Ray Tracing 22

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PRESENTED BY:MUGDHA CHHAPARWAL

ROLL NO. 22


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` Ifwe consider the line ofsight from a pixel positionon the view plane through ascene, we can determine

which objects in the sceneintersect this line.

` From the intersection pointswith different object, we can

identify the visible surfaceas the one whoseintersection point is closestto the pixel.

` In Ray tracing, instead ofidentifying for visiblesurface for each pixel, wecontinue to bounce the ray

around the picture.

` When ray bounces, itcontributes to intensityforthat surface.

` This technique is useful forobtaining global reflectionand transmission effects.


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` Pixel positions are designated

in x-y plane.` Projections reference point lie

on z axis.` With is coordinate system,

the contributions to a pixel isdetermined by tracing a lightpath BACKWARD from thepixel to the picture.

` For each pixel ray, eachsurface is tested, whicharises following cases:


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`

Ifsurface is intersected:(i) Smallest distance between pixel to surfaceintersection point identifies the visible surface for thatpixel.

(ii) Once visible surface identified, ray is reflected offthe

visible surface along a specular path where i =r.

` Secondary ray: The ray reflected from the visiblesurface or passed through the transparent surface inrefraction direction.

` The ray after reflection/refraction strikes anothervisible surface recursively producing Ray TracingTree.


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` Recursion depth is

determined by theamount ofstorageavailable or by the user.

` Ray path is terminated

when predetermineddepth is reached or theray strikes a light source.

` The surface intensities ofall the nodes are added

traversing the tree frombottom to top todetermine pixel intensity.


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` Path along L is calledShadow Ray.

` Specular reflection:R= u (2u.N)N

` In transparent material,intensity contribution

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Ray equation:P = Po + s.u

Ray equation gives thecoordinates ofany point Palong the ray path at adistance s from Po.

u = Ppixel Pprp

| Ppixel Pprp |

S = u. P +-(u. P)2 - | P|2+r2


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` Aliasing is used to definejagged edges i.e.boundaries are not thatsharp, reflections are not

that perfect and things canbe in and out offocus.` Three techniques are used

to perform Antialiased ray

tracing.` In these, pixel is treated as

finite square area instead ofsingle point.


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` Super Sampling: Instead ofshooting one ray

per pixel, shoot four raysthrough the corners ofa pixel.

Color at the pixel is the average ofthe colors at each corners.

` Adaptive Sampling: Compute the intensity variation

between the four corners withthe average

Shoot more rays throughcorners with higher intensityvariation

Compute final color as aweighted average rather thanthe regular average


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` Distributed Ray tracing /Stochastic sampling:

Is based on randomly distributed

rays over the pixel area used toreduce aliasing effect.

Multiple samples are taken andaveraged together.

Random distribution ofa numberofa number ofrays over the pixelsurface is achieved by Jittering.

Initially pixel area is divided into16 subareas. Then randompositions are obtained by jitteringthe center coordinates ofeachsub pixel area by small amounts(x and y)


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Ray Tracing 22