[X3D-Public] Why X3D 3.3 changes the default values of beamWidth and cutOffAngle

Don Brutzman brutzman at nps.edu
Sun Jan 29 03:05:41 PST 2012

Thanks for reporting this issue Michalis.

I believe that the allowed ranges for these two values were contradictory to the spec text.
The defaults likely got swapped to match when the ranges got swapped.  There was no intent
to break backwards compatibility, rather it appeared that the prose and the values were
logically inconsistent.

In general, if something is found to be erroneous, we do not necessarily attempt to maintain
backwards compatibility.

There should be a Mantis issue to track this change, it would provide background info.
Sounds like a good topic for an upcoming X3D teleconference; Len schedules the priorities
for each call.  Let's be diligent and check prose, values and practice carefully.  Your
detailed analysis is very helpful, we shall see.

I am traveling overseas (Dubai) through next Wednesday and can look at my book notes upon returning.

On 1/21/2012 6:22 PM, Michalis Kamburelis wrote:
> Hi,
> I was looking at the draft of X3D 3.3 specification, and noticed a change to the default SpotLight.beamWidth and cutOffAngle. Looks like in X3D 3.3:
> - cutOffAngle is by default pi/2
> - beamWidth is by default pi/4
> In X3D 3.2 and earlier, it was the other way around,
> - cutOffAngle by default pi/4
> - beamWidth by default pi/2
> This is a very bad change in my opinion...
> 1. It breaks compatibility for no good reason. Suddenly by changing version number from 3.2 to 3.3 on your X3D scene, you will have SpotLights with larger size (cutOffAngle) and smooth borders. (Unless you specified angles explicitly in the file, of course.)
> 2. The defaults in X3D 3.2 were also much safer and faster. When beamWidth is > cutOffAngle, you have a light without a falloff (constant intensity within light cone). This is implementable using fixed-function OpenGL spot lights. Cases when beamWidth is < cutOffAngle are not precisely implementable using standard OpenGL spot lights with fixed-function pipeline.
> I hope that this is just a typo in X3D 3.3 draft, and it's going to be corrected :)
> For reference:
> - X3D 3.3 about lighting component: http://www.web3d.org/files/specifications/19775-1/V3.3/Part01/components/lighting.html
> - X3D 3.2 lighting component SpotLight: http://www.web3d.org/files/specifications/19775-1/V3.2/Part01/components/lighting.html#SpotLight
> Michalis
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all the best, Don
Don Brutzman  Naval Postgraduate School, Code USW/Br       brutzman at nps.edu
Watkins 270,  MOVES Institute, Monterey CA 93943-5000 USA   +1.831.656.2149
X3D graphics, virtual worlds, navy robotics http://faculty.nps.edu/brutzman

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