1. Projective Geometry and Transformations of 2D
——2D摄影几何和变换
射影几何的概念和表示法是多视图几何分析的核心, 使用齐次坐标就能用线性矩阵方程来表示非线性映射(例如透视投影).
1.1 平面几何(Planar Geometry)
Point is Vector, Symmetric matrix is Conic.
1.2 2D射影平面(2D Projective Plane)
x = ( x , y ) T i n IR 2 \textit{\textbf{x}}=(x,y)^T\ in\ \textit{\textbf{IR}}^2 x = ( x , y ) T i n IR 2
1.2.1 Points and Lines
这种等价关系下的矢量等价类被称为齐次矢量,射影空间 IP 2 = IR 3 − ( 0 , 0 , 0 ) T \textit{\textbf{IP}}^2=\textit{\textbf{IR}}^3-(0,0,0)^T IP 2 = IR 3 − ( 0 , 0 , 0 ) T ;a x + b y + c = 0 ax+by+c=0 a x + b y + c = 0 l i n e : ( a , b , c ) T ~ k ( a , b , c ) T \\line:(a,b,c)^T~k(a,b,c)^T l i n e : ( a , b , c ) T ~ k ( a , b , c ) T p o i n t : ( x , y ) T ~ ( k x , k y , k ) T \\point:(x,y)^T~(kx,ky,k)^T p o i n t : ( x , y ) T ~ ( k x , k y , k ) T Result 1: x T l = 0 \textit{\textbf{x}}^T\textit{\textbf{l}}=0 x T l = 0 , dof =2([x , y x, y x , y ] or {a : b : c a :b :c a : b : c });Result 2 :x = l × l ′ , l = x × x ′ \textit{\textbf{x}}=
\textit{\textbf{l}}\times\textit{\textbf{l}}',
\textit{\textbf{l}}=
\textit{\textbf{x}}\times\textit{\textbf{x}}' x = l × l ′ , l = x × x ′ ;
1.2.2 Ideal Points and the Line at Infinity
由平行线交点定义 <idel points >,再由 <idel points > 定义 <infinity line >,<infinity line > 可以看作 <lines direction >集合;任意两条直线都相交于一点,而任意两个相异的点都在一条直线上;x = l × l ′ = ∣ i j k a b c a b c ′ ∣
\textit{\textbf{x}}=
\textit{\textbf{l}}\times\textit{\textbf{l}}'=
\left|
\begin{array}{ccc}
i & j & k \\
a & b & c \\
a & b & c' \\
\end{array}
\right|
x = l × l ′ = ∣ ∣ ∣ ∣ ∣ ∣ i a a j b b k c c ′ ∣ ∣ ∣ ∣ ∣ ∣ i = ( 1 , 0 , 0 ) , j = ( 0 , 1 , 0 ) , k = ( 0 , 0 , 1 )
\\\textit{\textbf{i}}=(1,0,0), \textit{\textbf{j}}=(0,1,0),\textit{\textbf{k}}=(0,0,1)
i = ( 1 , 0 , 0 ) , j = ( 0 , 1 , 0 ) , k = ( 0 , 0 , 1 ) x i d e l i n l ∞ → l ∞ × l = ( 0 , 0 , 1 ) ( a , b , c ) = ( b , − a , 0 ) = x i d e l → l d i r
\\\textit{\textbf{x}}_{idel}\ in\ \textit{\textbf{l}}_\infty \rightarrow \textit{\textbf{l}}_\infty\times \textit{\textbf{l}}=(0,0,1)(a,b,c)=(b,-a,0)=\textit{\textbf{x}}_{idel} \rightarrow \textit{\textbf{l}}_{dir}
\\
x i d e l i n l ∞ → l ∞ × l = ( 0 , 0 , 1 ) ( a , b , c ) = ( b , − a , 0 ) = x i d e l → l d i r
射影平面模型:
两相异射线共处于一张平面上,而任何两张相异平面相交于一条射线; 表示理想点的射线和表示无穷远直线平面都与该平面平行;Result 3:
(Dual )2维射影几何中的任何定理都有一个对应的对偶定理,它可以通过互换原定理中点和线的作用而导出;
1.2.3 Conics and Dual Conics
