brdf measurement and color appearance … 1 brdf measurement and color appearance simulation based...
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Page 1
BRDF measurement and color
appearance simulation based on
iccMAX framework
Wei-Chun HUNG, Pei-Li SUN 2017/06/28
Graduate Institute of Color and Illumination Tech., Nat. Taiwan Univ. of Sci. & Tech.
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Motivation
• ICC profiles have been widely used in digital archives to preserve the color information of the relics. However, their surface properties such as glossiness and texture have not yet been recorded using the conventional ICC system.
• ICC is currently working on iccMAX. The new ICC system can be used to simulate material appearance with BRDF parameters.
• The study aims to optimize BRDF measurement and color appearance simulation based on the iccMAX and providing real-world examples for evaluation.
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Contents
• BRDF measurement of real-world samples
• Evolution of BRDF parameters
• Blinn-Phong model as an example
• Color appearance simulation based on the iccMAX
framework
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BRDF measurement of real-world samples
1
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• PANTONEcoated/uncoated/metallic/cotton
• R/G/B/W/K/GY
Samples:
BRDF Measurements
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BRDF Measurements
A hemisphere scatter imager
test sample(contact
measurement)
movablelight source
CCD
convexmirror
Apparatus: IS-SA BRDF Scatterometer
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BRDF Measurements
The
project
aims to
Apparatus:
BYK-mac
𝟎° 𝟐𝟎° 𝟑𝟎°
𝟔𝟎°
−𝟑𝟎°
−𝟔𝟓°
𝑺𝒑𝒆𝒄𝒖𝒍𝒂𝒓
𝟗𝟎° 𝟐𝟕𝟎°
−𝟒𝟓°
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BRDF Measurements
Measurement Geometries :
• Illumination Angle :
Incident Angles10°~ 80°
r: 0°~ 80°(Zenith/Azumuth)
d: 0°~ 359° (Polar Angles)
IS-SA BYK-MacIncident Angles d=φ=𝟗𝟎° d=φ=𝟗𝟎°
Ɵ𝒊 𝟏𝟎°𝒕𝒐 𝟖𝟎° -𝟒𝟓°
Reflection Angles
d=φ=𝟎°𝐭𝐨 𝟑𝟓𝟗° d=φ=𝟗𝟎°/ 𝟐𝟕𝟎°
Ɵ𝒓 𝟎°𝐭𝐨 𝟖𝟎° 𝟎°, 𝟐𝟎°, 𝟑𝟎°, 𝟔𝟎°,−𝟑𝟎°, −𝟔𝟓°,
IS-SA BYK-Mac
data X Y Z L*a*b*
𝟗𝟎° 𝟐𝟕𝟎°
1𝟖𝟎°
𝟎°
d= azimuth angle, Ɵ𝒊= illumination angleƟ𝒓 = viewing angle
five aspecular angles
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BRDF data coordinate transform
30
(r,d) = (80, 30)
80
60
40
200 0180
270
90
Raw Data (polar coordinates)
drr =[0 90]
d=[0 360]
range
80
60
40
200 0180
270
90
r*cos(d)
dr*sin(d)
(x,y)=(r*sin(d), r*cos(d))r
Raw Data (Cartesian coordinates)
y
x
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Y stimulus
White samples with different glossiness
Uncoated paper Coated paper Metallic
CottonPerfect Diffuser
Incident Angles
Incident Angles Incident Angles
Incident AnglesIncident Angles
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Evolution of BRDF parameters
Blinn-Phong model as an example
2
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Where B is the output of the absolute transform, 𝐼𝑑 is the diffuse scaling factor, 𝐼𝑠 is the specular scaling factor, and 𝐼𝑔𝑠 is a global specular component.
𝐾𝑑 is the diffuse reflection constant for the material 𝐾𝑠 is the specular reflection constant for the material. n is the shininess constant for the material.
