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 https://github.com/orx/orx.git 

    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: http://orx-project.org/wiki/en/tutorials/cloning_orx_from_github 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 init.sh (Mac/Linux) command.

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

    init c:\temp\ufo


    init.sh ~/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 https://github.com/orx/orx/blob/master/code/bin/CreationTemplate.ini and here: https://github.com/orx/orx/blob/master/code/bin/SettingsTemplate.ini


    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  assets-for-orx-ufo-game.zip 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

    ..\..\..\lib;$(ORX)\lib\dynamic; doesn't exsist. Some some steps setting up new project are missing.

    Share this comment

    Link to comment
    Share on other sites
    8 hours ago, trsh said:

    ..\..\..\lib;$(ORX)\lib\dynamic; doesn't exsist. Some some steps setting up new project are missing.

    Hi Trsh, thanks for reporting. Under "Getting Orx" there is a part that mentions that git close auto-setup creates a project so that Orx can be used in your own projects. On reflection it's not 100% clear that you need to compile the projects. But I did include the link on compiling which takes you to the Orx wiki.

    Let me know if that doesn't solve the issue. However, once the orx libraries (debug, profile and release) are all compiled, the "dynamic" folder should then exist.

    Also check that after git clone step (the post setup step) managed to create an $ORX variable in your environment variables.

    Share this comment

    Link to comment
    Share on other sites
    On 3/15/2018 at 11:45 PM, trsh said:

    ..\..\..\lib;$(ORX)\lib\dynamic; doesn't exsist. Some some steps setting up new project are missing.

    I recently made the building of the Orx lib instructions more clear. Should be less of a barrier for new comers.

    Share this comment

    Link to comment
    Share on other sites

    Create an account or sign in to comment

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

    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

  • Advertisement
  • Game Developer Survey


    We are looking for qualified game developers to participate in a 10-minute online survey. Qualified participants will be offered a $15 incentive for your time and insights. Click here to start!

    Take me to the survey!

  • Advertisement
  • Latest Featured Articles

  • Featured Blogs

  • Advertisement
  • Popular Now

  • Similar Content

    • By kandarp_pandya
      Please check out my new game which is level based infinite target shooter.

      For Android

      For iOS

      Please let me know what you think about this. Also any suggestion feedback are most welcome.
    • By b2soft
      I have a problem with implementing mouse-look camera movement, like in FPS games. For me common solution is:
      Process WM_MOUSEMOVE event in WndProc Calculate delta movement from the window's center using event's lParam Rotate camera Return cursor back to window's center using SetCursorPos The problem is when SetCursorPos is called, another WM_MOUSEMOVE event is being fired. So camera rotates back.
      What is the common way to create such type of camera on Windows platform (using WinAPI)?
      I know that in WM_MOSEMOVE I can check is mouse.x == windowCenter.x and if it is - do nothing, but it's a hack from my point of view. Is there any "non-hacky" way to achieve the goal?
    • By Marscaleb
      Hello folks, I was hoping I could get some feedback on some character designs I've been working on for a game I'm making.
      This is for a 2D platformer with a cartoony design.  I'm making three playable characters.
      I'm looking for any feedback at all on these designs; let me know what you think.  Be honest; you don't need to sugar-coat anything.
      Also I have a couple different versions I've sketched out and I'd like to see which ones people like the best.
      One of the designs I'm implementing is that when the player has one hit left, part of their attire get blown off.  (Like in Ghosts n' goblins, when you lose your armor.)  The design on the far left is the one that represents this "no armor" state.
      First of all, this is the mage.  I'm pretty satisfied with his design so I never created different variations.  But I'm still interested in feedback about his design if you have anything you want to say.

      Next up is the reaper.  I was originally planning for her armored version to just be a generic cloak, but I've been thinking lately that I should have something more distinct; something more unique.  So I drew some different sketches of some different ideas, and I'd really like to know which of these people like.

      And finally, the ranger.  I just have a couple different variations on her outfit.  With and without a pauldron, and a shoulder cape.