非退化二次曲线是不同方向的平面与圆锥相交所产生的截线,退化的二次曲线由过锥顶的平面产生,五点定义一条二次曲线:a x 2 + b xy + c y 2 + d x + e y + f = 0
a\textit{\textbf{x}}^2+b\textit{\textbf{xy}}+c\textit{\textbf{y}}^2+d\textit{\textbf{x}}+e\textit{\textbf{y}}+f=0
a x 2 + b xy + c y 2 + d x + e y + f = 0 x T C x = 0
\\\textit{\textbf{x}}^T\textbf{C}\textit{\textbf{x}}=0
x T C x = 0 C = [ a b / 2 d / 2 b / 2 c e / 2 d / 2 e / 2 f ]
\\\textbf{C}=
\left[
\begin{array}{ccc}
a & b/2 & d/2 \\
b/2 & c & e/2 \\
d/2 & e/2 & f \\
\end{array}
\right]
C = ⎣ ⎡ a b / 2 d / 2 b / 2 c e / 2 d / 2 e / 2 f ⎦ ⎤
零空间拟合几何实体:s c a l e f a c t o r : { a : b : c : d : e : f }
scale factor:\{a:b:c:d:e:f\}
s c a l e f a c t o r : { a : b : c : d : e : f } c = ( a , b , c , d , e , f ) T
\textit{\textbf{c}}=(a,b,c,d,e,f)^T
c = ( a , b , c , d , e , f ) T [ x i 2 x i y i y i 2 x i y i 1 ] c = 0
\left[
\begin{array}{ccccc}
x^2_i & x_iy_i & y^2_i & x_i & y_i & 1
\end{array}
\right]\textit{\textbf{c}}=0
[ x i 2 x i y i y i 2 x i y i 1 ] c = 0 Result 4:
(Tangent )过(非退化)二次曲线C \textbf{C} C 上点x \textit{\textbf{x}} x 的切线l \textit{\textbf{l}} l 由 l = C x \textit{\textbf{l}}=\textbf{C}\textit{\textbf{x}} l = C x 确定;
(Dual Conics )l T C ∗ l = 0 \textit{\textbf{l}}^T\textbf{C}^*\textit{\textbf{l}}=0 l T C ∗ l = 0 *,C ∗ = C − 1 \textbf{C}^*=\textbf{C}^{-1} C ∗ = C − 1 ;
(Degenerate Conics )非满秩矩阵C \textbf{C} C 所定义的二次曲线称作退化二次曲线,退化的点二次曲线包含两条线(秩2)或一条重线(秩1),零矢量x = l × m \textit{\textbf{x}}=\textit{\textbf{l}}\times\textit{\textbf{m}} x = l × m ;
1.3 射影变换(Projective Transformations)
射影映射=>保线变换=>射影变换=>单应(homograph )homography : x ′ = H x \textit{\textbf{x}}'=\textbf{H}\textit{\textbf{x}} x ′ = H x , l ′ = H − T l \textit{\textbf{l}}'=\textbf{H}^{-T}\textit{\textbf{l}} l ′ = H − T l , C ′ = HC H − 1 \textbf{C}'=\textbf{H}\textbf{C}\textbf{H}^{-1} C ′ = H C H − 1 , C ∗ ′ = H C ∗ H T {\textbf{C}^*}'=\textbf{H}\textbf{C}^*\textbf{H}^T C ∗ ′ = H C ∗ H T (H \textbf{H} H - 逆变,H − 1 \textbf{H}^{-1} H − 1 - 协变);
1.4 变换层次(A Hierarchy of Transformations)
1.4.1 isometric
x ′ = H E x = [ R t 0 T 1 ] x
\textit{\textbf{x}}'=\textbf{H}_E\textit{\textbf{x}}=
\left[
\begin{array}{cc}
\textbf{R} & \textit{\textbf t}\\
\textbf 0^T & 1
\end{array}
\right]\textit{\textbf{x}}
x ′ = H E x = [ R 0 T t 1 ] x dof = 3,R T R = R R T = I \textbf{R}^T\textbf{R}=\textbf{R}\textbf{R}^T=\textbf{I} R T R = R R T = I ,保向等距矩阵∣ R ∣ = 1 |\textbf{R}|=1 ∣ R ∣ = 1 ,逆向等距矩阵∣ R ∣ = − 1 |\textbf{R}|=-1 ∣ R ∣ = − 1 ,不变量为长度、角度和面积;
