Difference between .pic colours and traced ray colours

Hi there
I'm wondering why when I view a .pic image I've created in HDRShop, the pixel
values aren't the same as when I generate a text file of their colours using
vwrays.

I wnat a green box's surface reflectances in the pic file so I do

./vwrays.exe -fd greenbox.pic | rtrace -fda `vwrays -d greenbox.pic`
-ovm `if(s='green_plastic')` testroom.oct | grep "green_plastic" > rays.txt

Is it because I need to put an I in there somewhere for irradiance values?

Thanks for any help

Tarik

···

--
Tarik Rahman
PhD student, Institute of Perception, Action and Behaviour
School of Informatics
University of Edinburgh

Hi Tarik,

I couldn't really follow your question:

I'm wondering why when I view a .pic image I've created in HDRShop, the pixel
values aren't the same as when I generate a text file of their colours using
vwrays.

I first read this as if you had created the .pic in HDRShop, which didn't make sense. I assume now you are only viewing it with HDRShop. What do you mean that the pixel values aren't the same? The same as what? The same as eachother? You are expecting them all to be the same? ??

I wnat a green box's surface reflectances in the pic file so I do

./vwrays.exe -fd greenbox.pic | rtrace -fda `vwrays -d greenbox.pic`
-ovm `if(s='green_plastic')` testroom.oct | grep "green_plastic" > rays.txt

Ignoring the `if(s='green_plastic')` nonsense, which I'm hoping doesn't do anything, this command might give you the luminance at each visible point on the green surface, but not its reflectance. If you want the reflectance, just look at the Radiance scene file. It's right there in the definition of "green_plastic".

-Greg

Sorry I meant that I have created the .pic and then I view it in HDRShop. Say
the first pixel of the green surface rendered, ( Aren't .pics rendered from
top down, left to right?) pixel 216, 238 has the luminance (sorry, not
reflectance) value of
(0.307, 0.541, 0.291)
Well, in the rays.txt file from

./vwrays.exe -fd greenbox.pic | rtrace -fda `vwrays -d greenbox.pic` -ovm
testroom.oct | grep "green_plastic" > rays.txt

I get (0.239, 0.443, 0.219) which are similar but not exactly the same values.

···

--
Tarik Rahman
PhD student, Institute of Perception, Action and Behaviour
School of Informatics
University of Edinburgh

Hi Tarik,

The difference could be due to the change in pixel center between vwrays and rpict. Rpict normally "jitters" pixel centers as an anti-aliasing measure. If you rerender your image, turning jitterinig off with -pj 0, you will get the exact pixel centers produced by vwrays. Being off by even a fraction of a pixel can make a difference in the result, especially near the object's visible boundaries.

-Greg

···

From: Tarik Rahman <[email protected]>
Date: March 16, 2005 3:22:14 AM PST

Sorry I meant that I have created the .pic and then I view it in HDRShop. Say
the first pixel of the green surface rendered, ( Aren't .pics rendered from
top down, left to right?) pixel 216, 238 has the luminance (sorry, not
reflectance) value of
(0.307, 0.541, 0.291)
Well, in the rays.txt file from

./vwrays.exe -fd greenbox.pic | rtrace -fda `vwrays -d greenbox.pic` -ovm
testroom.oct | grep "green_plastic" > rays.txt

I get (0.239, 0.443, 0.219) which are similar but not exactly the same values.