Advertisement Jump to content
  • Advertisement
  • 02/27/18 02:33 AM

    How to write a 2D UFO game using the Orx Portable Game Engine - Part 1

    General and Gameplay Programming
       (1 review)



    Welcome to the 2D UFO game guide using the Orx Portable Game Engine.

    My aim for this tutorial is to take you through all the steps to build a UFO game from scratch.

    The aim of our game is to allow the player to control a UFO by applying physical forces to move it around. The player must collect pickups to increase their score to win.

    I should openly acknowledge that this series is cheekily inspired by the 2D UFO tutorial written for Unity.

    It makes an excellent comparison of the approaches between Orx and Unity. It is also a perfect way to highlight one of the major parts that makes Orx unique among other game engines, its Data Driven Configuration System.

    You'll get very familiar with this system very soon. It's at the very heart of just about every game written using Orx.

    If you are very new to game development, don't worry. We'll take it nice and slow and try to explain everything in very simple terms. The only knowledge you will need is some simple C++.

    I'd like say a huge thank you to FullyBugged for providing the graphics for this series of articles.


    What are we making?

    Visit the video below to see the look and gameplay of the final game:

    Getting Orx

    The latest up to date version of Orx can be cloned from github and set up with:

    git clone 

    Once cloning has completed, the setup script in the root of the files will start automatically for you. This script creates an $ORX environment variable for your system. The variable will point to the code subfolder where you cloned Orx.

    Why? I'll get to the in a moment, but it'll make your life easier.

    The setup script also creates several projects for various IDEs and operating system: Visual Studio, Codelite, Code::Blocks, and gmake. You can pick one of these projects to build the Orx library.

    Building the Orx Library

    While the Orx headers are provided, you need to compile the Orx library so that your own games can link to it. Because the setup script has already created a suitable a project for you (using premake), you can simply open one for your chosen OS/IDE and compile the Orx library yourself.

    There are three configurations to compile: Debug, Profile and Release. You will need to compile all three.

    For more details on compiling the Orx lbrary at: at the Orx learning wiki.

    The $ORX Environment Variable

    I promised I would explain what this is for. Once you have compiled all three orx library files, you will find them in the code/lib/dynamic folder:

    • orx.dll
    • orxd.dll
    • orxp.dll

    Also, link libraries will be available in the same folder:

    • orx.lib
    • orxd.lib
    • orxp.lib

    When it comes time to create our own game project, we would normally be forced to copy these library files and includes into every project.

    A better way is to have our projects point to the libraries and includes located at the folder that the $ORX environment variable points to (for example: C:\Dev\orx\code).

    This means that your projects will always know where to find the Orx library. And should you ever clone and re-compile a new version of Orx, your game projects can make immediate use of the newer version.

    Setting up a 2D UFO Project

    Now the you have the Orx libraries cloned and compiled, you will need a blank project for your game. Supported options are: Visual Studio, CodeLite, Code::Blocks, XCode or gmake, depending on your operating system.

    Once you have a game project, you can use it to work through the steps in this tutorial.

    Orx provides a very nice system for auto creating game projects for you. In the root of the Orx repo, you will find either the init.bat (for Windows) or (Mac/Linux) command.

    Create a project for our 2D game from the command line in the Orx folder and running:

    init c:\temp\ufo

    or ~/ufo

    Orx will create a project for each IDE supported by your OS at the specified location. You can copy this folder anywhere, and your project will always compile and link due to the $ORX environment variable. It knows where the libraries and includes are for Orx.

    Open your project using your favourite IDE from within the ufo/build folder.

    When the blank template loads, there are two main folders to note in your solution:

    • config
    • src

    Firstly, the src folder contains a single source file, ufo.cpp. This is where we will add the c++ code for the game. The config folder contains configuration files for our game.


    What is config?

    Orx is a data driven 2D game engine. Many of the elements in your game, like objects, spawners, music etc, do not need to be defined in code. They can be defined (or configured) using config files.

    You can make a range of complex multi-part objects with special behaviours and effects in Orx, and bring them into your game with a single line of code. You'll see this in the following chapters of this guide.

