body's type to static. This ensures that the level never moves, so it doesn't fall down anymore.

Linking Felgo Entities with RUBE Objects

Relax now, the hardest part is already done! Before doing any more modifications, let's try loading the RUBE level with Felgo. Just replace your Main.qml with this implementation:


 import Felgo 4.0
 import QtQuick 2.0

 GameWindow {
   activeScene: scene

   // load level at game startup
   Component.onCompleted: {
     physicsWorld.running = true

   Scene {
     id: scene

     property alias level: level

     EntityManager {
       id: entityManager
       entityContainer: level

     Item {
       id: level

       PhysicsWorld {
         id: physicsWorld

         // physics is disabled initially, and enabled after the splash is finished
         running: false
         //gravity.y: 9.81 // this is set from rube anyways, no need to set it here
         z: 10 // draw the debugDraw on top of the entities

         // these are performance settings to avoid boxes colliding too far together
         // set them as low as possible so it still looks good
         updatesPerSecondForPhysics: 60
         velocityIterations: 5
         positionIterations: 5
         // set this to true to see the debug draw of the physics system
         // this displays all bodies, joints and forces which is great for debugging
         debugDrawVisible: false

     // rube parsers allows to load json files and dynamically create the game entities
     RubeParser {
       id: parser
       entitiesFolder: "entities/"

     Row {
       spacing: 4
       GameButton {
         text: "Reload Rube Level"
         onClicked: {
           console.debug("start loading level")
           // call clearLevel first to start with a new level

       GameButton {
         text: "Unload Rube Level"
         onClicked: {
           console.debug("start un-loading level")
       GameButton {
         text: "Zoom In"
         onClicked: {
           level.scale /= 0.9
       GameButton {
         text: "Zoom Out"
         onClicked: {
           level.scale *= 0.9

   // loads the rube-level from a json file
   function loadLevel() {
     parser.setupLevelFromJSON( Qt.resolvedUrl("../assets/sidescroller.json"), physicsWorld, level )

     level.y = scene.height / 2
     level.x = scene.width / 2

This sets up our game scene. We add a new Item to our Scene that will contain the RUBE level after we load it. The only thing we need here is our PhysicsWorld component. We also set up our EntityManager to use the level item as the entityContainer.

The most important component of the scene is the RubeParser. With the parser component, different RUBE levels can be loaded or unloaded. To test the parser, we also add a few GameButton instances within a Row.

The helper function loadLevel is used to load and create the JSON level. When starting the game, we use the Component.onCompleted signal of the GameWindow to initially load the level at startup.

If you export the complete RUBE level to assets/sidescroller.json and then start the game, you'll see a bunch of generic bodies with their correct fixtures and images attached. But those objects won't do anything special. Pingi just follows the gravity. But instead, he should be able to fly. A special QML entity is required, that let's us add our custom game code.

Penguin Entity

Before we create the Felgo entity for our penguin, keep in mind that RUBE and Felgo don't automatically work together. In order to decide which RUBE object should be linked to which QML entity, we have to introduce a few new properties in RUBE. Felgo can then act based on the values we configure there.

  • Open penguin.rube or switch to its tab if it's still open.
  • Open the scene settings by clicking on the menu item scene and then scene settings. Go to the custom properties tab, if it isn't already open.
  • Create a new property with these settings:

  • Select Pingi's body. Scroll down in the properties panel until you see our new property and enter "Penguin".
  • Save the scene and export the whole level again.

When Felgo loads the exported JSON of the penguin body, it sees the qmlType property and tries to create an entity from qml/entities/Penguin.qml for Pingi. Here is the implementation for this file:


 import QtQuick 2.0
 import Felgo 4.0

 RubeBody {
   id: penguin
   entityType: "penguin"

     // force applied to the body, drives the penguin forward (always) and up (when touched)
   force: Qt.point(60, isMouseDown ? -300 : 0)

   // torque to achieve the target rotation = deltaRotation * factor
   // this is like a proportional controller (see PID controller)
   torque: (targetRotation - rotation) * 60

   // target rotation depends on the vertical velocity
   property real targetRotation: linearVelocity.y / 6

   // current touch state
   property bool isMouseDown: false

   onXChanged: {
     // move "camera" of scene so that the penguin is 1/3 from the left screen border
     scene.level.x = -x + scene.width / 3

   Connections {
     target: scene.mouseArea
     onPressed: penguin.isMouseDown = true
     onReleased: penguin.isMouseDown = false

To control the penguin, also add a MouseArea to the Scene in your Main.qml:


 import Felgo 4.0
 import QtQuick 2.0

 Scene {
   // ...
   property alias mouseArea: mouseArea

   // ...

