Le 17-12-15 à 15:57, dick balaska a écrit :
> On 12/15/2017 07:50 AM, Bald Eagle wrote:
>>  argh.
>>
>> So, just so I fully understand what you want,
>> 1. You want to have a flashlight that always points at "Lookat"
>> 2. You want it offset by a certain amount from the camera position
>>
>> 3.  ... ? (if any)
> 
> Yes, like in Doom, where the gun is in a fixed position relative to the 
> game window.
> 
> (Wait, isn't there a text thingy in the object library that draws text 
> in a fixed position on the screen? ...)
> 
> 

Locate your light at the same location as the camera, with a slight 
offset if you want.
Make it a spot_light.
Spotlight have a point_at parameter, make it the same as your look_at. 
point_at default to <0,0,0> if you don't set it.

#declare Light_Offset = <0, -0.5, 0>;
Camera{Cam_Location look_at Somewhere}
light_source{Cam_location + Light_Offset
    rgb 1
    spotlight
    radius 5
    tightness 40
    point_at Somewhere
  }

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On 12/15/2017 07:50 AM, Bald Eagle wrote:
> dick balaska <dic### [at] buckosoftcom> wrote:
>> On 05/29/2017 03:42 PM, dick balaska wrote:
>>
>>   > Help, Math wizards, you're my only hope.
>>
>> I have started again to try to tackle this problem.
>>
>> Scott's solution gave me a vector, which is not what I need. :(  I need
>> an angle.
>>

http://www.buckosoft.com/tteoac/video/testRenders/test3.mp4

Eureka!

The solution is to rotate it with the Cartesian result and then 
translate it with scott's vcross-ish solution.  i.e.

object {
myCyl
#local flOfs = <-0.4,-0.3,0.7>; // flashlight offset from camera
rotate VtoA(Lookat-Camera)
translate Camera+ssOffset(flOfs)
}

// Convert this vector into its angle
#macro VtoA(V)
	#local _V=vnormalize(V);
	#local rZ=degrees(acos(_V.z));
	#local rY=0;
	#local rX=0;
	#if (_V.y != 0)
		#local rY=degrees(atan2(_V.x,_V.y));
	#else
		#local rX=rZ;
		#local rZ=0;
	#end
	<-rZ,rX,-rY>
#end


#macro ssOffset(bar)
	#local newZ = vnormalize( Lookat-Camera );
	#local newY = y;
	#local newX = vnormalize(vcross(newY,newZ));
	#local newY = vnormalize(vcross(newZ,newX));
	#local barOut = bar.x * newX + bar.y * newY + bar.z * newZ;
	barOut
#end

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dick balaska <dic### [at] buckosoftcom> wrote:
> On 12/15/2017 07:50 AM, Bald Eagle wrote:
> > dick balaska <dic### [at] buckosoftcom> wrote:
> >> On 05/29/2017 03:42 PM, dick balaska wrote:
> >>
> >>   > Help, Math wizards, you're my only hope.
> >>
> >> I have started again to try to tackle this problem.
> >>
> >> Scott's solution gave me a vector, which is not what I need. :(  I need
> >> an angle.
> >>
>
> http://www.buckosoft.com/tteoac/video/testRenders/test3.mp4
>
> Eureka!
>
> The solution is to rotate it with the Cartesian result and then
> translate it with scott's vcross-ish solution.  i.e.
> ...

See this post for an alternative solution:

From: Tor Olav Kristensen
Subject: Reorienting a flashlight along with the camera
Date: 17 Dec 2017 15:00:00
http://news.povray.org/povray.text.scene-files/thread/%3Cweb.5a36860f8dfaa90bb572bd120%40news.povray.org%3E/

It's interesting that no trigonometric functions are needed for this.

--
Tor Olav
http://subcube.com

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On 5/31/2017 2:06 AM, dick balaska wrote:
> Am 2017-05-29 15:42, also sprach dick balaska:
>> I'm trying to master converting a vector to an angle in 3D with no luck 
> 
> Help, Math wizards, you're my only hope.
> 
> Ok. I'm stuck. I'm missing something.
> 
> My goal is to position an object in a fixed position on the screen, like 
> carrying a flashlight. Or a gun, like in a video game.

You could create the flashlight and camera, and then apply the same 
transformations to both.

Mike

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