For the full colour Blinn-Phong function the three parameters shall be 𝐾𝑑 , 𝐾𝑠, and n . The order of the parameters in the transform shall be: 𝐾𝑑 , 𝐾𝑠, and n
The monochrome function combines the output of the absolute transform with three parameters to compute the Blinn-Phong parameters 𝐾𝑑=𝐼𝑑B𝐾𝑠=𝐼𝑠B+𝐼𝑔𝑠
BRDF Model
XYZ tristimulus value in x/0 viewing
ls are normally very small. We ignore them.
The following are the Blinn-Phong parameters that specify the material:
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BRDF Rendering Parameters
Convert IS-SA raw data to images
Least-squared curve fitting for Blinn-Phong Reflection model
BRDF parameter fitting:
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Coated Paper - Red
X stimulus
Normalized X stimuli with pseudo-colors
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Coated Paper - Red
X stimulus
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Coated Paper - Red
Y stimulus
Normalized Y stimuli with pseudo-colors
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Coated Paper - Red
Y stimulus
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Coated Paper - Red
Z stimulus
Normalized Z stimuli with pseudo-colors
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Coated Paper - Red
Z stimulus
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Summary
Monochrome BRDF parameters across X, Y and Z channels are very similar.
ls are normally very small. We ignore them.
Limit upper boundary of ld to 1 could improve the fitting.
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Coated Paper
Normalized Y stimuli with pseudo-colors
R G B
K Gray W
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R G B
K Gray W
Y stimulus
Coated Paper
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Uncoated Paper
Normalized Y stimuli with pseudo-colors
R G B
K Gray W
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Uncoated Paper
R G B
K Gray W
Y stimulus
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Metallic coated paper
Normalized Y stimuli with pseudo-colors
R G B
K Gray W
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Metallic coated paper
R G B
K Gray W
Y stimulus
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Cotton
R G B
K Gray W
Y stimulus
Normalized Y stimuli with pseudo-colors
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Cotton
R G B
K Gray W
Y stimulus
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Color appearance simulation based on the
iccMAX framework
3
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iccMAX application
1. Device signals to PCS conversion using an A2Bx profile.
2. Apply a monochrome BRDF profile for the material. It contains ld, ls, lsg and nparameters for each of XYZ channel.
sRGB XYZ
ld, ls, lsg, n
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iccMAX application
3. Calculate color BRDF parameters.Kd = ld*XYZKs = (ls*XYZ+lsg)
sRGB XYZ
ld, ls, lsg, n
kd, ks, n
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iccMAX application
4. Color rendering based on given illumination angle, viewing angle and the BRDF model.
• We have implemented Blinn-Phong and Cook-Torrance models.
sRGB XYZ
Blinn-Phong Model
kd, ks, n
brdfColorimetricParameterX/brdfSpectralParameterX
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iccMAX application
5. Highlight compression in XYZ space.• Non-linear tone mapping is needed to avoid
highlight clipping of shiny objects.
unchanged compressed
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iccMAX application
6. PCS to Device conversion using a B2Ax profile.
• In our case, use a sRGB profile for LCD simulation.
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Multi-Angle
The A Red coated paper - ld: 1, ls:1.6, n:189
view
ing
angl
e -9
0 ~
90
(q
r)
illumination angle -90 ~ 90 (qi)
(-90,-90)
(90,90)
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Correct the input values
BYK-mac
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Correct the input values
BYK-mac
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Correct the input values
MetallicBYK-mac
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Correct the input values
BYK-mac
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Correct the input values
IS-SA BRDF Scatterometer Incident Angles50^°
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Correct the input values
IS-SA BRDF Scatterometer
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Correct the input values
MetallicIS-SA BRDF Scatterometer
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Correct the input values
IS-SA BRDF Scatterometer
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Image simulation
Simulate a light tube above the painting
B2Ax Device
iccMAX approach
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Conclusions
We have done BRDF measurement of many real-world samples.
We also proposed a method for evaluating BRDF parameters of Blinn-Phong model.
Tone compression is needed to avoid highlight clipping. The BRDF parameters can be used to simulate color
appearance of a material based on the the iccMAXframework.
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