Fix VP2 rendering of UsdPreviewSurface with opacityThreshold #3947

jufrantz · 2024-10-07T09:32:41Z

This PR fixes VP2 support for the UsdPreviewSurface opacityThreshold input, aligning it with Pixar's UsdPreviewSurface Specification.

Note: When opacityThreshold is greater than zero, opacity values less than the threshold will not be rendered, while values equal to or greater than the threshold will be fully visible.

Included Changes:

Fixed the UsdPreviewSurfaceLightAPI*.xml fragment graphs to expose opacityThreshold. The ShadeFragment node input was not connected to the MShaderInstance parameter, which had no effect.
Updated fragment shader implementations so that non-discarded fragments are fully opaque when opacityThreshold is positive.
Aligned the C++ implementation PxrMayaUsdPreviewSurface::compute for software rendering.
Added unit tests.

Results:

Snaphots of the scene included in the unit tests. TestOpacityThreshold.usda features 5 planes with materials bound that have textured opacity, with opacityThreshold gradually increasing from 0.0 to 1.0 from left to right.

usdview/HdStorm

vp2RenderDelegate before PR

vp2RenderDelegate after PR

…urface fragment graphs.

…s greater than or equal to the opacityThreshold.

JGamache-autodesk

Nicely done! Thank you!

seando-adsk · 2024-10-08T18:03:27Z

@jufrantz Unfortunately the preflight failed two different build configurations:

Maya 2022, Interactive, Windows:

======================================================================
ERROR: testOpacityThreshold (__main__.testVP2RenderDelegateUSDPreviewSurface)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../maya-usd/test/lib/mayaUsd/render/vp2RenderDelegate\testVP2RenderDelegateUSDPreviewSurface.py", line 285, in testOpacityThreshold
    mayaUsdLib.PrimUpdaterManager.editAsMaya(stageShapeNode + ",/scene/camera")
AttributeError: 'module' object has no attribute 'PrimUpdaterManager'

I believe here you just need to conditionally run your new test:
@unittest.skipUnless(ufeFeatureSetVersion() >= 3, 'Test only available in UFE v3 or greater.')

Maya 2026, Interactive, Linux:

======================================================================
FAIL: testDrawModes (__main__.testVP2RenderDelegateDrawModes.testDrawModes)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../maya-usd/test/lib/mayaUsd/render/vp2RenderDelegate/testVP2RenderDelegateDrawModes.py", line 87, in testDrawModes
    self.assertSnapshotClose('%s_cross_all_positive.png' % self._testName, imageVersion)
  File ".../maya-usd/test/lib/mayaUsd/render/vp2RenderDelegate/testVP2RenderDelegateDrawModes.py", line 64, in assertSnapshotClose
    return self.assertImagesClose(baselineImage, snapshotImage)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../maya-usd/test/testUtils/imageUtils.py", line 208, in assertImagesClose
    self.fail(msg)
AssertionError: Images differed by 0.001973237790486565 (max allowed: 0.0001):
  .../maya-usd/test/lib/mayaUsd/render/vp2RenderDelegate/VP2RenderDelegateDrawModesTest/baseline/post-22_11-DrawModes_cross_all_positive.png
  .../maya-usd/test/lib/mayaUsd/render/vp2RenderDelegate/testVP2RenderDelegateDrawModesOutput/DrawModes_cross_all_positive.png

I'm not sure if your changes have affected this test. Maybe @JGamache-autodesk can help with this one?

Sean

JGamache-autodesk · 2024-10-08T20:03:10Z

The change also affects the card draw mode (which uses UsdPreviewSurface since 23.02 with an opacityThreshold of 0.1). This means @jufrantz will have to provide a set of updated baseline images for the testVP2RenderDelegateDrawModes.py unit test.

