Monthly Archives: February 2007

Large Focal Length compresses depth

camera的镜头随着focal length的增大,景象会被放大,同时,depth也被压缩了。跟调整aperture的效果不一样。

而且,一般照相的时候,应该使用的大的focal length, 然后离目标远一些,这时照出的效果好些,如图6.2,使用210mm的镜头。使用50mm的镜头照出来的效果有distortion,如图6.1. 这是因为,小的focal length等于拉长了depth,增加了projective的效果,使照片看的邪恶些.所以,下次照相的时候想整人的话,就使用小的focal length镜头. 

 

Fun With Focal Length! by Jim Sherwood

http://www.hash.com/users/jsherwood/tutes/focal/focal.html

An Introduction to Projective Geometry (for computer vision)

前几天在网上找了一个很完整的讲解projective geometry的tutorial,看完以后果然以前很多支离破碎的对epipolar geometry, homogeneous coordinate, plucker coordinate, essential/fundamental matrix的理解都联系起来了,感觉很爽。尤其是使用不同的方法推导essential/fundamental matrix。

An Introduction to Projective Geometry (for computer vision)

Stan Birchfield

http://ai.stanford.edu/~birch/projective/

Fourier Slice Theorem

The Fourier slice theorem is derived by taking the one-dimensional Fourier transform of a parallel projection and noting that it is equal to a slice of the two-dimensional Fourier transform of the original object. It follows that given the projection data, it should then be possible to estimate the object by simply performing the 2D inverse Fourier transform.

Start by defining the 2D Fourier transform of the object function as:

….

http://www.eng.warwick.ac.uk/oel/courses/undergrad/lec13/fourier_slice.htm