Overclockers Club published a review on the AMD & NVIDIA 4K Gaming with the ASUS PQ321Q
A quote from the article:
One of the challenges that presents itself is how to eliminate vertical and horizontal tearing in game. AMD is currently working on something, but still has a way to go as you will see a vertical line in fast paced games as you move left to right. Additionally I saw instances of horizontal tearing in specific games. Hopefully this gets addressed soon with a driver release that supports higher resolutions. Where the deficits are obvious with AMD, NVIDIA has stepped up and delivered two technologies that seem to eliminate these issues with its GPUs in Fliplock that "forces each GPU to flip frame buffers in sync and Scanlock that forces each GPU and each head to display scanlines in sync". Adjusting the refresh rate to 60Hz does help AMD some in this respect but does not fully eliminate the split in the panels. Again something that hopefully gets fixed sooner than later, and who knows, it might be introduced with AMD's next-gen cards.AMD & NVIDIA 4K Gaming with the ASUS PQ321Q Review @ OCC