Clearing a texture used as render target: issue on iOS with active anti-aliasing

[PrimaryFeather]

Problem Description

When activating anti-Aliasing on a Starling FragmentFilter on iOS, the filter contents accumulates in each frame, i.e. it seems as if context.clear (with a texture as render target) would not work any longer.

Background information:

Starlings FragmentFilters render objects into a texture so that they can then be processed by subsequent shaders. When you assign a vanilla FragmentFilter to a display object, it just displays that texture.

On iOS, it seems as if the texture (render target) wasn't cleared correctly as soon as antiAliasing is activated on the filter (which is forwarded to context.setRenderTexture).

• Tried with AIR 27.0.0.128 and AIR 28.0.0.120
• on an iPhone 6S running iOS 11.1.2
• using Starling 2.2
• Adobe Tracker Issue AIR-4198517

Steps to Reproduce

Run the following code as part of a Starling root object:

var quad:Quad = new Quad(100, 100, 0xff0000);
quad.alignPivot();
quad.x = quad.y = 100;
quad.filter = new FragmentFilter();
quad.filter.antiAliasing = 2;
addChild(quad);

Starling.juggler.tween(quad, 3, { rotation: Math.PI * 2, repeatCount: 0 });

The result, which should be a red rotating quad, looks like this:

Remarks:

• This does work fine on macOS and the AIR iOS emulator.
• I tried to reproduce the issue with pure Stage3D code, without Starling, but I could not do it. It must be some combination of settings Starling is using.

Known Workarounds

Starling's filters also support a resolution property that works similar to anti-aliasing, but doesn't rely on Stage3D features. That seems to work fine.