Tag Archives: performance

Avoiding Performance Cost of OnGUI

The Unity profiler revealed recently that Pawns for iPhone was spending 5% of its time in a UnityGUI script that I didn’t need any more. Removing it from my scene was easy enough, but it got me thinking. The script looked something like this:

void OnGUI()
{
   if (showDialogBox)
   {
      // lots of code that never executed
   }
}

Not much going on here! So it appeared that just having OnGUI in a script was expensive. This was worth knowing because I still have three other OnGUI scripts left in the scene. Each of them exits immediately if their dialog box is not showing. So in theory they should cost nothing during actual gameplay. But the profiler told a different story:

It turns out that there are some major setup costs associated with OnGUI. GUI.Repaint is listed as taking over 65% of the CPU! Of that, most of the time is spent in GUIUtility.BeginGUI. Note that almost no time at all is spent in my three OnGUI methods (TutorialController.OnGUI, DlgOKCancel.OnGUI, and DlgPuzzleFooter.OnGUI.) So the early exit clause was working, but the damage was done before OnGUI was even called.

To verify this I disabled the three objects in the inspector while the scene was running. The new profiler results are much healthier:

GUI.Repaint has been eliminated. The time is now mostly spent on rendering, physics, and in scripts.

Since disabling the scripts solved the problem, I did the obvious thing, which was to to disable the objects with the OnGUI scripts on them until needed:

footerDlg.gameObject.active = false;

But one trick is needed to reactivate them. Search functions like Find and FindObjectOfType do not return inactive objects. Instead, you must already have a reference to the object you want to activate.

One way is to start the scene with active = true. Your script can then Find the object, store a reference to it, and then set active = false until it is needed. (This is what I did because it worked well with the code I’d already written.)

	void Awake()
	{
		// cache reference to footer dlg
		footerDlg = (DlgPuzzleFooter) GameObject.FindObjectOfType(typeof(DlgPuzzleFooter));
		if ( footerDlg == null )
		{
			Debug.Log("Error: DlgPuzzleFooter object is missing.");
		}
		else
		{
			// deactivate it until needed, for performance
			footerDlg.gameObject.active = false;
		}
        }

A second way is simply to declare a public variable for the inactive object, and then use Unity’s Inspector to set the value at design-time.

A third approach is detailed in a post on UnityAnswers. It keeps a list of every object that gets deactivated.

The fact that OnGUI looks like any other Unity method is misleading. It’s an expensive operation, even if it returns immediately without drawing anything.

UnityGUI itself is not recommended where performance is important, particularly during gameplay on an iPhone or Android device. But it is a convenient way to define menus, dialog boxes, high score lists, and other parts of the user interface. By deactivating OnGUI scripts until they are needed, you can often avoid paying much for that convenience.

Reducing UnityGUI Draw Calls

When trying to improve the performance of a Unity iPhone game, time spent reducing the number of draw calls is usually time well spent.

Unfortunately one of the big draw call “hogs” is UnityGUI. A useful guideline is to only use UnityGUI for game menus and option screens where framerate isn’t important. For in-game controls such as a HUD, developers resort to other methods. Alternatives include GUITextures, SpriteManager, or its commercial sibling SpriteManager 2.

However, other tricks are sometimes possible, and I found an especially easy way to speed up a ScrollView. My game Pawns has a scrolling list that allows the player to pick a puzzle:

Pawns puzzle picker

Pawns puzzle picker

I used UnityGUI for it, and since the game is not actually being played on this scene I was hoping it would be fast enough. However, the screen was unusably slow on my iPhone, and number of draw calls reported by Unity was 165! So a serious rewrite or redesign appeared to be needed.

