RFR: 8234920: Add SpotLight to the selection of 3D light types

[Kevin Rushforth]

On Wed, 9 Dec 2020 01:06:45 GMT, Nir Lisker <nlisker at openjdk.org> wrote:

>> The main issue I have is that the specified equation matches the proposed implementation only for values that satisfy the range constraints: `0 <= innerAngle <= outerAngle <= 180`.
>> 
>> The reason for this is that the shader implementation compares cosines not angles, and clamps the result of the division rather than testing whether the angle is < inner or > outer. These are very reasonable implementation choices.
>> 
>> We have a few options that I can see:
>> 
>> 1. List the expected range, and specify that values outside the expected range are clamped on usage.
>> 2. List the expected range, and specify that the results are undefined for values outside the expected range.
>> 3. Describe the equation that is actually happening.
>> 4. Implement what the equation actually specifies, even for out-of-range values. This would constrain the implementation and likely require extra if tests in the shader.
>> 
>> Option 4 makes no sense to me. I don't care for 3, since it is less easy to explain and understand, and it precludes other implementations that are equivalent for in-range values but might behave differently for out-of-range values.
>> 
>> So that leaves option 1 or 2 as workable options. I prefer option 1, although I'll admit we aren't consistent on this point: we let the implementation do whatever it will do with out of range color values for PhongMaterial or Lights.
>
> I see.
> 
> I also dislike 3 and 4. I prefer 2, but if you are convinced that 1 is better I'll go with it.

I'm OK with option 2. So that would mean removing any mention of the "ring" effect, and documenting that the valid ranges are:

innerAngle: [0, 180]
outerAngle: [innerAngle, 180]

But we wouldn't clamp them or otherwise enforce it. Instead we could add a sentence that the results are undefined (or maybe "unpredictable"?) if the angles are outside that range.

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PR: https://git.openjdk.java.net/jfx/pull/334