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

[Andrea V. <and...@gm...>]

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)
-------------------------------------------------------------