The structure of my puzzle picker is a Gui.Window for the frame, containing a Gui.Scrollview. Inside of the scrolling view, I loop through the list of puzzles and paint the three elements that make up each line (a checkbox, text for the puzzle name, and a texture displaying the difficulty rating.) The code looks something like this:

scrollPosition = GUI.BeginScrollView (rScrollFrame, scrollPosition, rList, false, false);
 
for (int iPuzzle = Puzzles.GetFirstPuzzle(), numPuz = 1;
      iPuzzle >= 0;
      iPuzzle = Puzzles.GetNextPuzzle(iPuzzle), numPuz++)
{
    int difficulty = Puzzles.GetPuzzleDifficulty(iPuzzle)-1;
    if ( GUI.Button(rCheckbox, texSolved, checkboxStyle) ||
         GUI.Button(rBtn, Puzzles.GetPuzzleName(iPuzzle), puzzleButtonStyle) ||
         GUI.Button(rDifficulty, ratings[difficulty], puzzleButtonStyle) )
    {
        // Code to jump to the selected puzzle goes here...
    }
 
    // set up rectangles for the next line
    rBtn.y += lineHeight;
    rTexture.y += lineHeight;
    rDifficulty.y += lineHeight;
}
GUI.EndScrollView();

Each GUI element causes a draw call. (If you are using GUILayout instead of the GUI class, then double that.) The more puzzles in my list, the more draw calls.

For the total number to be as high as 165, even the lines which are currently scrolled off the screen are generating draw calls. On my screen if the player can’t see a line they can’t tap on it, and no code depends on them. In fact, I don’t need to draw them at all. If I was using GUILayout then I wouldn’t know the position of each element, but I’m using GUI, which requires me to position each element myself.

So it turns out it’s a simple matter to skip GUI elements that are not visible. You simply test the rectangle of each control before drawing it. In my case all scrolling is vertical, and the elements are arranged in lines, so I can skip entire lines by testing if either the top or bottom of each line is visible.

The code now looks something like this. Note the line marked *NEW*:

scrollPosition = GUI.BeginScrollView (rScrollFrame, scrollPosition, rList, false, false);
 
for (int iPuzzle = Puzzles.GetFirstPuzzle(), numPuz = 1;
      iPuzzle >= 0;
      iPuzzle = Puzzles.GetNextPuzzle(iPuzzle), numPuz++)
{
    // *NEW* don't actually draw the controls if this line's rectangle is not visible
    if ( rBtn.yMin >= scrollPosition.y && rBtn.yMax <= scrollPosition.y + rScrollFrame.height )
    {
        int difficulty = Puzzles.GetPuzzleDifficulty(iPuzzle)-1;
        if ( GUI.Button(rCheckbox, texSolved, checkboxStyle) ||
             GUI.Button(rBtn, Puzzles.GetPuzzleName(iPuzzle), puzzleButtonStyle) ||
             GUI.Button(rDifficulty, ratings[difficulty], puzzleButtonStyle) )
        {
            // Code to jump to the selected puzzle goes here...
        }
    }
 
    // set up rectangles for the next line
    rBtn.y += lineHeight;
    rTexture.y += lineHeight;
    rDifficulty.y += lineHeight;
}
GUI.EndScrollView();

This worked well; only 6 or 7 lines are ever visible at a time, so the draw calls now hovers around 27 no matter how puzzles are in the list. The screen is now usable on my iPhone, though I should probably cut the number of draw calls a little further.

Obviously not everyone will happen to be using ScrollViews so I’ll mention a few general approaches to improve UnityGUI’s performance:

  • Combine elements/textures. For example, if I painted each line with a single texture instead of three separate elements, that would cut the draw calls by two-thirds. I could go further and paint multiple lines, even entire pages in one texture.
  • Avoid GuiLayout. If you don’t use GuiLayout at all, and your GUI script sets
    [code language=”csharp”]useGUILayout = false;[/code] early, such as in the Awake method, then the number of UnityGUI draw calls will be cut in half.
  • Redesign. For my example I could have abandoned the scrolling interface and replaced it with page-up/page-down buttons.

I will probably need to try out alternatives such as GUITexture or SpriteManager to speed up my main game screen, so I may have more to say about them at a later date.