Gamemaker Studio: Debugging

 This section outlines the debug tool and how to correct and detect errors in your game.

GameMaker: Studio has a built-in programming language called GML, and once you become more familiar with how GameMaker works, you will want to use it to its fullest extent, meaning that you will need to program aspects of your game in this language.

However, when programming code it can be very easy to make mistakes - using the wrong variables, passing the wrong arguments, or using the wrong functions - which are not always detected by the syntax checker that is included with the script editor (for more information on the script editor see Scripts). This means that these mistakes will only become apparent when you actually run your game, and even then it can sometimes be difficult to know exactly what is going wrong! It can therefore be of vital importance to know how your game is performing on the chosen device, as well as see what instances are in the room, what global variables have been created and a whole host of other details.

Now, you can check a lot of these things yourself by adding in "debug code" to your games to display on-screen the information you require, or that shows messages when there is an error, but this is often a lot of work and if you aren't sure where the problem is in the first place then it can be a time consuming case of trial and error to track down your bug. For that, GameMaker: Studio has a special run mode called Debug Mode which will start the Debug Module, and there is also a comprehensive error reporting system which displays messages at run time to help you track down any problems with your game.

Error Reporting

When an error occurs during the execution of a script this is reported with an onscreen message, much like the one shown below:

Most error messages follow the same outline, with a message stating the type of error, the instance and event in which the error was encountered, then a series of compiler details that explain the error in a lot more depth and, finally, a list of all the declared global, instance and local variables in your game at that point. At the very bottom is the Abort button which will abort the game, as well as the Copy button (which copies the error into the clipboard) and the Clear button which clears the error messages from the window. The information provided by the error reporting is very important as it detects and reports the large, syntax related bugs that are going to cause your finished game to crash, but it won't report more subtle bugs, nor does it give feedback on performance... for that you need to use the Debug Mode.

Apart from these compile errors, you can sometimes get compiler errors. These will be shown (where possible) in a separate window, with a list of the possible problems. These types of errors are more general than compiler errors and may be due to some subtle error in the game.

You can find further information on the different compiler errors and runner errors that are shown from here.

Debug Mode

If you need to check things more carefully, you can run the game in debug mode. When you do, the Debug Module will open along with your game. This module permits you to monitor how everything is working in your game, from the global performance, right down to the values of individual variables in individual instances. You can find out further information about the Debug Module and it's possibilities and how to use it from the following page:

  • The Debug Module

Checking Performance

When running a game for debugging, you can also see how your game performs and get an overview of how everything is performing by using the Profile option in the Debug Module. This will give very detailed information on how a game is performing and how long it takes to perform specific function calls or events (see the section on Profiling for further details). However this requires that the debug module is running which may not be what you want, especially if you wish to test final executables on devices etc...

In these cases, and as a compliment to the profiler too, you can display the debug bar in your game, which will display much memory, CPU power and GPU power your game is using.

This Debug Bar shows in a helpful graphic of CPU/GPU usage in the actual game window itself, and this can be toggled on and off in your game using the function show_debug_overlay(). This bar is split into sections, with each section being 1/60th of a second. As you can see from the image below, the bar is made up of various colours, with each one signifying a different aspect of the games performance, where the larger the bar, the more time that aspect requires and the larger the performance hit from it:


  • Green - Input / Output processing (ie: keyboard, mouse, gamepad, networking etc...)
  • Red - The update speed of the step event
  • Yellow - The time required for the draw event
  • Orange - Debug update time, which is only normally visible when you use the debug module
  • White - GPU left over time, which is the time spent waiting for the GPU to finish the rendering of the frame before the next one can start
  • Cyan - The text rendering time
  • Grey - The time required to clear screen each draw step
  • Dark Red - The GPU flush, which is how long the GPU takes to clear images from memory


Apart from the coloured bars, there are also a few values at the top. These values represent the following:

FPS - This is the "Frames per Second" of your game, and basically shows how fast your game is running. This value is not clamped to the room speed of your game, and so shows you how much "overhead" you have to play with. The larger this value compared to your room speed, the smoother your game will run and the more overhead you have for adding further things. Note that this value may vary a lot during a session, but when you close your game, the compiler window within GameMaker: Studio will show you the minimum and maximum as well as the average values for this (note that the minimum value can be negative! this is not a bug, but due to the startup times and the way that the game is processed when initialised).

Texture Swaps - This number shows how many times your game is swapping textures per game tick (the time it takes GameMaker: Studio to run all events is a game tick, and the number of game ticks per second is set by the room speed). Textures (sprites and backgrounds) are stored on texture pages, and if you have a lot of image assets in your game, then GameMaker: Studio has to swap between texture pages to draw them all, therefore if this value is high (over 15-20, for example) you really should look into ways to optimise this as it will affect the performance of your game.

Vertex Batches - This is how many times the game is sending off texture batches to the GPU per game tick, and (as with Texture Swaps) you want this to be as low as possible. Texture batches, are basically bundles of data that are sent all at once to the GPU for rendering. The batch is "broken" when you change a font, a colour, a blend mode, or any other draw status that affects how things are drawn on a global scale, so you should try to limit these things to as few objects as possible since a high number of batches will adversely affect your games performance.

