Camera parameters

[Macha]

I am writing a python script to convert voodoo camera tracking data to a Houdini camera. Getting the data across seems to work OK. The camera moves and wriggles in kind of the right manner.

However, I am a bit stuck with what exactly the corresponding Houdini parameters are. Voodoo gives me the following:

# f               : Focal Length [mm]
# fov             : Horizontal Field of View [degree] = (2*atan(0.5*Width*sx/f)*180/PI;

How do they relate to Houdini Focal Length and Aperture?

........................

Then, it also gives me the rotation axis.

# (Ax, Ay, Az)    : RotationAxis2  [unit vector]
# (Hx, Hy, Hz)    : RotationAxis0 [pel] (including FocalLength, PixelSizeX, and Principal Point offset) 
# (Vx, Vy, Vz)    : RotationAxis1 [pel] (including FocalLength, PixelSizeY, and Principal Point offset) 

There's probably a neat matrix way to convert this into rx ry rz rotation angles. But for now I calculated angles this way:

def getangle(vec1, vec2):
    v1 = hou.Vector3( vec1 )
    v2 = hou.Vector3( vec2 )
    angle = v1.angleTo(v2)
    return angle

So far so good. Now for the confusion: Which two vectors do I choose to calculate the angles? Like this?

camera_rotation_rx = getangle( A, world_y_axis )
camera_rotation_ry = getangle( H, world_z_axis )
camera_rotation_rz = getangle( V, world_x_axis )

Or did I muddle up things there?

Edited March 11, 2010 by Macha

[Macha]

BTW, the complete camera model specification is here, in case I missed an important piece of info:

http://www.digilab.uni-hannover.de/docs/manual.html#cahvmodel

[mrice]

There's some info hidden in the docs about camera aperture, fov, etc:

http://www.sidefx.com/docs/houdini10.0/ref/cameralenses

Just glancing at that page, maybe

(H0x, H0y, H0z) : RotationAxis0 [unit vector]
(V0x, V0y, V0z) : RotationAxis1 [unit vector]

are more useful?

It says there that there's a python export script for blender, maybe that's worth a look if you don't mind cheating

[Macha]

Oh, stealing from Blender. Good idea Michael

It appears their camera transformation matrix setMatrix(Mathutils.Matrix(stuff)) with the values of this camera model looks like this:

[HOx,   HOy,  HOz, 0]
[-VOx, -VOy, -VOz, 0]
[-Ax,   -Ay,  -Az, 0]
[ Cx,    Cy,   Cz, 1]

My matrix skills are a bit, err, basic yet, so I guess in Houdini I'd have to use:

cam_mat = hou.Matrix4((
(HOx, HOy, HOz, 0),
(VOx, VOy, VOz, 0),
(Ax,   Ay,  Az, 0),
( Cx,  Cy,  Cz, 1)))

and then

mrot = cam_mat.extractRotates()

#or this
mrot = cam_mat.explode()

to get a rotation vector.

Does that sound about right?

Edited March 12, 2010 by Macha

[Macha]

Wohooo! I got it working! Well chuffed now. The camera appears to move the way it should.

Only thing is that f and fov thing that doesn't fit... Hmmm... I'll keep digging...

It looks like a trig problem. According to the docs this should be correct:

tan( fov*.5 ) = (aperture*.5)/f

so then

aperture = 2*f*tan(fov/2)

but it gives me a negative aperture...

Here's what I got so far (with manual fov)

Edited March 12, 2010 by Macha

[edward]

What happens if you just take the absolute value of aperture?

[Macha]

What happens if you just take the absolute value of aperture?

Hmm, in that case I can only see 50% of what I should see. Hmmm. Hmmm.

Oh, hold on! Maybe it's a degrees/radians issue! I'll go try that.

Edited March 12, 2010 by Macha

[Macha]

Degrees/radians does not appear to be the problem.

What does aperture actually stand for in Houdini? The helpcard says it is " Width of the visible field."

In millimiters? Degrees? Is it a ratio?

[edward]

It is a distance measure in whatever units you choose as long as it is consistent with all the other values you use in your scene.

[Macha]

Oh, thanks.

It also appears as if voodoo is giving me nonsense focal lengths, depending on its settings. No wonder everything I tried went wrong.