1.4.2 similarity
x ′ = H S x = [ s R t 0 T 1 ] x
\textit{\textbf{x}}'=\textbf{H}_S\textit{\textbf{x}}=
\left[
\begin{array}{cc}
\textit{s}\textbf{R} & \textit{\textbf t}\\
\textbf 0^T & 1
\end{array}
\right]\textit{\textbf{x}}
x ′ = H S x = [ s R 0 T t 1 ] x dof = 4,不变量为角度和比率,度量结构就是确定到只相差一个相似变换的结构;
1.4.3 affine
x ′ = H A x = [ A t 0 T 1 ] x
\textit{\textbf{x}}'=\textbf{H}_A\textit{\textbf{x}}=
\left[
\begin{array}{cc}
\textbf{A} & \textit{\textbf t}\\
\textbf 0^T & 1
\end{array}
\right]\textit{\textbf{x}}
x ′ = H A x = [ A 0 T t 1 ] x A = R ( θ ) R ( − ϕ ) DR ( ϕ ) , D = [ λ 1 0 0 λ 2 ]
\textbf{A} = \textbf{R}(\theta)\textbf{R}(-\phi)\textbf{D}\textbf{R}(\phi)
,\ \textbf{D} =
\left[
\begin{array}{cc}
\lambda_1 & 0\\
0 & \lambda_2
\end{array}
\right]
A = R ( θ ) R ( − ϕ ) D R ( ϕ ) , D = [ λ 1 0 0 λ 2 ] SVD :A = UD V T = ( U V T ) ( VD V T ) = R ( θ ) ( R ( − ϕ ) DR ( ϕ ) )
\textbf{A} =
\textbf{U}\textbf{D}\textbf{V}^T=(\textbf{U}\textbf{V}^T)(\textbf{V}\textbf{D}\textbf{V}^T)=
\textbf{R}(\theta)(\textbf{R}(-\phi)\textbf{D}\textbf{R}(\phi))
A = U D V T = ( U V T ) ( V D V T ) = R ( θ ) ( R ( − ϕ ) D R ( ϕ ) ) dof = 6,不变量为平行线、平行线段长度比(直线段的长度缩放仅与该线段方向和缩放方向之间的夹角有关λ 1 2 cos 2 α + λ 2 2 sin 2 α \sqrt{\lambda_1^2\cos^2\alpha+\lambda_2^2\sin^2\alpha} λ 1 2 cos 2 α + λ 2 2 sin 2 α )、面积比(d e t A = λ 1 λ 2 detA=\lambda_1\lambda_2 d e t A = λ 1 λ 2 );
仿射矩阵 A 被看成是一个旋转( ϕ ) (\phi) ( ϕ ) ,加上在(已旋转)的x x x 和y y y 方向分别进行按比例因子( λ 1 ) (\lambda_1) ( λ 1 ) 和( λ 2 ) (\lambda_2) ( λ 2 ) 缩放,再加上一个回转( − ϕ ) (-\phi) ( − ϕ ) 和最后一个旋转( θ ) (\theta) ( θ ) 的复合变换,(比相似变换多了缩放方向角度ϕ \phi ϕ 和缩放比率λ 1 : λ 2 \lambda_1:\lambda_2 λ 1 : λ 2 两个*度);
1.4.4 projective
x ′ = H P x = [ A t 0 T 1 ] x
\textit{\textbf{x}}'=\textbf{H}_P\textit{\textbf{x}}=
\left[
\begin{array}{cc}
\textbf{A} & \textit{\textbf t}\\
\textbf 0^T & 1
\end{array}
\right]\textit{\textbf{x}}
x ′ = H P x = [ A 0 T t 1 ] x dof = 8,不变量为交比,射影变换可对消影点建模;
1.4.5 decomposition
H = H S H A H P = [ s R t ∖ v 0 T 1 ] [ K 0 0 T 1 ] [ I 0 v T v ] = [ A t v T v ]
\textbf{H}=
\textbf{H}_S\textbf{H}_A\textbf{H}_P=
\left[
\begin{array}{cc}
\textit{s}\textbf{R} & \textit{\textbf{t}}\setminus\textit{v}\\
\textbf{0}^T & 1
\end{array}
\right]
\left[
\begin{array}{cc}
\textbf{K} & \textbf{0}\\
\textbf{0}^T & 1
\end{array}
\right]
\left[
\begin{array}{cc}
\textbf{I} & \textbf{0}\\
