On 9/15/2016 2:08 PM, Alain wrote:
> Le 16-09-14 à 21:33, Mike Horvath a écrit :
>> On 9/12/2016 10:43 AM, Mike Horvath wrote:
>>> On 9/12/2016 7:56 AM, clipka wrote:
>>>> Am 12.09.2016 um 12:45 schrieb Jim Holsenback:
>>>>> On 9/11/2016 1:55 PM, clipka wrote:
>>>>>> Am 11.09.2016 um 16:23 schrieb Mike Horvath:
>>>>>>
>>>>>>> Looks_like works fine with parallel lights as long as you position
>>>>>>> the
>>>>>>> light correctly to start off with instead of placing it at the
>>>>>>> origin
>>>>>>> and then translating it at some later time.
>>>>>>>
>>>>>>> Unless I made a mistake.
>>>>>>
>>>>>> Judging from the code, I see no reason why that should make any
>>>>>> difference.
>>>>>>
>>>>>
>>>>> Pulled directly from the previous version of this section:
>>>>>
>>>>> "For point lights and spotlights you could also place the light source
>>>>> and the looks_like object at the origin and translate them together
>>>>> later. But for parallel lights this will not work."
>>>>>
>>>>> Did I get it wrong? Just trying to honor what was previously
>>>>> mentioned!
>>>>
>>>> I can't say for sure at the moment. All I can say is that I didn't find
>>>> anything in the code that would explain that wording, but I might have
>>>> overlooked something. I guess experiments may be needed.
>>>>
>>>> Maybe the sentence was placed there not so much due to unexpected
>>>> effects on the looks_like object, but unexpected effects on the light
>>>> source itself. (For instance, whether or wheter not POV-Ray translates
>>>> the `point_at` location along with the parallel light source, either
>>>> way
>>>> the effect may be surprising to some users.)
>>>>
>>>
>>>
>>> I posted a test scene in p.t.s-f.
>>>
>>>
>>> Mike
>>
>> Oops, I was wrong. But translation is *dangerous*. As per clipka's
>> response to my post:
>>
>>> With a parallel light source, the direction of all light rays depends
>>> only on the /relative/ location of two points: The light source's
>>> nominal center and the point_at parameter. But transformations act on
>>> both these points (I've just checked the code), so a translation
>>> operation changes only the absolute location of the points, not their
>>> relative location to one another. And thus the light direction
>>> remains unimpressed.
>>
>> I.e. translating a light source translates the location of the source of
>> the light *and* the location the light is pointing at, which could
>> produce unwanted effects. This should be mentioned in the article. Or at
>> least state that you should explicitly configure point_at to keep this
>> from happening.
>>
>> You might want to update this page as well:
>>
>> http://wiki.povray.org/content/Knowledgebase:Language_Questions_and_Tips#Topic_1
>>
>>
>>
>> It recommends translating the light source. Quote:
>>
>>> It's a good idea to define both things, the light_source and the
>>> looks_like object, at the origin, and then translate them to their
>>> right place...
>>
>>
>> Mike
>
> This also apply to spotlight and cylinder light.
>
> Whenever you translate a spotlight, a parallel light or a cylinder
> light, it's orientation always remain unchanged. If you want to change
> the orientation, you need to rotate it.
> For me, it's pretty obvious, but it seems that it's not the case for
> everybody.
>
>
> Alain

I haven't seen /point_at/ used in many demo scenes either. I don't think 
the "Ready made scenes" that come with POV-Ray use /point_at/. I never 
knew it was so important.

Mike

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