Why the OS calls your view again after the app is already in the background

When the user presses the Home button iOS

1. sends your application the usual life-cycle messages
2. immediately takes an image of all the windows that are on-screen (this is the picture that is shown if the user re-enters the application without having killed it)
3. still while your process is alive, asks your view hierarchy to re-layout itself for every orientation / size class that might be needed later (portrait, landscape, Slide-Over widths …).

Those last layout passes are not there so that you can render with OpenGL again – they are only there so that you can put ordinary UIKit content in the hierarchy if you want the system to take an additional snapshot that looks better than the one it already has.
Because Core Animation will not rasterise a CAEAGLLayer that has no current contents, the second (or third…) snapshot often ends up being a scaled version of the first one. That is what you are seeing.

The rule “Background Apps May Not Execute Commands on the Graphics Hardware” is therefore not contradicted: the system is giving you the opportunity to substitute some non–OpenGL content before it takes those extra pictures; it is not asking you to draw with OpenGL.

What shipping OpenGL games do

Almost every game that looks good in the App-Switcher does exactly this:

• Just before it stops using OpenGL it puts an ordinary UIKit view – most of the time an UIImageView that contains a prerendered picture – on top of the GL view.
• When the game becomes active again it simply removes that UIImageView and starts using OpenGL once more.

Because the overlay is a normal UIKit view, Core Animation can render it again and again during the additional “pre-snapshot” layouts and the snapshot that iOS stores for each orientation is sharp.

Step-by-step recipe

1. Add a static image (or a different image for every orientation / Slide-Over width) to your asset catalogue.

2. Keep a pointer to an UIImageView that will be used as an overlay
   UIImageView *_snapshotCover;

3. In your application delegate

   - (void)applicationWillResignActive:(UIApplication *)application
   {
       // 1. build or reuse the overlay
       if (!_snapshotCover) {
           UIImage *img = [self _snapshotImageForCurrentTraitCollection];
           _snapshotCover = [[UIImageView alloc] initWithImage:img];
           _snapshotCover.contentMode = UIViewContentModeScaleAspectFill;
           _snapshotCover.autoresizingMask =
                 UIViewAutoresizingFlexibleWidth |
                 UIViewAutoresizingFlexibleHeight;
       }
   
       _snapshotCover.frame = self.window.bounds;
       [self.window addSubview:_snapshotCover];
   }
   
   - (void)applicationDidBecomeActive:(UIApplication *)application
   {
       [_snapshotCover removeFromSuperview];
       // the UIImageView is kept in memory so it can be reused next time
   }
   

4. (Optional) If you want the picture to follow size-class changes while you are still active (Slide-Over) adjust the image in viewWillTransitionToSize: or in traitCollectionDidChange:.

Nothing in the code above talks to the GL context, so it fully respects Apple’s rule about GPU calls after applicationDidEnterBackground:. At the same time the system has crisp UIKit content to snapshot for every orientation or size class it asks for, which means:

• A good looking thumbnail in the iOS App-Switcher
• No ugly scaled picture when the user launches the app again
• Still 100 % compliant with the multitasking rules for OpenGL ES.

Launch-image fallback

If you do nothing special the system will try to fall back to the launch screen storyboard / launch images when it cannot obtain a fresh snapshot. Therefore, as a minimal fix, make sure that your launch resources exist for every orientation you intend to support. Using the overlay technique above gives you full control and a better result, but supplying the correct launch images is enough to avoid the grotesquely scaled picture you described.

Summary

1. You really must not issue OpenGL commands once the app is in the background.
2. Add ordinary UIKit content (for instance an UIImageView) on top of your GL view in applicationWillResignActive:; remove it in applicationDidBecomeActive:.
3. That UIKit content will be rendered during the extra layout passes and becomes the crystal-clear snapshot that is shown in the App-Switcher for every orientation and for Slide-Over.