NOTE: texture swaps and vertex batches will never be zero and will normally show values of 2 or 3, since even with an empty room an no objects GameMaker: Studio still has to draw and batch things.

With these tools you can see how well your game is performing and what areas need "tweaked" to increase this performance. You can find out more information on performance issues and ways to optimise your game on the YoYo Games Helpdesk - Optimising Your Games.

Clearing the Cache

GameMaker: Studio has an asset compiler cache which stores sprites, sounds, object data, etc... between builds of your game. This greatly increases build time as each asset only has to be re-created only when it has changed. However, this cache can become "stale" and odd behaviour can creep into your games, like the wrong sound playing when something happens, or graphics becoming corrupted. When this happens it's time to clear the asset compiler cache using the "broom" icon at the top of the screen:

This will clear all cached files and the next test run or final build will recreate all the files for your game from scratch. Note that you should generally do this anyway every few builds, and always before building a final executable package.

Command Line Parameters

When creating games it is important that you play test the executable file to make sure that the final compile is correct and has no unforeseen errors or differences. However creating an executable means that since you have no access to the debug console - nor to the compiler window - it can be difficult to pin down and debug any problems that you may find. However there is a fix for this, and that is to use command line parameters to set certain properties when the executable version of the game is run, in turn permitting you to create output log files and test other special situations.

NOTE: This is available on the standard Windows target module only.

To get this working you must first create a Windows executable of your game and have it installed on your test machine. Once you have that done, you need to create a Shortcut to the game on the desktop. This is where we will add in the parameters to be passed to the *.exe (to create a shortcut, right click the game executable and then select Send to > Desktop (create shortcut)) .

Once you have made your shortcut you can then use any of the following parameters in the target path:

-noaudio: This switches off all audio in your game, no matter whether you are using the legacy sound functions or the new audio ones.


-inawindow: Forces the game to start in a window, even when set to run in fullscreen mode.


-output : sends console output to the filename


-debugoutput : sends debug messages and console output to the filename


-software: Will force the game to use Software Vertex Processing instead of hardware for rendering the game graphics. This is especially useful for those PCs that are using an on-board Intel GFX chipset or show display issues on older machines.


-intel: This will switch on a fix for bad Intel drivers, no matter what GPU is being used. This work around for those bad drivers will slow things down, giving a noticeable performance hit for your game, so if you do not need the fix do not use it.


-vanillaGFX: Using this switches off any check for driver manufacturer, and can be very useful for testing your game "as is".

It's worth noting that the in -output and -debugoutput can be the same one if desired, as the file is opened and closed on each write.

You should now go to where you created the shortcut to the game executable, and then right click on it and select Properties:

Command Line Shortcut This will open a window similar to the one above. Here you can add into the Target path your command line. Simply scroll along to the end of the given path, add a space, then the parameter string. So, for example, to create a debug file output you would have:

D:\Users\Me\GameMaker\CustomLoadBar-Default-\CustomLoadBar.exe -debugoutput debug.txt

This will now create a file called "debug.txt" in the same location as the executable (not the shortcut location, but where the game is installed) with debug and compiler information from your finished game. Click on "OK" and then use the shortcut to run your game to have it use these parameters and help with your debugging.


A/B Testing Using The Command Line

The YoYo runner also benefits from a special command line parameter:

-game : loads the given game file

Why would you use this? Well, normally you wouldn't, but if you wish to have more than one version of your game running at a time it can be done using this, making A/B comparative tests far easier. With this method you could focus test with someone else or, as the designer, get a better feel for things by doing side-by-side tests.

To set this up, you will need to know a couple of things beforehand. To start with you will need the path to the Runner itself. This can be found in the Appdata Roaming folder, using the following in Explorer:


You will also need the path to the .win file, as this is what we are going to point the runner to. The easiest way to find this is to go to the Temp folder and just search for *.win, as that will show you all those available and you can simply pick that which you need to get the full path from.

Now, we could create a shortcut to the runner like we did for the exe, and then change the parameters as necessary, but that means having to open and edit it between tests, which defeats the purpose of our using this functionality for side-by-side testing. Instead we are going to create a couple of tiny "batch" files to handle this.

To do this, first create a new text document on your desktop and open it in Notepad. Add the following:

START %appdata%\GameMaker-Studio\Runner.exe -game \.win

You need to to set the temp path to point to your game *.win file, and then save this as "GM_Test_1.bat" to your desktop (be aware that Notepad will save automatically as a *.txt file unless you choose "all files" from the save dialogue options). An example of how your final command should look would be:

START %appdata%\GameMaker-Studio\Runner.exe -game D:\Users\Me\AppData\Local\Temp\gm_ttt_44657\

 Now, create another text file and add the same line, only this time you point it to the path for the second version of the game that you want to do side-by-side testing with, and you would save this as "GM_Test_2.bat". You can double click either of them to bring up the different versions of your game, and having them as batch files means that you can easily open them again to edit them and change the game being tested.