      And for context, here's a sample scene with some slightly older versions of these characters if you want to see what kind of visual environment they are designed for: https://imgur.com/cZ4oR6L
      Let me know what you think!
    • By gustavo rincones
      Hi, this is my first forum and I want to do it: quick way to calculate the square root in c ++ with floating data types. These types of functions are very useful to gain some CPU time, especially when used continuously. I will show you 3 similar functions and indicate the advantages and disadvantages of each of them. The last of these three functions was written by me. If you notice that the writing is a bit out of grammar, it is because I do not speak English and I am using a support tool. My native language is Spanish. Well, let's start:

      The First method is very famous was used in the video game quake III arena and you can find a reference in Wikipedia as: :https://en.wikipedia.org/wiki/Fast_inverse_square_root.
      The Function was optimized for improvements in computing times.
      float sqrt1(const float &n) { static union{int i; float f;} u; u.i = 0x5F375A86 - (*(int*)&n >> 1); return (int(3) - n * u.f * u.f) * n * u.f * 0.5f; }  
      * When Root of 0 is calculated the function returns 0.
      * The convergence of the function is acceptable enough for games.
      * It generates very good times.
      * The Reciprocal of the root can be calculated by removing the second “n” from the third line. According to the property of: 1 / sqrt (n) * n = sqrt (n).
      * Convergence decreases when the root to be calculated is very large.
      The second method is not as famous as the first. But it does the same function calculate the root.
      float sqrt2(const float& n) { union {int i; float f;} u; u.i = 0x1FB5AD00 + (*(int*)&n >> 1); u.f = n / u.f + u.f; return n / u.f + u.f * 0.25f; }  
      * The convergence of the function is high enough to be used in applications other than games.
      * Computing times are much larger.
      * The square root of “0” is a number very close to “0” but never “0”.
      * The division operation is the bottleneck in this function. because the division operation is more expensive than the other arithmetic operations of Arithmetic Logic Units (ALU).
      The third method takes certain characteristics of the two previous methods.
      float sqrt3(const float& n) { static union {int i; float f;} u; u.i = 0x2035AD0C + (*(int*)&n >> 1); return n / u.f + u.f * 0.25f; }  
      * The convergence of the function is greater than that of the first method.
      * Generates times equal to or greater than the first method.
      * The square root of “0” is a number very close to “0” but never “0”.
      The 3 previous methods have something in common.
      They are based on the definition of the Newton-Raphson Method. according to the function of the square root > f (x) = x ^ 2 - s.
      well thanks to you for reading my forum.
      well thanks to you for reading my forum.
    • By Cringey Boy
      Looking for a 2D artist to make with me a top-down game with cool features, guns, spells, and powerups. I'm a programmer, and I already made the code for the guns, different spells, powerups and basic mechanics like shooting and moving and stuff like that. I just don't have any assets to use so I'm looking for 1-2 2D artists, can be a pixel artist or anything that you want. Compensation will be 50% for you and 50% for me if we are only 2 and will be different if we are gonna be a trio, you are not working for me (or volunteering) we are a team. The only thing that I control and you not is the money, but you can argue with me and I will probably give you the amount that you think that you deserve. It doesn't have a name yet, we will decide about the name together. You can create guns with no coding because of a system that I created so you will also be able to create content for the game, besides ideas and art. I really need an artist so if you are interested please contact me in discord: #1615Cringey Boy
      I will leave a video to see the game and also the build to try and actually play the game that I have right now. I don't have any art so it looks bad (;
      https://drive.google.com/drive/folders/1Na3JKPBYXuUpxtP-lBUO-hIl0xO1ujSj?usp=sharing < this is the build, just download the folder called "Dungeon" and in there press on Dungeon.exe to open the game.
      1. Switch to the main gun
      2. Switch to the secondary gun
      Mouse Left Click. Shoot (you also aim with the mouse)
      R. Reload the gun that you are holding.
      E. Use main spell (currently, fireball which explodes and deals damage. And you also aim that with the mouse)
      Q. Use secondary spell (currently, heal aura which heals you pretty fast. You don't need to aim)
      (In the video there is no restart but in the build, there will be a restart button when you die)

      Desktop 2019.10.21 - Desktop 2019.10.21 - )

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

GameDev.net 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!