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USD: improve light units conversion #109795

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Brecht Van Lommel merged 4 commits from brecht/blender:usd-light-units into main 2023-07-07 18:27:19 +02:00
Conversation 4 Commits 4 Files Changed 5 +86 -50
Brecht Van Lommel commented 2023-07-06 20:14:42 +02:00
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  • Use pi factor to convert between radiant flux and intensity
  • Mark lights as normalized on export
  • Add spot light export support
  • Add treatAsPoint support for import and export
  • Empirically match normalized distant light
  • Fix wrong unnormalized point/sphere/disk light unit in Cycles

Overall it should be much closer now for all light types. Point and distant
light units are inconsistent between renderers, so not possible to match
everything there.

Ref #109404

* Use pi factor to convert between radiant flux and intensity * Mark lights as normalized on export * Add spot light export support * Add treatAsPoint support for import and export * Empirically match normalized distant light * Fix wrong unnormalized point/sphere/disk light unit in Cycles Overall it should be much closer now for all light types. Point and distant light units are inconsistent between renderers, so not possible to match everything there. Ref #109404
Brecht Van Lommel added 1 commit 2023-07-06 20:14:52 +02:00 
Use pi factor to convert from radiant flux to intensity, and fix wrong
unnormalized point/sphere light unit in Cycles.

Overall it should be much closer now for all lights. But point and distant
lights are still off and seemingly without a consistent convention in other
Hydra renderers.

Ref #109404
Brecht Van Lommel requested review from Michael Kowalski 2023-07-06 20:15:27 +02:00
Brecht Van Lommel requested review from Weizhen Huang 2023-07-06 20:15:27 +02:00
Brecht Van Lommel added 1 commit 2023-07-06 21:04:00 +02:00
Brecht Van Lommel added 1 commit 2023-07-06 21:21:54 +02:00
Michael Kowalski approved these changes 2023-07-07 02:01:21 +02:00
Michael Kowalski left a comment
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@brecht The changes look consistent to me, so I'm approving this PR. Just so I understand the motivation behind this, the goal appears to be to better match Karma lighting, is that right?

@brecht The changes look consistent to me, so I'm approving this PR. Just so I understand the motivation behind this, the goal appears to be to better match Karma lighting, is that right?
Brecht Van Lommel commented 2023-07-07 09:45:22 +02:00
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I'm trying to match all Hydra renderers, but unfortunately there is no documented convention and there are inconsistencies in implementations.

  • For sphere, spot, area and disk lights, PRMan and Karma are the same and we match them. Storm can not accurately render these so difficult to compare to, but the old factor of 100 makes no sense and the new one seems okay-ish.
  • For distant lights, the PRMan unit is weird (see #109404). So I matched it to Karma. Storm doesn't support distant lights.
  • For zero radius points lights, I will have another look to see if we can do better. PRMan doesn't support them, Karma for some reason normalizes them differently than sphere lights with very small radius.
I'm trying to match all Hydra renderers, but unfortunately there is no documented convention and there are inconsistencies in implementations. * For sphere, spot, area and disk lights, PRMan and Karma are the same and we match them. Storm can not accurately render these so difficult to compare to, but the old factor of 100 makes no sense and the new one seems okay-ish. * For distant lights, the PRMan unit is weird (see #109404). So I matched it to Karma. Storm doesn't support distant lights. * For zero radius points lights, I will have another look to see if we can do better. PRMan doesn't support them, Karma for some reason normalizes them differently than sphere lights with very small radius.
👍 1
Weizhen Huang reviewed 2023-07-07 11:10:26 +02:00
source/blender/io/usd/intern/usd_writer_light.cc Outdated
@ -10,6 +10,7 @@
#include <pxr/usd/usdLux/sphereLight.h>
#include "BLI_assert.h"
#include "BLI_math_rotation.h"
Weizhen Huang commented 2023-07-07 11:10:26 +02:00
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Seems unnecessary?

Seems unnecessary?
Brecht Van Lommel commented 2023-07-07 15:03:36 +02:00
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Indeed, though after the latest changes I did need it for spot lights.

Indeed, though after the latest changes I did need it for spot lights.
brecht marked this conversation as resolved
Weizhen Huang approved these changes 2023-07-07 11:10:49 +02:00
Brecht Van Lommel added 1 commit 2023-07-07 15:00:27 +02:00
Brecht Van Lommel commented 2023-07-07 16:17:03 +02:00
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I did various fixes for zero radius point lights, they match Storm now. For Karma it's off by a factor of 4, and the same inconsistency exists in Houdini when switching Karma and Storm. So no hope of matching both and I think matching a sphere light with small radius makes most sense.

I did various fixes for zero radius point lights, they match Storm now. For Karma it's off by a factor of 4, and the same inconsistency exists in Houdini when switching Karma and Storm. So no hope of matching both and I think matching a sphere light with small radius makes most sense.
👍 1
Brecht Van Lommel manually merged commit 05c97df2de into main 2023-07-07 18:27:19 +02:00
Pablo Delgado commented 2023-07-11 15:15:54 +02:00
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Hello @brecht, I want to draw attention to the current ongoing effort to streamline light handling in USD/Hydra-supporting renderers over at the ASWF slack (usd-lux-discussion channel).

As part of this effort, a comparison between different renderers has been conducted, and a tighter specification is being worked out.

Hello @brecht, I want to draw attention to the current ongoing effort to streamline light handling in USD/Hydra-supporting renderers over at the ASWF slack (usd-lux-discussion channel). As part of this effort, a [comparison between different renderers](https://github.com/anderslanglands/light_comparison) has been conducted, and a tighter [specification](https://github.com/anderslanglands/light_comparison/blob/main/specification/specification.md) is being worked out.
Brecht Van Lommel commented 2023-07-11 18:10:14 +02:00
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Thanks, I will follow the discussion in usd-lux.

Thanks, I will follow the discussion in usd-lux.
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