The upcoming alpha gamma adjustment should also be available to particles
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Frionil Fang
I've noticed that there will be way to adjust the gamma of the alpha channel for textures to compensate for the change from sRGB to linear gamma, e.g. referenced in this issue on github https://github.com/secondlife/viewer/issues/2655
I have not seen mention that the adjustment would be available for particles, but it definitely should be as they too can be just as affected by the change in rendering. Unfortunately there would be no easy UI-based way to add such an adjustment by end-users, but at least a new parameter for llParticleSystem would let allow scripters to fix their effects.
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Dan Linden
Frionil, If all particles used the alpha as it existed before PBR, would that fulfill your request?
Frionil Fang
Dan Linden That would at least fix all existing effects, yes. I made an image to explain my justification why it's important: made 3 particle effects (color slide, alpha slide, color+alpha slide in additive blend used for double-ended fades) on a black background.
Left is PBR on, right is PBR off.
With PBR, the alpha slide has barely any effect until the very end due to its linear gamma, and the double-fade has its brightest point very much brigher (color picker says ~50%) and offset from the middle because the alpha vs. color gammas don't match anymore. Flipping the directions of color and alpha slides just slants it on the other side.
Without PBR, the color and alpha slides have the same curve and are indistinguishable since gammas are the same, and the exact middle point on the double fade is the brightest and less intense (~25%). This would be the preferred default situation and retain the effects' original look.
Spidey Linden
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Issue tracked. We have no estimate when it may be implemented. Please see future updates here.