Emerging realtime photorealism:
path tracing and friends
COSC342
Lecture 24
21 May 2015
Before we talk about path tracing: point clouds
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Sampling the real world:
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Depth sensing cameras, e.g. Microsoft Kinect (2)
Laser scanners used in geospatial work
Don’t get polygons: get points in space
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But polygons can be fitted to point clouds . . .
. . . usable in games and real-time graphics?
Perhaps? https://www.youtube.com/watch?v=Irf-HJ4fBls
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Converging ends of a spectrum?
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We have frequently contrasted:
1. Real-time, scan-line graphics
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Z-buffers, Gouraud and Phong shading, . . .
2. Raytracing
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“Photorealistic”, refraction, reflection, subsurface scattering, global
illumination, depth of field (DoF) and other lens effects, etc.
Modern hardware challenges this split!
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GPGPUs return to graphics
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We’ve seen GPUs do scan line graphics
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OpenCL/CUDA has GPGPUs do maths
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(General Purpose computing on GPUs)
But raytracing involves lots of maths . . .
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(That’s what they were explicitly built for.)
. . . can GPGPUs can help raytracing?
How long does raytracing take?
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Level of Detail (LoD) in scan-line graphics
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Scene ‘difficulty’ in scan-line graphics with hardware acceleration
depends on:
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Number of polygons and their size
Textures and fragment processing
Lighting calculation complexity
Frame will be ready after some time
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Determines frame-rate alongside the display’s refresh frequency
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Level of Detail in raytracing?
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Raytracing difficulty depends on view
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Mirror reflections of mirror reflections . . .
But for a complex scene, we want to be able to ‘bail out’ in an
elegant manner
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But want some idea of scene appearance . . .
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Runtime Path Tracing: Brigade
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Watch carefully the video at
https://www.youtube.com/watch?v=pXZ33YoKu9w
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Think about what you see within it:
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both in terms of good effects . . .
. . . and also problematic artefacts
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What did you notice?
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Good:
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Mirror reflections / glossy surfaces
Indirect illumination
Depth of field
Bad:
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Speckled noise in the display
Frame rate
Still polygons (. . . is that actually bad?)
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“May I have some path tracing?”
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The Brigade system is proprietary code
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Valuable commercially while new technology
(and that’s for runtime Path Tracing)
Look no further than Blender:
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The ‘Cycles’ engine does Path Tracing
. . . planned to replace the internal ray-tracer
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OK, so what is Path Tracing?
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Simplified: for a ‘sample’ of the scene:
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Set up primary ray as usual.
Form new ray from random vector in hemisphere of hit-point normal.
Recurse on that new ray (with depth+1).
Add object emittance to recursion result, scaled by surface BRDF and
reflectance.
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Repeat the above steps ‘many times’.
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Take the average colour of all samples.
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Bidirectional Reflectance Distribution Function (BRDF)
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BRDF explains how light behaves at the surface of an opaque object.
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It is a function of light angle and view angle.
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Thus a generalisation of both diffuse and specular lighting.
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It is a four dimensional function (why four?).
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BRDF vs. lighting we discussed
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Lambertian?
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n
just ignore ωr
Phong?
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more than cosn :
get angular effects
ωi
ωr
Figure derived from Wikimedia Commons CC-SA media http://en.wikipedia.org/wiki/File:BRDF_Diagram.svg
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Our path tracing was “simplified”
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Showed algorithm for ‘gathering rays’
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Can also use ‘shooting rays’ instead
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Go from light source into the scene
Better: ‘bidirectional path tracing’
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Both gathering and shooting; connect paths
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We said “repeat many times”
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OK, so approximately how many times?
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For gathering rays we want thousands of samples per surface point.
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For shooting rays we want orders of magnitude more than that.
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Samples: “use average colour”
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Raytracers average supersampled colours
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But scenes look OK without supersampling—
lighting models fairly completely explained (maybe poorly) the
illumination at the hit-point.
Path Tracing process is more of a statistical process.
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This is the cause of the speckle noise.
‘Eventually’ the image will converge.
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Beyond BRDF
Specular
Reflection
Diffuse
Reflections
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Want refraction?
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Need BTDF: the
Bidirectional
Transmittance
Distribution
Function
Incident
Light Beam
Reflected
Scatter
Distribution
BRDF
Transmitted
Scatter
Distribution
BTDF
Specular
Transmission
Derived from public domain Wikipedia image
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Limitations
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Sub-surface scattering:
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Pathological caustics:
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Need the Bidirectional surface scattering reflectance distribution
function (BSSRDF)
Can use Metropolis Light Transport
Colour effects: basic Path Tracing still won’t simulate a prism
correctly
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