JGamache-autodesk · 2024-10-08T20:18:42Z

You can also take these:
updated_test_images.zip

… UsdPreviewSurface VP2 shader changes.

jufrantz · 2024-10-09T09:55:49Z

Thank you @seando-adsk @JGamache-autodesk.
I updated baseline images with those you provided for USD > 22.11.
I fixed the test with ufe v2 (editAsMaya was not really needed here, it now uses a maya native camera).
Julien

seando-adsk · 2024-10-11T14:55:33Z

@jufrantz I re-ran the preflight and it still failed on "Maya 2022, Python 2, interactive, Windows"

======================================================================
FAIL: testOpacityThreshold (__main__.testVP2RenderDelegateUSDPreviewSurface)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../maya-usd/test/lib/mayaUsd/render/vp2RenderDelegate\testVP2RenderDelegateUSDPreviewSurface.py", line 310, in testOpacityThreshold
    self.assertSnapshotClose('opacityThreshold.png')
  File ".../maya-usd/test/lib/mayaUsd/render/vp2RenderDelegate\testVP2RenderDelegateUSDPreviewSurface.py", line 60, in assertSnapshotClose
    return self.assertImagesClose(baseline_image, snapshot_image)
  File "...\maya-usd\test\testUtils\imageUtils.py", line 208, in assertImagesClose
    self.fail(msg)
AssertionError: Images differed by 0.0262658122277 (max allowed: 0.0001):

jufrantz · 2024-10-11T18:13:03Z

Sorry for this @seando-adsk,
I tried on maya2022 python2 linux and the test is passing.
This might be failing only on windows and I won't have access to a windows setup in the short term.
Are there any additional relevant logs ? Would it be possible to share the snapshot generated by the failing test?
Thank you,
Julien

jufrantz · 2024-10-14T10:16:41Z

Hello,
I found the issue. I was testing with Maya 2022.5 (Maya light API v2), I just tried Maya 2022.0 (API v1) and got the same image diff ratio than the preflight. Since I didn't add support for opacityThreshold with API v1 and adding it would require more work due to fragment graph changes in V2, I'll just disable the test in this case.
Julien

UsdPreviewSurface fragment graph for v1 does not support opacityThreshold input.

…dered as opaque. As discussed in PR [Autodesk#3952](Autodesk#3952).

JGamache-autodesk

LGTM. Only one minor comment.

JGamache-autodesk · 2024-10-16T21:22:22Z

lib/usd/pxrUsdPreviewSurface/usdPreviewSurface.cpp

+            PxrMayaUsdPreviewSurfaceTokens->OpacityThresholdAttrName.GetText());
+        const MDataHandle opacityThresholdData
+            = dataBlock.inputValue(opacityThresholdAttr, &status);
+        CHECK_MSTATUS(status);


Can you replicate the opacityConnected code you see for opacity, but on opacityThreshold? A connected opacityThreshold should also activate masked mode.

Thank you @JGamache-autodesk for your feedback,

Interesting point, but I think I disagree with this solution.

As you know, marking the whole material as transparent informs the rasterizer to use alpha blending for proper rendering (e.g. GL_BLEND and object sorting). Conversely, opaque material allows the rasterizer to perform simple depth testing and overwrite fragments. Moreover for masked materials, some fragments will be discarded without writing to the depth or color buffers, which still works with simple depth testing.

OpacityThreshold functions similarly to a combination of glTF alphaMode and alphaCutoff. For a material with authored opacity:

A zero opacityThreshold is equivalent to glTF.alphaMode = BLEND

Positive values would translate to glTF.alphaMode=MASK and glTF.alphaCutoff=opacityThreshold.

Texturing opacityThreshold is akin to texturing both alphaCutoff AND alphaMode: since you can get zero-valued threshold texels, you would expect a partially masked and partially translucent material. In this scenario, to ensure proper rendering of the semi-transparent regions, the only viable solution to me is to render these objects in the transparent pass (that would also still work with alpha cut/discarded regions).

For reference, HdStorm’s implementation of masked detection for GLSLFX UsdPreviewSurface only tests for a constant positive opacityThreshold value, while for translucent it tests opacity input connections and decides that a varying opacity differs from plain one, like the current VP2 shading node implementation.

https://github.com/PixarAnimationStudios/OpenUSD/blob/59992d2178afcebd89273759f2bddfe730e59aa8/pxr/imaging/hdSt/materialNetwork.cpp#L75-L83

// Next check for authored terminal.opacityThreshold value > 0 for (auto const& paramIt : terminal.parameters) { if (paramIt.first != _tokens->opacityThreshold) continue; VtValue const& vtOpacityThreshold = paramIt.second; if (vtOpacityThreshold.Get<float>() > 0.0f) { return HdStMaterialTagTokens->masked; } }

Moreover, I may have over-interpreted it but - as I understand it - the USDPreviewSurface Specification discourages a material with a texture used for translucency and mask.

