Even if you’re doing this on the Lumen side of things, it’s still going to use similar exponential math and still going to be prone the same problems as what I said in my previous post.
Here are are the only updates to the Lumen implementation that was introduced in November 2024:
April 15th, commit ea535de:
“Lighting Only View Mode - override materials for all bounces in Lumen and Path Tracer. Having this only on primary rays made it akward to investigate lighting as it unbalances direct and indirect lighting. This is also what lighting artists prefer.”
April 30th, commit 5fac3c0:
DiffuseColor = min(saturate(DiffuseColor * (DiffuseLumBoost / max(DiffuseLum, 1e-8f))), 0.99f);
(they added the max(DiffuseLum, 1e-8f) check so that it can’t divide by zero).
What you’re probably seeing are the changes to the lighting only view mode. You could test this pretty easily by making a backup copy(store it on your desktop our somewhere outside of the UE folder structure to play it safe) of the LumenDiffuseColorBoost.ush shader file and commenting out the DiffuseColor = DiffuseColor.rgb * View.DiffuseOverrideParameter.w + View.DiffuseOverrideParameter.xyz; line with // , then relaunch the editor to recompile shaders and test again. You might also have to edit out the two lines that changed in the same commit in PathTracingCore.ush, if you’re testing with path tracing.
Also, if you went back to UE5.5 and added that line to the same file, you’d likely see similar results in your testing (5.6 still has much more accurate lighting, so they’d still differ to some degree)