[Radiance-general] Using vwright/rcalc to do -pj

[Mark Stock]

In my toying around with pixel-wise depth-of-field blur, I am
encountering a need for pixel jitter. I want to run

vwrays -fd -vf tempvf -x 740 -y 480 | rcalc -id6
  -e `vwright i < tempvf`
  [expressions to modify $1..$6, the ray origin and direction]
  -od | rtrace -fdc -x 740 -y 480 @opts scene.oct > img.pic

but there is no facility to incorporate pixel jitter, as defined
in rpict(1), save the use of the variables returned by vwright:

------------
       Nt:  view type ('v'==1,'l'==2,'a'==3,'h'==4,'c'==5)
       Npx: view point x value
       Npy: view point y value
       Npz: view point z value
       Ndx: view direction x value (normalized)
       Ndy: view direction y value (normalized)
       Ndz: view direction z value (normalized)
       Nux: view up vector x value (normalized)
       Nuy: view up vector y value (normalized)
       Nuz: view up vector z value (normalized)
       Nh:  view horizontal size
       Nv:  view vertical size
       Ns:  view shift
       Nl:  view lift
       No:  view fore clipping distance
       Na:  view aft clipping distance
       Nhx: derived horizontal image vector x value (normalized)
       Nhy: derived horizontal image vector y value (normalized)
       Nhz: derived horizontal image vector z value (normalized)
       Nhn: derived horizontal image vector multiplier
       Nvx: derived vertical image vector x value (normalized)
       Nvy: derived vertical image vector y value (normalized)
       Nvz: derived vertical image vector z value (normalized)
       Nvn: derived vertical image vector multiplier
------------

My question is: how do I use these variables to incorporate pixel
jitter into these renderings? I do not know how to scale the
Nhx,Nhy,Nhz and Nvx,Nvy,Nvz vectors to jitter over one pixel's
width. Do I multiply by Nh,Nv and divide by 740 and 480,
respectively? Or does it depend on the view type?

Mark