    There are three ufo config files in the config folder but for this guide, only one will actually be used in our game. This is:


    All our game configuration will be done there.

    Over in the Orx library repo folder under orx/code/bin, there are two other config files:

    • CreationTemplate.ini
    • SettingsTemplate.ini

    These are example configs and they list all the properties and values that are available to you. We will mainly concentrate on referring to the CreationTemplate.ini, which is for objects, sounds, etc. It's good idea to include these two files into your project for easy reference.

    Alternatively you can view these online at and here:


    The code template

    Now to take a look at the basic ufo.cpp and see what is contained there.

    The first function is the Init() function.

    This function will execute when the game starts up. Here you can create objects have been defined in the config, or perform other set up tasks like handlers. We'll do both of these soon.

    The Run() function is executed every main clock cycle. This is a good place to continually perform a task. Though there are better alternatives for this, and we will cover those later. This is mainly used to check for the quit key.

    The Exit() function is where memory is cleaned up when your game quits. Orx cleans up nicely after itself. We won't use this function as part of this guide.

    The Bootstrap() function is an optional function to use. This is used to tell Orx where to find the first config file for use in our game (ufo.ini). There is another way to do this, but for now, we'll use this function to inform Orx of the config.

    Then of course, the main() function. We do not need to use this function in this guide.

    Now that we have everything we need to get start, you should be able to compile successfully. Run the program and an Orx logo will appear slowly rotating.


    Great. So now you have everything you need to start building the UFO game.

    If you experience an issue compiling, check the troubleshooting article for Orx projects    for help.


    Setting up the game assets

    Our game will have a background, a UFO which the player will control, and some pickups that the player can collect.

    The UFO will be controlled by the player using the cursor keys.

    First you'll need the assets to make the game. You can download the file which contains:

    The background file (background.png😞


    The UFO and Pickup sprite images (ufo.png and pickup.png😞


    And a pickup sound effect (pickup.ogg😞


    Copy the .png files into your data/texture folder

    Copy the .ogg file into your data/sound folder.

    Now these files can be accessed by your project and included in the game.


    Setting up the Playfield

    We will start by setting up the background object. This is done using config.

    Open the ufo.ini config file in your editor and add the following:


    Texture = background.png 
    Pivot   = center


    The BackgroundGraphic defined here is called a Graphic Section. It has two properties defined. The first is Texture which has been set as background.png.

    The Orx library knows where to find this image, due to the properties set in the Resource section:


    Texture = ../../data/texture


    So any texture files that are required (just like in our BackgroundGraphic section) will be located in the ../../data/texture folder.

    The second parameter is Pivot. A pivot is the handle (or sometimes “hotspot” in other frameworks). This is set to be center. The position is 0,0 by default, just like the camera. The effect is to ensure the background sits in the center of our game window.

    There are other values available for Pivot. To see the list of values, open the CreationTemplate.ini file in your editor. Scroll to the GraphicTemplate section and find Pivot in the list. There you can see all the possible values that could be used.

    top left is also a typical value.

    We need to define an object that will make use of this graphic. This will be the actual entity that is used in the game:


    Graphic = BackgroundGraphic 
    Position = (0, 0, 0)


    The Graphic property is the section BackgroundGraphic that we defined earlier. Our object will use that graphic.

    The second property is the Position. In our world, this object will be created at (0, 0, 0). In Orx, the coordinates are (x, y, z). It may seem strange that Orx, being a 2D game engine has a Z axis. Actually Orx is 2.5D. It respects the Z axis for objects, and can use this for layering above or below other objects in the game.

    To make the object appear in our game, we will add a line of code in our source file to create it.

    In the Init() function of ufo.cpp, remove the default line:


    and replace it with:


    Compile and run.

    The old spinning logo is now replaced with a nice tiled background object.


    Next, the ufo object is required. This is what the player will control. This will be covered in Part 2.

      Report Article

    User Feedback

    Create an account or sign in to leave a review

    You need to be a member in order to leave a review

    Create an account

    Sign up for a new account in our community. It's easy!

    Register a new account

    Sign in

    Already have an account? Sign in here.

