计算机视觉 Lecture 02 《Light and Shading》

Lecture 02 《Light and Shading》
像素点的强度值主要因素为：
1.光照的强度和方向
2.表面几何图形
3.表面的材料
4.附近的表面
5.摄像机曝光


Image filters in spatial domain


Filter is a mathematical operation of a grid of numbers
Smoothing, sharpening, measuring texture




Image filters in the frequency domainFiltering is a way to modify the frequencies of images
Denoising, sampling, image compression


Convolution with self is another Gaussian
So can smooth with small-width kernel, repeat, and get same result as larger-width kernel would have
。Convolving two times with Gaussian kernel of width σ is same as convolving once with kernel of width  σ√2 




Canny edge detector
1.Filter image with x, y derivatives of Gaussian 
2.
Find magnitude and orientation of gradient
3.Non-mimum suppression:
——
Thin multi-pixel wide “ridges” down to single pixel width
4.Thresholding and linking (hysteresis):
——
Define two thresholds: low high
——
Use the high threshold to start edge curves and the low threshold to continue them

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