图像形成

• Color balance: • Gamma:
• Other color spaces: YIQ,YUV
• YCbCr:
Hue, Saturation, Value(HSV)
Compression
• Discrete cosine transform(DCT):
谢谢！
• If the light sources are discrete (a finite number of point light sources), we can replace the integral with a summation,
Diffuse reflection
• The amount of light depends on the angle between the incident light direction and the surface normal θi.
Lighting
• Light sources can generally be divided into point and area light sources. • Point light source: location,intensity,color spectrum. • Area light source: • Environment map:maps incident light directions v^ to color values.
The digital camera
• CCD: photons are accumulated in each active well during the exposure time. • CMOS: photons hitting the sensor directly affect the conductivity (or gain) of a photodetector, which can be selectively gated to control exposure duration.
Phong shading
• In the Phong model, the ambient term does not depend on surface orientation, but depends on the color of both the ambient illumination La(λ) and the object ka(λ),
Di-chromatic refl the apparent color of a uniform material lit from a single source depends on the sum of two terms,
Global illumination (ray tracing and radiosity)
Vignetting
• Natural vignetting:
• Mechanical vignetting: it caused by the internal occlusion of rays near the periphery of lens elements in a compound lens.
Image Formation
Photometric image formation
• Lighting • Reflectance and shading • Optics
The digital camera
• Sampling and aliasing • Color • Compression
• The shading equation for diffuse reflection can thus be written as
Specular reflection
• It depends strongly on the direction of the outgoing light. Incident light rays are reflected in a direction that is rotated by 180 around the surface normal n. We can compute the specular reflection direction Si as
• Putting all of these terms together, we arrive at the Phong shading model,
• Note also that the diffuse component of the Phong model (or of any shading model) depends on the angle of the incoming light source v^i, while the specular component depends on the relative angle between the viewer vr and the specular reflection direction s^i (which itself depends on the incoming light direction v^i and the surface normal n^).
Reflectance and shading
• The Bidirectional Reflectance Distribution Function(BRDF)
• The function can be written in terms of the angles of the incident and reflected directions relative to the surface frame as
• The main factors affecting the performance of a digital image sensor: shutter speed sampling pitch fill factor chip size analog gain sensor noise resolution of the analog-to-digital converter
• The amount of light reflected in a given direction V^r thus depends on the angle θs=cos-1 (V^r *S^i) between the view direction V^r and the specular direction S^i.
Optics
• A diagram of the most basic lens model.
• Chromatic aberration: The tendency for light of different colors to focus at slightly different distances(and hence also with slightly different magnification factors)
• Ray tracing or Path tracing: it associates a light ray with each pixel in the camera image and finds its intersection with the nearest surface. • Radiosity: It works by associating lightness values with rectangular surface areas in the scene(including area light sources).
• For an isotropic material,we can simplify the BRDF to
• To calculate the amount of light exiting a surface point p in a direction V^r under a given lighting condition
Sampling and aliasing
• PSF:
Color
• XYZ: the transformation from RGB to XYZ is given by
• L*a*b* color space:
• Color cameras:
• Color filter arrays: