Intent to Ship: WebGPU: WGSL Fragment Depth

[Chromestatus]

Contact emails

dsin...@google.com

Explainer

https://github.com/gpuweb/gpuweb/blob/main/proposals/fragment-depth.md

Specification

https://github.com/gpuweb/gpuweb/pull/6299

Summary

Adds the ability to provide a `less` or `greater` modifier to the `@builtin(frag_depth)` in WGSL. The current `@builtin(frag_depth)` can potentially introduce a performance penalty due to disabling the early-Z optimizations on a draw call. The new modifiers allow the explicit setting of the buffer mode and allow the early-Z optimizations to be applied.

Blink component

Blink>WebGPU

Web Feature ID

webgpu

Motivation

In the current WGSL specification, the mere act of writing to @builtin(frag_depth) often incurs a significant performance penalty because driver heuristics cannot guarantee that the fragment shader output will adhere to the depth written by the rasterizer's interpolated depth. Consequently, writing to frag_depth typically forces the GPU to disable crucial early-Z optimizations for the entire draw call. The introduction of a new depth_mode built-in parameter for the @builtin(frag_depth) with modes less, and greater directly addressing this performance limitation by letting the developer express their intent to the hardware.

Initial public proposal

https://github.com/gpuweb/gpuweb/blob/main/proposals/fragment-depth.md

TAG review

No information provided

TAG review status

Pending

Goals for experimentation

None

Risks

Interoperability and Compatibility

This feature has been approved in W3C GPU for the Web WG meetings including participants from Safari and Firefox.

Gecko: No signal (https://github.com/gpuweb/gpuweb/pull/6299) Feedback indicated via WebGPU standardization process.

WebKit: Positive (https://github.com/gpuweb/gpuweb/pull/6299) Specification approval constitutes a positive signal, per https://github.com/WebKit/standards-positions/issues/294#issuecomment-1877411933

Web developers: No signals

Other signals:

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

No information provided

Debuggability

No information provided

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

Yes
All platforms will eventually have support. Will immediately be available on Android, Android WebView, ChromeOS, Mac, and Windows, where hardware support is available. Linux is planned to have WebGPU support in the future, so this feature will become available when WebGPU does.

Is this feature fully tested by web-platform-tests?

Yes
WebGPU/WGSL have a conformance test suite (https://github.com/gpuweb/cts) that is regularly pulled into Chromium and part of the testing of Dawn/Tint in Chromium. While the CTS can be embedded in WPT, the WebGPU team opted to keep it separate in Chromium testing to use a customized harness for robustness and performance. * https://github.com/gpuweb/cts/pull/4679 * https://github.com/gpuweb/cts/pull/4689

Flag name on about://flags

No information provided

Finch feature name

WebGPU

Rollout plan

Will ship enabled for all users

Requires code in //chrome?

False

Tracking bug

https://crbug.com/457993779

Estimated milestones

No milestones specified

Anticipated spec changes

Open questions about a feature may be a source of future web compat or interop issues. Please list open issues (e.g. links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (e.g., changing to naming or structure of the API in a non-backward-compatible way).

No information provided

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5663304168112128?gate=5657119415205888

This intent message was generated by Chrome Platform Status.

[Alex Russell]

Are there really no signals from developers? Is nobody asking for this?

[dan sinclair]

There are signals, but indirectly. Developers have requested having access to early-z-test (https://github.com/gpuweb/gpuweb/issues/4891#issuecomment-3341897141). But, if you write to the `frag_depth` it will disable the early-z test. Adding the `less` or `greater` attribute allows the fragment shader to write to the fragment depth, while maintaining the early-z optimization because the shader knows you're only writing in the depth in a given direction.

So, while developers did not specifically comment on this, it is a component of providing a comprehensive early-z feature in the future.

dan

[Ashley Gullen]

FWIW as developers of the browser game engine Construct (https://www.construct.net/) we want this to be able to optimize 3D rendering in WebGPU. We already use EXT_conservative_depth in the WebGL renderer where available, and this brings the same feature to WebGPU.

Ashley

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[Yoav Weiss (@Shopify)]

LGTM1

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[Daniel Bratell]

LGTM2

/Daniel

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[Vladimir Levin]

LGTM3

LGTM1

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