[Merged by Bors] - bevy_pbr2: Improve lighting units and documentation #2704
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# Objective A question was raised on Discord about the units of the `PointLight` `intensity` member. After digging around in the bevy_pbr2 source code and [Google Filament documentation](https://google.github.io/filament/Filament.html#mjx-eqn-pointLightLuminousPower) I discovered that the intention by Filament was that the 'intensity' value for point lights would be in lumens. This makes a lot of sense as these are quite relatable units given basically all light bulbs I've seen sold over the past years are rated in lumens as people move away from thinking about how bright a bulb is relative to a non-halogen incandescent bulb. However, it seems that the derivation of the conversion between luminous power (lumens, denoted `Φ` in the Filament formulae) and luminous intensity (lumens per steradian, `I` in the Filament formulae) was missed and I can see why as it is tucked right under equation 58 at the link above. As such, while the formula states that for a point light, `I = Φ / 4 π` we have been using `intensity` as if it were luminous intensity `I`. Before this PR, the intensity field is luminous intensity in lumens per steradian. After this PR, the intensity field is luminous power in lumens, [as suggested by Filament](https://google.github.io/filament/Filament.html#table_lighttypesunits) (unfortunately the link jumps to the table's caption so scroll up to see the actual table). I appreciate that it may be confusing to call this an intensity, but I think this is intended as more of a non-scientific, human-relatable general term with a bit of hand waving so that most light types can just have an intensity field and for most of them it works in the same way or at least with some relatable value. I'm inclined to think this is reasonable rather than throwing terms like luminous power, luminous intensity, blah at users. ## Solution - Documented the `PointLight` `intensity` member as 'luminous power' in units of lumens. - Added a table of examples relating from various types of household lighting to lumen values. - Added in the mapping from luminous power to luminous intensity when premultiplying the intensity into the colour before it is made into a graphics uniform. - Updated the documentation in `pbr.wgsl` to clarify the earlier confusion about the missing `/ 4 π`. - Bumped the intensity of the point lights in `3d_scene_pipelined` to 1600 lumens. Co-authored-by: Carter Anderson <mcanders1@gmail.com>
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Objective
A question was raised on Discord about the units of the
PointLightintensitymember.After digging around in the bevy_pbr2 source code and Google Filament documentation I discovered that the intention by Filament was that the 'intensity' value for point lights would be in lumens. This makes a lot of sense as these are quite relatable units given basically all light bulbs I've seen sold over the past years are rated in lumens as people move away from thinking about how bright a bulb is relative to a non-halogen incandescent bulb.
However, it seems that the derivation of the conversion between luminous power (lumens, denoted
Φin the Filament formulae) and luminous intensity (lumens per steradian,Iin the Filament formulae) was missed and I can see why as it is tucked right under equation 58 at the link above. As such, while the formula states that for a point light,I = Φ / 4 πwe have been usingintensityas if it were luminous intensityI.Before this PR, the intensity field is luminous intensity in lumens per steradian. After this PR, the intensity field is luminous power in lumens, as suggested by Filament (unfortunately the link jumps to the table's caption so scroll up to see the actual table).
I appreciate that it may be confusing to call this an intensity, but I think this is intended as more of a non-scientific, human-relatable general term with a bit of hand waving so that most light types can just have an intensity field and for most of them it works in the same way or at least with some relatable value. I'm inclined to think this is reasonable rather than throwing terms like luminous power, luminous intensity, blah at users.
Solution
PointLightintensitymember as 'luminous power' in units of lumens.pbr.wgslto clarify the earlier confusion about the missing/ 4 π.3d_scene_pipelinedto 1600 lumens.