    Sign In Now



    Great tutorial, I have been using ORX for a long time now for my hobby projects, it takes a little while to get used to the config files, but it is most definitely worth it.

    I remember the first time I used ORX for a project, after I got hold of the config files, I added particle effects (the game had four elemental arrows, each required different particles and each particle had some extra effect - rotating and scaling, for instance) and death effects (enemy blinking + disappearing) to my game in a couple of hours.

    ORX also has a very active community and any questions get answered pretty fast.

    Share this review

    Link to review

  • Advertisement
  • Advertisement
  • What is your GameDev Story?

    In 2019 we are celebrating 20 years of! Share your GameDev Story with us.

    (You must login to your account.)

  • Latest Featured Articles

  • Featured Blogs

  • Advertisement
  • Popular Now

  • Similar Content

    • By Josheir
      I am needing to test a program on multiple window operating systems.  Somewhere I read that the way to do this is using virtual machines.  Is this dependable?  Also, can I use a vm to successfully determine all the additional files I will need to install, for example dlls and runtimes, on my clients computers?  Is it true that the VM doesn't see anything on the real computer?
      Thanks so much; easy if you can please,
    • By Josheir
      I am working on multiple projects, one of which is using windows API.  😉    I am trying to catch a WM_DISPLAYCHANGE in WndProc switch for message.
      LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam) { switch (message) { case WM_DISPLAYCHANGE: {  
      The problem is that when I catch the event for a resolution not supported by my application, there is  flash with this screen before the event is caught.  Is there any way to bypass this?  Oh, and I tried changing the WM_PAINT code too.
      Thanks for any help,
    • By Fahrenheit.MVP
      Hi guys, check out MY FIRST GAME its still in development
      feedback is welcome
      Download on Google Play :
      Accept the challenge and survive to achieve as high of a score as you can!

      Simple and addictive game! Just hop in and balance the log to save yourself from the lava!!


      -Endless fun with challenging enemy monsters falling from the sky
      -Clean art style for a simple and enjoyable experience
      -Simple screen controls
      -Leaderboards to compete with strangers across the globe (to be added)

