Present with damage region (VkPresentRegionKHR)

[valpackett]

Is your feature request related to a problem? Please describe.

Looks like there is no way to pass the VkPresentRegion (and equivalents on other platforms) when presenting a frame.

The thing the Vulkan WSI-side code gets as the last arg here:

static VkResult
wsi_wl_swapchain_queue_present(struct wsi_swapchain *wsi_chain,
                               uint32_t image_index,
                               const VkPresentRegionKHR *damage)

Describe the solution you'd like

Some way to do that. I can't find anything like that in the WebGPU spec :(
Are custom extensions allowed in wgpu?

Additional context

Damage tracking allows applications to only render changed parts between frames, saving power. In this case, I'm only talking about the compositor-side thing, where applications tell the compositor which regions of the window have changed so the compositor can only update them. This is called EGL_KHR_swap_buffers_with_damage in the GL world.

[kvark]

Thank you for filing! It's great to see a well made issue 👍

I think damage tracking is only feasible if you have full control over your swapchain. I.e. if you rendered to swapchain frame X before, and you render to it again, you know which parts you can rely on being preserved.

In WebGPU, every frame is a new frame. So I don't think with the current swapchain model the partial present is an option. An issue on https://github.com/webgpu-native/webgpu-headers would help to kick off the discussion. cc @Kangz

[Kangz]

We'll certainly need more and more presentation extensions. In a way we can do whatever we want for now, as long as the "core" API can be translated to GPUSwapChain, then coalesce APIs later.

[valpackett]

you know which parts you can rely on being preserved

Here, I'm not talking about preserving buffers in the webgpu application, just about the damage hint to the windowing system so the system compositor would preserve parts of the whole monitor output's buffer. Again:

In this case, I'm only talking about the compositor-side thing, where applications tell the compositor which regions of the window have changed so the compositor can only update them. This is called EGL_KHR_swap_buffers_with_damage in the GL world.

This works just fine with every frame being a new frame, in fact Firefox's legacy GL compositor always renders a full frame and I have added SwapBuffersWithDamage to it.

[kvark]

@myfreeweb the damage region needs to be specified relative to something. We could accept it relative to the previous frame, an internally accumulate the regions between frames of the swapchain. Is this close to what you are thinking?

[valpackett]

Why would there be a need to accumulate anything? o_0 It already is defined as

rectangle of the given image that has changed since the last image was presented to the given swapchain

https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VkPresentRegionKHR.html

It's just straightforward data passing, nothing clever is required.

[kvark]

o_0 It already is defined as

If I read it correctly, the spec specifically describes what regions of a particular VkImage are dirty:

VkPresentRegionKHR - Structure containing rectangular region changed by vkQueuePresentKHR for a given VkImage

WebGPU doesn't expose the fact that a swapchain contains multiple images that are recycled. User always gets a new image (from their perspective). So wgpu would internally acquire images from the swapchain.

Let's consider this example:

• first frame is acquired image 1 (of the Vulkan swapchain)
• next frame is image 2, presented with dirty rect A
• next frame is image 0 presented with dirty rect B
• next frame is image 1, presented with dirty rect C

If webgpu-native specifies the dirty rect relative to the previous frame, then the damage regions we pass to the last Vulkan present (internally) would have to include all of A | B | C.

[valpackett]

Nah, you really have to read the pRectangles member description carefully:

rectangle of the given image [yes, as in the struct description] that has changed since the last image [any image — does not say this one!] was presented to the given swapchain

This is a convoluted way of saying "relative to the previous frame".

And so, Wayland WSI code in Mesa literally just turns this into wl_surface_damage_buffer:

      for (unsigned i = 0; i < damage->rectangleCount; i++) {
         const VkRectLayerKHR *rect = &damage->pRectangles[i];
         assert(rect->layer == 0);
         wl_surface_damage_buffer(chain->surface,
                                  rect->offset.x, rect->offset.y,
                                  rect->extent.width, rect->extent.height);
      }

Wayland also doesn't care about VkImages, this is also just "damage since the last frame".

[kvark]

Ok, that's good to know! I find Vulkan spec wording to be confusing here.

[cwfitzgerald]

Note that currently wgpu doesn't allow you to write to a swapchain image without clearing it first, so if things like iced want to take full advantage of it, we would have to allow that somehow.

[valpackett]

@cwfitzgerald yeah, that's kind of an orthogonal issue. "telling the compositor what changed" and "not redrawing stuff on buffers" do not depend on each other, they just use the same data (damage tracking)

[cwfitzgerald]

Ah yes, you're right. I somehow thought you had to do one to do the other, 'pologizes

[cwfitzgerald]

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