129 lines
2.6 KiB
Markdown
129 lines
2.6 KiB
Markdown
# Lecture 6
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## Continue on Light, eye/camera, and color
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### BRDF (Bidirectional Reflectance Distribution Function)
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$$
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\rho(\theta_i,\phi_i,\theta_o,\phi_o)
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$$
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#### Diffuse Reflection
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- Dull, matte surface like chalk or latex paint
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- Most often used in computer vision
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- Brightness _does_ depend on direction of illumination
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Diffuse reflection governed by Lambert's law: $I_d = k_d N\cdot L I_i$
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- $N$: surface normal
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- $L$: light direction
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- $I_i$: incident light intensity
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- $k_d$: albedo
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$$
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\rho(\theta_i,\phi_i,\theta_o,\phi_o)=k_d \cos\theta_i
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$$
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#### Photometric Stereo
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Suppose there are three light sources, $L_1, L_2, L_3$, and we have the following measurements:
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$$
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I_1 = k_d N\cdot L_1
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$$
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$$
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I_2 = k_d N\cdot L_2
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$$
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$$
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I_3 = k_d N\cdot L_3
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$$
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We can solve for $N$ by taking the dot product of $N$ and each light direction and then solving the system of equations.
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Will not do this in the lecture.
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#### Specular Reflection
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- Mirror-like surface
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$$
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I_e=\begin{cases}
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I_i & \text{if } V=R \\
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0 & \text{if } V\neq R
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\end{cases}
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$$
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- $V$: view direction
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- $R$: reflection direction
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- $\theta_i$: angle between the incident light and the surface normal
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Near-perfect mirror have a high light around $R$.
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common model:
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$$
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I_e=k_s (V\cdot R)^{n_s}I_i
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$$
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- $k_s$: specular reflection coefficient
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- $n_s$: shininess (imperfection of the surface)
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- $I_i$: incident light intensity
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#### Phong illumination model
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- Phong approximation of surface reflectance
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- Assume reflectance is modeled by three compoents
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- Diffuse reflection
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- Specular reflection
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- Ambient reflection
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$$
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I_e=k_a I_a + I_i \left[k_d (N\cdot L) + k_s (V\cdot R)^{n_s}\right]
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$$
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- $k_a$: ambient reflection coefficient
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- $I_a$: ambient light intensity
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- $k_d$: diffuse reflection coefficient
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- $k_s$: specular reflection coefficient
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- $n_s$: shininess
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- $I_i$: incident light intensity
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Many other models.
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#### Measuring BRDF
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Use Gonioreflectometer.
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- Device for measuring the reflectance of a surface as a function of the incident and reflected angles.
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- Can be used to measure the BRDF of a surface.
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BRDF dataset:
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- MERL dataset
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- CURET dataset
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### Camera/Eye
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#### DSLR Camera
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- Pinhole camera model
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- Lens
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- Aperture (the pinhole)
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- Sensor
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- ...
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#### Digital Camera block diagram
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Scanning protocols:
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- Global shutter: all pixels are exposed at the same time
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- Interlaced: odd and even lines are exposed at different times
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- Rolling shutter: each line is exposed as it is read out
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