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Index › Programming Questions & Help › Syntax Questions › Deciding on inclusion within the field of vision
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Deciding on inclusion within the field of vision (Read 515 times)
olivier
Deciding on inclusion within the field of vision
Dec 11th, 2007, 9:29pm  
Hi,
I'm using Processing to create a 3D animation composed of 150 objects, each of them composed of about 100 facets.
Each object has a draw() method that does calculation before to draw the vertex.
To speed up the rendering I would like not to process the draw() method of the object if the object is not in the field of vision of the camera. I will consider the center of the object as a reference (in a next release I could compute a bounding box)
I'm a bit confused on how to decide if a point at (x,y,z) is or is not in the field of vision of the camera located at (ex,ey,ez) and targeting at (tx,ty,tz) ?
Who could help me ?
Regards
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JohnG
Re: Deciding on inclusion within the field of visi
Reply #1 - Dec 11th, 2007, 9:38pm  
One quick (but not excessively accurate) method would be to do a dot product of the normalised vectors (camera position->camera target) and (camera position->object) then compare that to cos(fieldOfView + aBit);
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olivier
Re: Deciding on inclusion within the field of visi
Reply #2 - Dec 11th, 2007, 10:58pm  
Hi, thanks but I sould have missed something.

What do you mean by :
Quote:
a dot product of the normalised vectors (camera position->camera target) and (camera position->object)
??

Thanks
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JohnG
Re: Deciding on inclusion within the field of visi
Reply #3 - Dec 11th, 2007, 11:41pm  
Quick vector primer:

Vector from point a->b = b.x-a.x , b.y-a.y , b.z-a.z
Normalised vector: length=sqrt(v.x*v.x+v.y*v.y+v.z*v.z); v.x=v.x/length; v.y=v.y/length; v.z=v.z/length;

Dot product of vector a and b: a.x*b.x + a.y*b.y + a.z*b.z

(All this assumes you habve some simple class that has an x/y/z variable storing positions.)
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olivier
Re: Deciding on inclusion within the field of visi
Reply #4 - Dec 12th, 2007, 10:19pm  
Thanks for refreshing my mind on vectors ! Seems that I needed it ! BTW, I was confused by the english expression "dot product" (in french, this is "scalar product")

My "scene" where object are moiving around is flat, my objects are all at z=0
My cam position is defined by the angle, alphaZ, with the Z axis (0: the camera is at the vertical of the plane, PI/2 it is at the  horizontal) and by the angle with the Y axis, alphaY, and by the distance D from origin

The target point is always at (0,0,0)
As such, I can compute the normalized vector cam->target by :
(-cos(alphaZ), -sin(alphaZ)*cos(alphaY), -sin(alphaZ)*sin(alphaY))

I compute the vector cam->object at x,y,z by :
(x-D*cos(alphaZ), y-D*sin(alphaZ)*cos(alphaY), z-D*sin(alphaZ)*sin(alphaY))

Then I can compare the dot product of normalized vector which is equals to cos(angle between the vectors) with cos(half(?) of angle of field of view)

Am I right ?

If so, unfortunately, this does not work... yet.
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olivier
Re: Deciding on inclusion within the field of visi
Reply #5 - Dec 16th, 2007, 4:46pm  
Hi, ok I got it. After fix of several mistakes, I succeed in computing the dot product and compare with angle of field of view. Great !
But... as the screen is not a square, the angle of fov is not the same in every direction. I should compute the relative direction to know the angle to compare to...

Fortunately, there is another, quick and accurate, way to do it.
I simply use screenX() and screenY() for the (x,y,z) points that I want to know if they are visible or not. I just have to  check that 0<=screenX()<=width and 0<=screenY()<=height
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