The projection matrix from prman only stores a FOV, or the relationship
between focal-length and horizontal aperture.
If you want the "real" focal length values, you will need to know the size
of your filmback.
If you don't care about the "real" focal length value, you can still get the
right projection values by setting the focal length to 1, and getting the
filmback size like this (proj_matrix is a Matrix4 object filled from the
exr/worldToNDC metadata key):
lx = (-1 - proj_matrix[12] - proj_matrix[8]) / proj_matrix[0]
by = (-1 - proj_matrix[13] - proj_matrix[9]) / proj_matrix[5]
rx = (1 - proj_matrix[12] - proj_matrix[8]) / proj_matrix[0]
ty = (1 - proj_matrix[13] - proj_matrix[9]) / proj_matrix[5]
swW = max( lx , rx ) - min( lx , rx ) # Screen Window Width
swH = max( by , ty ) - min( by , ty ) # Screen Window Height
If you have access to the real filmback size (either from custom metadata or
because you can assume you'll always be dealing with cameras that have the
same filmback size), you could then get a "real" focal length value from
filmBack.width / swW.
Hope that helps.
Cheers,
Ivan
2011/4/28 Lucien Fostier <[email protected]>
> Hi fellow tds
>
> I m trying to create a camera from exr metadata coming from prman.
>
> So i use the projection matrix world to camera which give me position and
> orientation in space.
>
> Id like to extract the focal lenght from this data but i cant find how to
> do it.
>
> May someone help me about this or give me some pointer?
>
> Thanks
>
> Lucien
>
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