Am 06.07.2012 21:11, schrieb spitz:
> There was a nice thread on LuxRender about this
>
> http://www.luxrender.net/forum/viewtopic.php?f=14&t=4766
>
> In this case the CCD sensor or image plane simply is a plane with translucent
> material with transmission set to 1 and reflection set to 0. An orthographic
> camera was used to sample this image plane.
>
> Would this be possible in POV-Ray...?

In this quality? Pretty likely so.

The key issue here is how to model the sensor; I can think of two 
approaches:


(A) Use a non-solid object (e.g. a disc, or a union of two triangles); 
give it a perfectly white pigment, no highlights, no reflection, and 
"diffuse 0.0, 1.0" (sic!); this syntax introduced in POV-Ray 3.7 
specifies that 0% of incident illumination should be scattered /back/ 
diffusely (first parameter), while 100% should be scattered /through/ 
the surface instead (second parameter); this makes sure your sensor does 
not scatter light back into the scene.

To actually make use of this approach, you'll need to use radiosity, and 
you need to use special settings that make sure that you get a lot of 
radiosity samples; "maximum_reuse" is of particular importance in this 
context and should be very low. Radiosity sample density will be a 
problem though (as it limits the resolution of your sensor), and you 
might actually end up with a high memory footprint.


(B) Actually, the "sensor" used in the Luxrender example does nothing 
but apply pretty random perturbations to the rays going through it. It 
so happens that POV-Ray has a dedicated feature to perturb camera rays, 
aptly referred to as "Camera Ray Perturbation" in the docs, which works 
by adding a "normal" statement to the camera statement. If the specified 
perturbation is fine enough and has a suitable distribution, and you 
make generous use of anti-aliasing, it'll make the /perfect/ 
sensor-imitation screen for your camera: It'll cause no backscatter at 
all, you don't have to worry about positioning it relative to the 
camera, you don't need to use radiosity if you don't like it, and so on.


That said, I'm not yet sure what formula is used for the perturbation 
effect (whether it is equivalent to looking at a correspondingly 
perturbed mirror, or whether the ray directions are modified directly as 
if they were the normals), nor what kind of angular dependency is 
realistic for a CCD's response to incoming light (lambertian law might 
be a first approximation, but I guess it's not that simple), let alone 
what pattern would most faithfully model the scattering effect. But for 
starters I'd try with "normal { bumps 0.5 }" or something along these 
lines. There's always the possibility of changing the pattern later to 
make it more realistic once you're convinced that it's worth the effort 
and have figured out what actually /is/ realistic anyway.


As another alternative, you could try with very out-of-focus focal blur, 
setting the focal point maybe halfway between the camera and the lens, 
and "aperture" to the difference between the camera diagonal size and 
the lens' diameter. I don't know whether this is a suitable model for 
the anular dependency of a CCD's response though, and you can't just 
tweak it very much as you can do with the camera perturbation approach.


Oh, and then I just read about the mesh camera: You /can/ use it for 
your purposes, using distribution type 0. Use multiple meshes, each 
defining the same points from which to shoot a ray, but use different 
randomized surface normals for each mesh to jitter the directions in 
which rays are shot (you can use the very same normal for all triangles 
in a mesh, or randomize them even within one mesh). This gives you more 
direct control over the sample directions as compared to the perturbed 
camera approach. See the docs or the wiki for more details on mesh cameras.

Post a reply to this message