   MouseArea {
     id: mouseArea
     anchors.fill: scene.gameWindowAnchorItem

   Row { }

Our penguin is derived from the RubeBody, which gives you a simplified access to its fixture's contact signals RubeBody::beginContact, RubeBody::contactChanged, RubeBody::endContact and the RubeBody::initialized signal. Additionally, it gives direct access to its underlying ColliderBase.

Pingi always accelerates forward. He also accelerates upward if you touch the screen or press the left mouse button. Pingi's rotation depends on his vertical velocity.

Checking Collisions

Pingi now reacts to inputs from the player but it doesn't react to contact with the saws. To change this, we need another QML type for the saws and check for collision with the penguin. Let us add the same custom property qmlType for saw.rube. Set the qmlType to Saw and add this tiny QML file to your project:


 import QtQuick 2.0
 import Felgo 4.0

 RubeBody {
   id: saw
   entityType: "saw"

With this piece of code, we mark the saw as a saw entity. When checking for collisions in our penguin entity, we can then access the entityType and react accordingly.


 import QtQuick 2.0
 import Felgo 4.0

 RubeBody {


   onXChanged: {
     // move "camera" of scene so that the penguin is 1/3 from the left screen border
     scene.level.x = -x + scene.width / 3

     // test until endless scrolling is enabled
     if (x > 3200){

   onInitialized: {

   onBeginContact: (other, contactNormal) => {
     var entityType = other.getBody().target.entityType
     if (entityType === "saw") {

   function reset(){
     var resetPos = Qt.point(0, 0);

     // only one startPos can exist
     var startPosEntities = entityManager.getEntityArrayByType("startPos")
     if (startPosEntities.length > 0 && startPosEntities[0]) {
       resetPos = Qt.point(startPosEntities[0].x, startPosEntities[0].y)

     x = resetPos.x
     y = resetPos.y
     linearVelocity = Qt.point(0,0)

     rotation = 0
     angularVelocity = 0

   // ...

We now introduced collision checking and a new reset() function, it is called:

  • When the penguin's creation is finished.
  • When it flies out of the level.
  • And when it touches a saw.

The function moves Pingi to to the position of a certain StartPos entity (we will add this entity later) and resets its rotation, angular and linear velocity to 0. The StartPos entity is just a hidden object. We will use it to define the starting point of our penguin in the level.

Let us open RUBE again and add the StartPos object:

  • First, create the well-known custom property qmlType for the bodies of sidescroller.rube.
  • Then, add a new body without fixtures and set its qmlType to StartPos.
  • Place it where the penguin should start in the cave.
  • Save the scene and export it again.

Then add this code as StartPos.qml:


 import QtQuick 2.0
 import Felgo 4.0

 RubeBody {
   id: startPos
   entityType: "startPos"

If you try your game again it might already be fun by now. If not, try tweaking the force values of the penguin in its QML file, change its damping or move / scale the saws.

Background and Layers

Do you still see that big unpainted area in the middle of the screen? Or are you already used to it? I think we should fill this hole with a nice background.

  • Drag and drop the icy background image into the scene sidescroller.rube. Don't attach it to any body.
  • Add a custom property qmlType for the image, and set it to BGImage.

Then add the entity BGImage.qml:


 import QtQuick 2.0
 import Felgo 4.0

   id: bgImage
   objectName: "bgImage"

   onInitialized: {
     parent = scene;
     anchors.fill = parent;

This image is derived from the RubeImage. This is why the RubeBody::initialized signal can be used. When its creation is finished it attaches itself to the scene and fills it completely.

You might see that the background image is actually in front of everything. You can change this by modifying the render order property of images in RUBE. This is a nice opportunity to improve a few things. Try these settings for the render order:

  • -1: The BGImage in sidescroller.rube should be behind everything else.
  • 0: This is the default value for all new images in RUBE.
  • 1: The saw should be behind the level
  • 2: The level just needs any value in between.
  • 3: Pingi should be on top of everything.

Export everything again and you're all done!

What Now?

Congratulations, you have followed this tutorial till the end and hopefully created a simple but fun game. But what now - can you still improve the game? Here are some ideas:

  • Add sounds with the BackgroundMusic and GameSoundEffect components.
  • Add an endless mode.
  • Add a giant saw behind Pingi following him faster and faster.
  • Add upgrades for speed and steering.
  • Add collectible points.
  • Add temporary boosts through collectibles.
  • Add nicer graphics.
  • Let the saws grow over time or after each passed level.
  • Let Badland and Jetpack Joyride inspire you.

And I've got one last pro tip for your work with RUBE: always make sure that the width of the tiles is exactly one.

If you have any questions regarding this tutorial, don't hesitate to visit the support forums.

Visit Felgo Games Examples and Demos to gain more information about game creation with Felgo and to see the source code of existing apps in the app stores.

If you are interested in the full source code of this demo game, see R.U.B.E. Editor - Side Scroller.

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