CODEWITHSUNDEEP
vulkan
CromfCromf
CromfCromf

Reputation: 49

In Vulkan, using FIFO, happens when you don't have an image ready to present when the vertical blank arrives?

I'm new to graphics, and I've been looking at Vulkan presentation modes. I was wondering: in a situation where we've only got 2 images in our swapchain (one that the screen's currently reading from and one that's free), what happens if we don't manage to finish drawing to the currently free image before the next vertical blank? Do we do the presentation and get weird tearing, or do skip the presentation and draw the same image again (I guess giving a "stuttering" effect)? Do we need to define what happens, or is it automatic?

As a side note, is this why people use longer swap chains? i.e. so that if you managed to draw out 2 images to your swap chain while the screen was displaying the last image but now you're running late, at least you can present the newer of the 2 images from before?

I'm not sure how much of this is specific to FIFO or mailbox mode: I guess with mailbox you'll already have used the newest image you've got, so you're stuck again? [2-image swapchain][1] [1]: https://i.sstatic.net/rxe51.png

Upvotes: 2

Views: 1512

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473447

Tearing never happens in regular FIFO (or mailbox) mode. When you present an image, this image will be used for all subsequent vblanks until a new image is presented. And since FIFO disallows tearing, in your case, the image will be fully displayed twice.

If you are using a 2-deep swapchain with FIFO, you have to produce each image on time in order to avoid stuttering. With longer swapchains and FIFO, you have more leeway to avoid visible stuttering. With longer swapchains and mailbox, you can get a similar effect, but there will be less visible latency when your application is running on-time.

Upvotes: 3

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