I don't want an elliptic sun.... (advanced patch placement in 3d)

[Andrea Viarengo]

Hi,

Probably my knowledge of trigonometry are a bit rusty,
but I was not able to put a patch so that it is always orthogonal to the
segment connecting the center view (nx,ny,nz=0,0,0) with its center.

Imagine that I want to draw the sun in a cubic node,
the sun is simply an image representing a yellow circle,
and I want draw it passing azimuth and elevation to a function that
place and orienting the patch so that the sun is never distorted,
but always it appear perfectly circular.

(You know that in the reality the sun is a sphere,
so it should always appear circular from anywhere I look at it,
and never should be appear elliptical ...)

This would be useful if I wanted that my node can be visited at different
hours of day and I want to render the idea of the time flow
by the position of the sun (and possibly also the brightness of the scene).

How could I do?

I tried the code that follows, you can move the sun using arrow,
and you can notice distorsion with elevation>45

The distorsion is due to the system used to rotate patches, as
the first rotation (along x) also amends the orientation of
the second axis of rotation (along y).

and I believe that, in this case, the problem will disappear if you
rotate first along the Y axis and then along the axis X, (perhaps
the ideal would be able to choose the order of rotation axis ...)

What do you think?

-Andrea

---------------------------------------
cubic {
        "front.jpg", --front
        "right.jpg", --right
        "back.jpg",  --back
        "left.jpg",  --left
        "top.jpg",   --top
        "bottom.jpg" --bottom
}

function setpos(p,dis,az,el)
  local nz=math.cos(math.rad(az))*dis*math.cos(math.rad(el))
  local nx=math.sin(math.rad(az))*dis*math.cos(math.rad(el))
  local ny=math.sin(math.rad(el))*dis

  p:move{nx=nx, ny=ny ,nz=nz, anglex=el2, angley=az }
end

onkeydown (
    function(key)
      local press="no"
      if key == pipmak.key_arrow_right then az=az+10 press="yes" end
      if key == pipmak.key_arrow_left  then az=az-10 press="yes" end
      if key == pipmak.key_arrow_up    then el=el+10 press="yes" end
      if key == pipmak.key_arrow_down  then el=el-10 press="yes" end    
     
      if press=="yes" then setpos(sun,di,az,el) end
   end
   )

pipmak.setverticalfov(70)

-- initial position of the sun:
az =  0
el = 45
di =  2

-- "sun.png" is simply a yellow circle on trasparent png 80x80 pixel

sun=patch {nx=0, ny=0 ,nz=0, anchorh=40,anchorv=40,
           w=80, h=80,image="sun.png",visible=true }

setpos(sun,di,az,el)
-------------------------------------------------------------




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[Andrea Viarengo]

ERRATA:

in the function setpos(...) put anglex=el ... (remove"2")

sorry.

Andrea



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[Andrea Viarengo]

AAArgh!!!!

   in function setpos(...)
     anglex=-el  

This works!

I'm apologize twice!!!


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[Christian Walther]

Andrea Viarengo wrote:
> I was not able to put a patch so that it is always orthogonal to the
> segment connecting the center view (nx,ny,nz=0,0,0) with its center.

I haven't had a chance to look at your code yet - I'll get to that
shortly - but I wanted to mention the following right away:

> (You know that in the reality the sun is a sphere,
> so it should always appear circular from anywhere I look at it,
> and never should be appear elliptical ...)

No. That's not true. Projecting a sphere onto a plane will not get you a
circle unless the projection direction is perpendicular to the plane
(i.e., you're looking straight at the sun, so that, for a non-skewed
camera, it is in the center of the image). Think about it!

(The same thing said a bit more verbosely:
<http://tag.povray.org/povQandT/misconceptions.html#perspectivespheres>.)

  -Christian


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[Christian Walther]

Andrea Viarengo wrote:

> I tried the code that follows, you can move the sun using arrow,
> and you can notice distorsion with elevation>45
>
> The distorsion is due to the system used to rotate patches, as
> the first rotation (along x) also amends the orientation of
> the second axis of rotation (along y).
>
> and I believe that, in this case, the problem will disappear if you
> rotate first along the Y axis and then along the axis X, (perhaps
> the ideal would be able to choose the order of rotation axis ...)

Yes, you're right. Swapping the default order to Y first, then X would
be easy. I wonder if there are any other applications that would be made
more difficult by that? I agree that the application of orienting a
patch to the viewer given azimuth and elevation is a useful one and
should be made easy. Doing this would break backwards compatibility with
0.2.7, however. (I guess we could still do it, as there probably aren't
many people using X and Y rotation yet.) Implementing an arbitrarily
specifiable rotation order would be a bit more involved (though it may
be worth it - I'll look into it). We could also introduce another
property "anglex2", so that rotation would happen in the order anglex,
angley, anglex2, angle.

Opinions, anyone?


In the current system, you can't get away without doing some
trigonometry. Going through the cartesian coordinates seems easiest:

  x =  cos el sin az  =  sin dl
  y =  sin el         =  cos dl sin bz
  z = -cos el cos az  = -cos dl cos bz

Here's what I've come up with, modifying your example, and it seems to
work well:

function setpos(p,dis,az,el)
   local nz=-math.cos(math.rad(az))*math.cos(math.rad(el))
   local nx=math.sin(math.rad(az))*math.cos(math.rad(el))
   local ny=math.sin(math.rad(el))

   local dl = math.deg(math.asin(nx))
   local bz = math.deg(math.atan2(ny, -nz))

   pipmak.printinplace("az ", az, "   el ", el, "   bz ", bz,"   dl ",dl)

   p:move{nx=nx, ny=ny, nz=nz, anglex=bz, angley=-dl }
   pipmak.setviewdirection(az, el)
end


  -Christian


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[Andrea Viarengo]

Great Chriss!!

It works well!!

What do you think about adding to methods "patch" and "move" the
optional parameter:

rotationorder = one of {"xya","yxa","xay",....}

with a default value rotationorder="xya",
so that the backwards compatibility was not compromised.

I think it isn't too difficult to implement.

You're right about the perspective of a sphere,
but did not want to do things too complicated!!

Thank you

Andrea

P.S. I will try to put this example in the Wiki, because I think it
could be interesting for others.


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