[Interest] Tearing in Quick2 flickable-based items with clipping enabled

[Sletta Gunnar]

If this consistently reproducible across desktop and device, then I would appreciate a bugreport with an example that reproduces it. It is not a known issue to me at least. (bugreports.qt-project.org)

Clipping is implemented using scissor or stencil depending on the complexity of the mask, no intermediate texture is involved.

cheers,
Gunnar

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Fra: interest-bounces+gunnar.sletta=digia.com at qt-project.org [interest-bounces+gunnar.sletta=digia.com at qt-project.org] på vegne av Ola Røer Thorsen [ola at silentwings.no]
Sendt: 13. november 2013 15:29
To: interest at qt-project.org
Emne: [Interest] Tearing in Quick2 flickable-based items with clipping enabled

Hi all,

I'm seeing tearing-effects when scrolling in ListView and other items based on Flickable when clipping is enabled. This is running on both a Linux desktop as well as an embedded Linux device (eglfs). The Qt version is 5.1.1.

The systems are running with vsync enabled and double-buffering. There is no tearing in any other items except the ones with clipping enabled. Tearing is especially noticeable on the embedded device. The GPU is a PowerVR SGX chip which is tile-based, so the tearing "lines" are actually vertical. Also the framerate is mostly somewhere between 30-60.

Tearing appears maybe 20% of the time spent scrolling.

I'm wondering if the core issue is this:

I assume clipping is implemented as rendering the item into a temporary texture, and the clipped area is of this is finally rendered to the screen buffer. This temporary texture is not double-buffered. This means it would be possible that the render pipeline starts writing into a texture (next frame) that is being drawing into the screen buffer (current frame).

Any thoughts? The tearing is really bad sometimes, and I don't really have the option of disabling clipping either.

Cheers,
Ola



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