Thus,` the opacityThreshold serves as a switch for how the opacity input is interpreted; this 'translucent or masked' behavior is common in engines and renderers, and makes the UsdPreviewSurface easier to interchange. It does imply, however, that it is not possible to faithfully recreate a glassy/translucent material that also provides an opacity-based mask… so no single-polygon glass leaves.

If we want to improve this, an alternative could be to disallow connections to the opacityThreshold parameter during VP2 shader translation. This restriction can be implemented with MPxShadingNodeOverride.

maya-usd/lib/usd/pxrUsdPreviewSurface/usdPreviewSurfaceShadingNodeOverride.cpp

Line 86 in dae864e

void PxrMayaUsdPreviewSurfaceShadingNodeOverride::getCustomMappings(

Please let me know if this approach meets your requirements,
Best,
Julien

Good explanation. I agree with you. This would mean the implementation of _GetUsdPreviewSurfaceMaterialTag() I added to materialXFilter.cpp is buggy since it considers textured opacityThreshold as masked.

Probably no need to forbid connections since the result is not fatal and will render correctly, albeit slowly, in a transparency pass.

jufrantz added 3 commits October 4, 2024 19:30

Fix opacityThreshold was not exposed as a property in the UsdPreviewS…

3a29045

…urface fragment graphs.

Fix UsdPreviewSurface should be fully opaque when the input opacity i…

7fd21a5

…s greater than or equal to the opacityThreshold.

Test UsdPreviewSurface opacityThreshold cut-outs.

43d61cc

seando-adsk requested a review from JGamache-autodesk October 8, 2024 13:38

seando-adsk added the vp2renderdelegate Related to VP2RenderDelegate label Oct 8, 2024

JGamache-autodesk previously approved these changes Oct 8, 2024

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seando-adsk assigned jufrantz Oct 8, 2024

jufrantz mentioned this pull request Oct 8, 2024

Optimize VP2 rendering of USD objects with geometric cut-outs #3952

Open

jufrantz added 2 commits October 9, 2024 10:41

Updated baseline images for testVP2RenderDelegateDrawModes to reflect…

2319620

… UsdPreviewSurface VP2 shader changes.

Fix testVP2RenderDelegateUSDPreviewSurface for maya 2022 (ufe v2).

02c65c1

jufrantz dismissed JGamache-autodesk’s stale review via 02c65c1 October 9, 2024 09:54

seando-adsk requested a review from JGamache-autodesk October 9, 2024 13:25

JGamache-autodesk previously approved these changes Oct 9, 2024

View reviewed changes

seando-adsk assigned jufrantz and unassigned jufrantz Oct 10, 2024

Merge branch 'dev' into fix_preview_surface_opacity_threshold

5bc1a66

jufrantz added 2 commits October 14, 2024 12:34

Skip testOpacityThreshold with Maya light API v1.

1ca8787

UsdPreviewSurface fragment graph for v1 does not support opacityThreshold input.

UsdPreviewSurface shading nodes with opacityThreshold are now VP2 ren…

661ea9d

…dered as opaque. As discussed in PR [Autodesk#3952](Autodesk#3952).

jufrantz dismissed JGamache-autodesk’s stale review via 661ea9d October 14, 2024 11:30

jufrantz requested a review from JGamache-autodesk October 14, 2024 11:32

JGamache-autodesk assigned jufrantz and unassigned jufrantz Oct 16, 2024

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Fix VP2 rendering of UsdPreviewSurface with opacityThreshold #3947

Fix VP2 rendering of UsdPreviewSurface with opacityThreshold #3947

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Fix VP2 rendering of UsdPreviewSurface with opacityThreshold #3947

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Included Changes:

Results:

usdview/HdStorm

vp2RenderDelegate before PR

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