Siggraph Course 2017
Path Tracing in Production
Part 1

Manuka: Weta Digital's Spectral Renderer

Johannes Hanika, Weta Digital

Metameric spectra for the XYZ CMF observer

photographed by different cameras

spectrum

XYZ

c00

c01

c02

c03

c04

c05

c06

c07

c08

c09

c10

c11

Metameric spectra for the XYZ CMF observer

photographed by different cameras

spectrum

XYZ

c00

c01

c02

c03

c04

c05

c06

c07

c08

c09

c10

c11

Metameric spectra for the XYZ CMF observer

photographed by different cameras

spectrum

XYZ

c00

c01

c02

c03

c04

c05

c06

c07

c08

c09

c10

c11

Indirect lighting in a renderer

for a simple single-bounce path \[ I_r = \int_\mathcal{P} h_p(\mathbf{X}) \int_\Lambda \bar{r}(\lambda) \cdot L_e(\lambda) \cdot G \cdot f_r(\lambda) \cdot G \cdot W(\lambda) \cdot d\lambda d\mathbf{X} \]
we can swap the integrals
- first convert to RGB and then accumulate in an RGB frame buffer
but RGB rendering: \[ \underbrace{\int_\Lambda \bar{r}(\lambda) \cdot L_e(\lambda) \cdot f_r(\lambda) d\lambda}_{\scriptsize{\mathsf{\mbox{reflected light in RGB}}}} \neq \underbrace{\int_\Lambda \bar{r}(\lambda) \cdot L_e(\lambda) d\lambda}_{\scriptsize{\mathsf{\mbox{light in RGB}}}} \cdot \underbrace{\int_\Lambda \bar{r}(\lambda) \cdot f_r(\lambda) d\lambda}_{\scriptsize{\mathsf{\mbox{Bsdf in RGB}}}} \]
Ouch!

Indirect lighting, reflectance in rec709 (1.00, 0.95, 0.90)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
Adobe RGB
rec709
spectral

illuminant E white
(whitepoint does not match rec709)

Indirect lighting, reflectance in rec709 (1.00, 0.95, 0.90)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACEScg
Adobe RGB
rec709
spectral

illuminant E white
(whitepoint does not match rec709)

Indirect lighting, reflectance in rec709 (0.97, 0.01, 0.00)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACEScg
Adobe RGB
rec709
spectral

most saturated red
(gamut mapped to valid reflectance)

Indirect lighting, reflectance in rec709 (0.96, 0.00, 0.72)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACEScg
Adobe RGB
rec709
spectral

magenta
(not saturated)

## Colour pipeline

* well defined input colour space for rgb textures
  * upsampling rgb to spectra (lights and Bsdf parameters)
  * map to gamut of valid reflectances
* light transport in spectral domain
* map to output gamut for final frames (render is all colours!)  
<table class="packed"><tbody><tr>
<td><img style="height:200px" data-src="images/srgb-gamut.png"/></td>
<td><img class="plain" style="height:40px;width:160px" data-src="images/arrow.svg"/></td>
<td><img style="height:100px;width:280px" data-src="code/spectrum_8.svg"/></td>
<td><img class="plain" style="height:40px;width:160px" data-src="images/arrow.svg"/></td>
<td><img style="height:200px" data-src="images/CIE1931xy_blank.svg"/></td>
</tr><tr>
<td><img style="height:200px" data-src="images/sample-texture.png"/></td>
<td><img class="plain" style="height:40px;width:160px" data-src="images/arrow.svg"/></td>
<td><img style="height:200px" data-src="images/sample-render3.png"/></td>
<td><img class="plain" style="height:40px;width:160px" data-src="images/arrow.svg"/></td>
<td><img style="height:200px" data-src="code/heroes/emilyhero_64_8_fb00.jpg"/></td>
</tr></tbody></table>
<aside class="notes">
so far talked about colour from scene to sensor (getting colour /out/). what about
the other way from artist to scene definition (getting colour /in/).
we heard about massive texturing, increasing memory to store full spectra may not be a good idea
<br/>
<br/>
materialx and usd support colour management now
</aside>

Thank you for listening!

thanks to the wikihuman project for the digital emily asset used in the skin renders

Indirect lighting, reflectance in rec709 (1.00, 0.95, 0.90)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

illuminant E white
(whitepoint does not match rec709)

Indirect lighting, reflectance in rec709 (0.97, 0.01, 0.00)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

most saturated red
(gamut mapped to valid reflectance)

Indirect lighting, reflectance in rec709 (0.00, 0.86, 0.00)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

saturated green
(gamut mapped to valid reflectance)

Indirect lighting, reflectance in rec709 (0.00, 0.00, 0.68)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

most saturated blue
(gamut mapped to valid reflectance)

Indirect lighting, reflectance in rec709 (0.11, 0.09, 0.85)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

softer blue to reduce peaks

Indirect lighting, reflectance in rec709 (0.00, 0.81, 0.78)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

cyan

Indirect lighting, reflectance in rec709 (1.00, 0.00, 0.74)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

magenta

Indirect lighting, reflectance in rec709 (0.96, 0.00, 0.72)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

magenta
(not saturated)

Indirect lighting, reflectance in rec709 (1.00, 0.89, 0.00)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

yellow

Indirect lighting, reflectance in rec709 (0.93, 0.06, 0.00)

#bounces	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14
XYZ
ACES
ACEScg
Adobe RGB
rec709
spectral

measured spectrum

Siggraph Course 2017 Path Tracing in Production Part 1

Manuka: Weta Digital's Spectral Renderer

Metameric spectra for the XYZ CMF observer

Metameric spectra for the XYZ CMF observer

Metameric spectra for the XYZ CMF observer

Indirect lighting in a renderer

Indirect lighting, reflectance in rec709 (1.00, 0.95, 0.90)

Indirect lighting, reflectance in rec709 (1.00, 0.95, 0.90)

Indirect lighting, reflectance in rec709 (0.97, 0.01, 0.00)

Indirect lighting, reflectance in rec709 (0.96, 0.00, 0.72)

Thank you for listening!

Indirect lighting, reflectance in rec709 (1.00, 0.95, 0.90)

Indirect lighting, reflectance in rec709 (0.97, 0.01, 0.00)

Indirect lighting, reflectance in rec709 (0.00, 0.86, 0.00)

Indirect lighting, reflectance in rec709 (0.00, 0.00, 0.68)

Indirect lighting, reflectance in rec709 (0.11, 0.09, 0.85)

Indirect lighting, reflectance in rec709 (0.00, 0.81, 0.78)

Indirect lighting, reflectance in rec709 (1.00, 0.00, 0.74)

Indirect lighting, reflectance in rec709 (0.96, 0.00, 0.72)

Indirect lighting, reflectance in rec709 (1.00, 0.89, 0.00)

Indirect lighting, reflectance in rec709 (0.93, 0.06, 0.00)

Siggraph Course 2017
Path Tracing in Production
Part 1