Fix Area Light Specular Over-Brightness #22372

aevyrie · 2026-01-04T07:09:49Z

Objective

Fix the over bright specular in our area (point) light implementation.
Close Specular from area lights is disproportionately bright #13327

Solution

Apply the modified (a_prime) specular roughness instead of the base roughness. This allows specular to correctly broaden and fade with distance per Karis 2013.
The only downside of this approach is that specular reflections on smooth materials are over-attenuated. This is a known issue of Karis 2013, and the solution is to implement a popular solution like Linearly Transformed Cosines to handle area lights more correctly.
In the meantime, I've applied a correction that lerps between the base roughness and the modified area light roughness, to retain sharpness and brightness for glossy materials. This was tuned from the reference renders.

Testing

Reference (Blender)

Before

After

Sharp Reference (Blender)

Sharp Before

Sharp After

Showcase

github-actions · 2026-01-04T07:17:43Z

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-22372

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

aevyrie · 2026-01-04T10:12:54Z

Before

Screen.Recording.2026-01-04.at.2.11.09.AM.mov

After

Screen.Recording.2026-01-04.at.2.11.44.AM.mov

alice-i-cecile · 2026-01-04T21:21:20Z

Yay! Progress towards finally fixing our specular highlights is huge. I've marked this as Controversial because I want SME-Rendering to decide whether we ship this as a stop-gap, or wait for a better fix (probably Linear Transformed Cosines) to avoid altering the appearance of this twice for our users.

alice-i-cecile · 2026-01-04T23:07:33Z

Per discussion in the Discord thread I opened to discuss this, consensus seems to be that is safest to ship in 0.19, but a last minute 0.18 fix would be tolerable. Leaving out of the 0.18 milestone, but @cart or @mockersf can overrule me if they feel strongly.

The assorted experts feel strongly that we should ship this, and not wait for LTC to further improve things.

pcwalton · 2026-01-05T02:35:49Z

Can you link me Karis 2013 (ideally the part of it where it says to do what you're doing)?

Also it'd probably be good to mention that linearly transformed cosines is a better solution in the comment you left, preferably with a link so that people who want to contribute the right solution know where to look.

janhohenheim · 2026-01-05T12:59:15Z

crates/bevy_pbr/src/render/pbr_lighting.wgsl

+    // This is a modification to Karis 2013 for area lights to fix an issue where the specular reflection on smooth materials
+    // looks too rough and dim. We lerp between the base roughness and Karis2013 roughness with a lerp factor tuned by looking at reference renders.
+    // The goal is to preserve sharp specular highlights on smooth materials, without blowing out specular highlights on rough materials.
+    let cc_lerp = 1.0 - (1.0 - clearcoat_a) * (1.0 - clearcoat_a) * (1.0 - clearcoat_a) * (1.0 - clearcoat_a);


Would you mind putting these selected lines into a function to deduplicate them? Otherwise I can see someone in the future tweaking them at one place and not the other

Mmm, and with an intermediate variable this a^4 could be done in two operations instead of 3.

janhohenheim · 2026-01-05T13:03:09Z

@pcwalton looks like page 14 of https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf

This is the reason why the commented out code is... commented out.

The pages after that deal with the sphere light formula used here:

with the exception of the custom modification cc_lerp = 1.0 - (1 - a)^4.

janhohenheim · 2026-01-05T13:04:17Z

crates/bevy_pbr/src/render/pbr_lighting.wgsl

+
+    // a' = saturate( a + sourceRadius / (2 * distance) )
+    // see Karis 2013
+    let a_prime = saturate(a + light_position_radius / (2.0 * distance));


Karis talks about 3.0 * distance, not 2.0 * distance. Assuming this is intentional on your part, it would be good to add a comment about this :)

I've seen this both ways, I think there is an error in some slides/papers, not sure which is correct.

Add a link to https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf and say "page 14".

JMS55 · 2026-01-05T17:11:44Z

See also https://seblagarde.wordpress.com/wp-content/uploads/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf section 4.7.2.2 @aevyrie

janhohenheim

I don't have thaaaaat much knowledge here, but from comparing it with the paper it looks correct, and the manual tweak on top looks good in your test scenes :)

pcwalton

Awesome, let's get this in. Big improvement that addresses a long-standing problem.

pcwalton · 2026-01-05T19:59:42Z

crates/bevy_pbr/src/render/pbr_lighting.wgsl

+
+    // a' = saturate( a + sourceRadius / (2 * distance) )
+    // see Karis 2013
+    let a_prime = saturate(a + light_position_radius / (2.0 * distance));


Add a link to https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf and say "page 14".

pcwalton · 2026-01-05T20:01:24Z

crates/bevy_pbr/src/render/pbr_lighting.wgsl

+    // This is a modification to Karis 2013 for area lights to fix an issue where the specular reflection on smooth materials
+    // looks too rough and dim. We lerp between the base roughness and Karis2013 roughness with a lerp factor tuned by looking at reference renders.
+    // The goal is to preserve sharp specular highlights on smooth materials, without blowing out specular highlights on rough materials.
+    let lerp = 1.0 - (1.0 - a) * (1.0 - a) * (1.0 - a) * (1.0 - a);


As mentioned by others, factor this out into a function. Also, this would be faster:

let inv_a_sq = (1.0 - a) * (1.0 - a); let lerp = 1.0 - inv_a_sq * inv_a_sq;

(There's no need to pull the 1.0 - a into a variable as CSE shader optimizations will do that automatically.)

pcwalton · 2026-01-05T20:01:53Z

crates/bevy_pbr/src/render/pbr_lighting.wgsl

+
+    // This is a modification to Karis 2013 for area lights to fix an issue where the specular reflection on smooth materials
+    // looks too rough and dim. We lerp between the base roughness and Karis2013 roughness with a lerp factor tuned by looking at reference renders.
+    // The goal is to preserve sharp specular highlights on smooth materials, without blowing out specular highlights on rough materials.


Mention that you want linearly transformed cosines as the "real" solution and preferably provide a link. Might as well add a link to this PR too so people who want to improve this can see the discussion.

fix specular?

8670786

alice-i-cecile added this to the 0.18 milestone Jan 4, 2026

alice-i-cecile modified the milestones: 0.18, 0.19 Jan 4, 2026

Attempt to fix specular dimming and roughness on smooth materials

bfb6384

aevyrie added the M-Deliberate-Rendering-Change An intentional change to how tests and examples are rendered label Jan 4, 2026

alice-i-cecile added C-Bug An unexpected or incorrect behavior A-Rendering Drawing game state to the screen X-Controversial There is active debate or serious implications around merging this PR labels Jan 4, 2026

github-project-automation bot added this to Rendering Jan 4, 2026

alice-i-cecile added the S-Needs-Review Needs reviewer attention (from anyone!) to move forward label Jan 4, 2026

alice-i-cecile requested review from IceSentry, cart, pcwalton, robtfm and superdump January 4, 2026 21:21

alice-i-cecile added X-Uncontroversial This work is generally agreed upon and removed X-Controversial There is active debate or serious implications around merging this PR labels Jan 4, 2026

janhohenheim reviewed Jan 5, 2026

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janhohenheim approved these changes Jan 5, 2026

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pcwalton reviewed Jan 5, 2026

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Fix Area Light Specular Over-Brightness #22372

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