    • By I3DI
      I have spent many hours dedicated to advanced blending, shading, modeling and texture art for a game concept that will requires stages.  Think of this in the context of the sims, but far more advanced.  I need someone with a server that can host the Unreal server.  The plant life, very advanced.  The terrain textures very advanced and using blending, GIS, varying liquid textures I am building the foundation for a economic sim first.  The idea initially will be to build businesses in cities and have a character you can customize with basic animations.  Later down the road once world economies are in motion, we will add simulations, politics, governments, and finally combat simulations.  The idea is you can assume any role you would in life, build a life in modern civilization, all connected world wide.  Initially it's very basic, businesses and economies with a customizable male or female character, house or business building.
      My company has secured a contract that generates about 120,000.00 USD a year, and those willing to vest in this project will ultimately be paid.  Where is the money?  Well, the idea is to take subscriptions initially but then, money is deposited by players to build their virtual life, career, and each nation will have a government that builds on taxation.  I will add stock markets, housing markets, medical markets, technology markets and resource markets.  The beauty is, since players invest real cash, they can make real cash thru inventions, politics, banking, travel, whatever they set up in the virtual world.  Simply working a job for another players business that pays.  As the economy grows and more players are added, then they supply cash to begin their lives in our virtual world and the ones that make money can simply withdraw into their real world bank accounts.  We simply make our money off fees in transactions that occur in our virtual world.
      So to be clear, this is a beginning try out with stages of development.  I will be doing a early release and as players are added, money is invested, this will fund further development.  
      Stage 1:
      Government, Business and World Setup.  I have completed all texture art necessary to generate a complete earth in very vivid detail.  I am making preset buildings.  But I require assistance in the development of a in game GIS terrain generator.  I will also need help finding a database of cities and their building and maps so we can procedurally generate the cities.  I have iClone and could use another person to help me develop the basic animations for people.  I would like to make a building designer in game, setup zoned properties, making clothing, furniture, and get the economic part of the game setup.
      I am seeking someone with a server that can run 24/7, a Unreal game server.  I am seeking one or two artists that can help speed up the basic modeling process for many models.  I personally will deal with the terrain, city and various programming aspects.  I already have done all the architectural textures, detail and base, PBR.  I am doing models, like furniture, fountains, but being able to shift this work to several artists that are consistent would be of real help so I can focus on the programming aspect of this first stage.  I also have a merchant processor so I can run the credit cards and accounts thru them.  If you are willing to vest in this one project, one piece at a time, then I can guarantee once you are proven and consistent, there will be pay.  There are several other stages to building this advanced simulation but this first stage is the simplest and will at least begin a very lucrative money income for the company.
      A additional programmer would be of real help that is familiar with C++, Unreal, Blueprints, Materials, GitHub is hard working consistent, and wants to get their foot in the door in game development.
      Seeking two artists, 2D knowledgeable in Adobe 
      Seeking two artists 3D knowledgeable in Substance, Adobe, Blender, iClone is a bonus
      Seeking one individual with a 24/7 server for Beta Testing at no cost.
      Tools and accounts are setup and paid for by me.
    • By isu diss
      How can I find collision point and normal using sat? I read that sat can do that. Please help me?
      int AABB::supportFaceCount() { // there are only three directions for every face of an AABB box. return 3; } XMVECTOR AABB::supportFaceDirection(int i) { // the three axes of an AABB box. along the x, y and z axis. static const XMVECTOR s_aabbAxes[] = { XMVectorSet(1, 0, 0, 0), XMVectorSet(0, 1, 0, 0), XMVectorSet(0, 0, 1, 0) }; return s_aabbAxes[i]; } int AABB::supportEdgeCount() { // there are only three directions for every edges of an AABB box. return 3; } XMVECTOR AABB::supportEdgeDirection(int i) { // every edge go along the x y, or z axis. static const XMVECTOR s_aabbEdges[] = { XMVectorSet(1, 0, 0, 0), XMVectorSet(0, 1, 0, 0), XMVectorSet(0, 0, 1, 0) }; return s_aabbEdges[i]; } void AABB::supportInterval(XMVECTOR direction, float& min, float& max) { XMVECTOR centre = XMVectorSet(Center[0], Center[1], Center[2], 1); // projection of the box centre float p = XMVector3Dot(centre, direction).m128_f32[0]; // projection of the box extents float rx = fabs(direction.m128_f32[0]) * Radius[0]; float ry = fabs(direction.m128_f32[1]) * Radius[1]; float rz = fabs(direction.m128_f32[2]) * Radius[2]; // the projection interval along the direction. float rb = rx + ry + rz; min = p - rb; max = p + rb; } bool ObjectsSeparatedAlongDirection(XMVECTOR& direction, AABB* a, AABB* b) { float mina, maxa; float minb, maxb; a->supportInterval(direction, mina, maxa); b->supportInterval(direction, minb, maxb); return (mina > maxb || minb > maxa); } bool ObjectsIntersected(AABB* a, AABB* b) { // test faces of A for(int i = 0; i < a->supportFaceCount(); i++) { XMVECTOR direction = a->supportFaceDirection(i); if(ObjectsSeparatedAlongDirection(direction, a, b)) return false; } // test faces of B for(int i = 0; i < b->supportFaceCount(); i++) { XMVECTOR direction = b->supportFaceDirection(i); if(ObjectsSeparatedAlongDirection(direction, a, b)) return false; } // test cross product of edges of A against edges of B. for(int i = 0; i < a->supportEdgeCount(); i++) { XMVECTOR edge_a = a->supportEdgeDirection(i); for(int j = 0; j < b->supportEdgeCount(); j++) { XMVECTOR edge_b = b->supportEdgeDirection(j); XMVECTOR direction = XMVector3Cross(edge_a, edge_b); if(ObjectsSeparatedAlongDirection(direction, a, b)) return false; } } return true; }  

Important Information

By using, you agree to our community Guidelines, Terms of Use, and Privacy Policy. is your game development community. Create an account for your GameDev Portfolio and participate in the largest developer community in the games industry.

Sign me up!