Joe Doliner

Next up are transparency and reflection. In keeping with the them: we get these by first adding more information to the Ray Paths and then writing shaders that leverage this new information. Recall that Ray Paths contain information about how a ray from the camera travels through the scene, currently it just records what it hits and how that point is illuminated. To that we’ll be adding a Ray Path for the ray constructed by reflecting the incoming ray over the intersections normal and a Ray Path for if the ray continued going through the object. Here’s the new Ray Path:

data Ray_Path = Ray_Path {hit :: Intersection, 
                          ref :: Ray_Path, 
                          thru :: Ray_Path, 
                          light_hits :: [Light_Castback]}

Now we have the data in the Ray Paths, we need to write some shaders that use them:

shader_lit_red_ref :: Shader
shader_lit_red_ref depth rp 
    | depth > 10 = (0,0,0)
    | otherwise = blend [0.7, 0.3] 
                        [(shader_lit_red depth rp), (shade (depth + 1) (ref rp))]

What this does is blend (a weighted average) the color we would get if we just did a simple diffuse shading with a red color and the color we get from the reflection. Notice that we’ve needed to add a counter for the depth to occur the infinite recursion you’d get if you held 2 mirrors up facing each other. This shader is the same idea but blends both the reflected and the through ray:

glass :: Shader
glass depth rp 
    | depth > 10 = (0,0,0)
    | otherwise = blend [0.2, 0.4, 0.4] 
                        [(shader_lit_grey depth rp), (shade (depth + 1)(ref rp)), (shade (depth + 1) (thru rp))]

And here’s an image of a reflective red plane with a glass sphere (which really doesn’t look much like glass) on it:

A reflective transparent sphere sitting on a reflective plane.

Shadows are the first place where ray tracing has an advantage pipelined graphics (OpenGL). Not that shadows in pipelined graphics are impossible, but they can be a bit of a struggle (or at least they have been for me.) But as we’ll see: with ray tracing they’re actually quite pleasant.

We actually already have everything we need with the castback idea; we just need to extend them a bit so that we give shaders empty castbacks when the path to the light is blocked. What’s great is that we already have exactly that function built in, it’s the same function we use to intersect the scene. Here’s what the castback construction code looks like:

light_castback :: Scene -> Intersection -> Light -> Light_Castback
light_castback scene (Intersection _ inter_loc _ _ _) (Point light_loc color intensity) =
    if ((miss scene_xn) || light_t < (t scene_xn))
    then (Light_Castback (light_loc <-> inter_loc) color intensity)
    else empty_light_castback
 
    where ray = (ray_from_to inter_loc light_loc)
          scene_xn = (intersect_scene scene ray) --intersection with the scene
          light_dir = (light_loc <-> inter_loc)
          light_t = (len light_dir) --parameter at which we hit the light

Simple enough: we intersect with the scene and if the intersection is closer the to castbacks origin than the light we’re aiming at, we have an empty castback. So let’s see how it works:

Something is very wrong.

Hmm, not so well at all, although that is a cool looking bug. This type of effect is generally called surface acne and it’s normally down to a floating point error somewhere. In this case the problem is that the castback ray that I shoot out is, depending on roundoff errors, hitting or missing the object whose illumination we’re computing (the object it originally hit.) The easiest way to solve this is to just nudge the ray’s origin points off of the objects they’re intersecting using a function like this:

epsilon = (10 ** (-4))
 
nudge :: Ray -> Ray
nudge ray = (Ray (ray `at` epsilon) (dir ray))

Bug fixed.

Up until now I’ve been cheating in creating images (although cheating is really the point of graphics). As we all know there can be no images without light so let’s add some. First off we need a few structures to represent our lights:

data Light = Point {loc :: Vec3f, 
                    color :: Color, 
                    intensity :: Float} |
             Spot {loc :: Vec3f, 
                   look_at :: Vec3f, 
                   color :: Color, 
                   angle :: Float, 
                   intensity :: Float} deriving (Show)

A point light is the simplest light, it simply shines light in every direction from a single point. A spot light is a little more complicated: it shines light in a cone (like a spotlight).

So now what can we do with these things? In an ideal world we would do with them exactly what the real world does with them: We would simulate lightrays spraying out of them bouncing around the scene and eventually entering our simulated camera. However a randomly shot lightray has a very small chance of making it to the camera so it’s computationally infeasible to start from the lights and make our way to the cameras. So we do exactly the opposite, we start from the Camera and anytime we hit a solid we try to find the lights that this ray could have started from. And we call what we find “Light castbacks”

Where castbacks come from

Light castbacks are an extra piece of information that we can pass in to our shaders; they look like this:

data Light_Castback = Light_Castback {dir :: Vec3f, 
                                      color :: Color, 
                                      intensity :: Float} deriving (Show)

With these things we have everything we need to implement some basic lighting. Here’s what the shader looks like:

shader_lit :: Shader
shader_lit (Ray_Path (Intersection _ _ norm _ False) cbs) = ((clamp 0 255 (floor (220 * foldr (+) 0 (map (\x -> (normed norm) <.> (normed (dir x))) cbs)))) <*> (1,1,1)) <+> (35,35,35)

What this does is take all the castbacks from the scene and compute the dot product of the direction of the light and the normal of the surface (clamping it to within an acceptable range and boosting it to a minimum (hacky way to make there be some ambient lighting.) The fruits:

Diffuse Lighting

Article available at: http://www.rethinkdb.com/blog/2010/08/multi-slicing/