\textit{\textbf{v}}^T & \textit{v}
\end{array}
\right]=
\left[
\begin{array}{cc}
\textbf{A} & \textit{\textbf{t}}\\
\textit{\textbf{v}}^T & \textit{v}
\end{array}
\right]
H = H S H A H P = [ s R 0 T t ∖ v 1 ] [ K 0 T 0 1 ] [ I v T 0 v ] = [ A v T t v ] Result 5:
与函数无关的不变量数等于或大于配置的*度数减去变换的*度数;
1.5 1D射影几何(The Projective Geometry of 1D)
x ˉ ′ = H 2 × 2 x ˉ
\bar{\textit{\textbf{x}}}'=
\textbf{H}_{2\times2}\bar{\textit{\textbf{x}}}
x ˉ ′ = H 2 × 2 x ˉ
交比是IP 1 \textbf{IP}^1 IP 1 的基本射影不变量:C r o s s ( x ˉ 1 , x ˉ 2 , x ˉ 3 , x ˉ 4 ) = ∣ x ˉ 1 x ˉ 2 ∣ ∣ x ˉ 3 x ˉ 4 ∣ ∣ x ˉ 1 x ˉ 3 ∣ ∣ x ˉ 2 x ˉ 4 ∣
Cross(\bar{\textit{\textbf{x}}}_1,\bar{\textit{\textbf{x}}}_2,\bar{\textit{\textbf{x}}}_3,\bar{\textit{\textbf{x}}}_4)=
\frac{|\bar{\textit{\textbf{x}}}_1\bar{\textit{\textbf{x}}}_2||\bar{\textit{\textbf{x}}}_3\bar{\textit{\textbf{x}}}_4|}{|\bar{\textit{\textbf{x}}}_1\bar{\textit{\textbf{x}}}_3||\bar{\textit{\textbf{x}}}_2\bar{\textit{\textbf{x}}}_4|}
C r o s s ( x ˉ 1 , x ˉ 2 , x ˉ 3 , x ˉ 4 ) = ∣ x ˉ 1 x ˉ 3 ∣ ∣ x ˉ 2 x ˉ 4 ∣ ∣ x ˉ 1 x ˉ 2 ∣ ∣ x ˉ 3 x ˉ 4 ∣ ∣ x ˉ i x ˉ j ∣ = d e t [ x i 1 x j 1 x i 2 x j 2 ]
|\bar{\textit{\textbf{x}}}_i\bar{\textit{\textbf{x}}}_j|=
det\left[
\begin{array}{cc}
\textit{x}_{i1} & \textit{x}_{j1}\\
\textit{x}_{i2} & \textit{x}_{j2}
\end{array}
\right]
∣ x ˉ i x ˉ j ∣ = d e t [ x i 1 x i 2 x j 1 x j 2 ]
共点线是直线上共线点的对偶,任何四条共点线都有一个确定的交比;
1.6 射影平面的拓扑(Topology of the Projective Plane)
x = ( x 1 , x 2 , x 3 ) \textit{\textbf{x}}=(x_1,x_2,x_3) x = ( x 1 , x 2 , x 3 ) 乘以一个非零因子归一化为IR 3 \textbf{IR}^3 IR 3 单位球面x 1 2 + x 2 2 + x 3 2 = 1 x^2_1+x^2_2+x^2_3=1 x 1 2 + x 2 2 + x 3 2 = 1 ,任何相差一个乘数因子-1的矢量x \textit{\textbf{x}} x 和− x -\textit{\textbf{x}} − x 表示同一个点,IR 3 \textbf{IR}^3 IR 3 单位球面和IP 2 \textbf{IP}^2 IP 2 之间存在一种二对一的对应;由于包含一条逆向路径,所以射影平面不可定向;
1.7 从图像恢复仿射和度量性质(Recovery of Affine and Metric Properties from Images)
射影变换比相似变换多了4个*度:无穷远直线l ∞ \textit{\textbf{l}}_\infty l ∞ (2dof )和在l ∞ \textit{\textbf{l}}_\infty l ∞ 上的两个虚圆点 (2dof );
1.7.1 the line at infinity
Result 6:
在射影变换H \textbf{H} H 下,无穷远直线l ∞ \textit{\textbf{l}}_\infty l ∞ 为不东直线的充要条件是H \textbf{H} H 是仿射变换;
1.7.2 recovery of affine properties from images
H = H A [ 1 0 0 0 1 0 l 1 l 2 l 3 ]
\textbf{H}=\textbf{H}_A
\left[
\begin{array}{ccc}
1 & 0 & 0\\
0 & 1 & 0\\
\textit{l}_1 & \textit{l}_2 & \textit{l}_3
\end{array}
\right]
H = H A ⎣ ⎡ 1 0 l 1 0 1 l 2 0 0 l 3 ⎦ ⎤
仿射矫正:世界平面上的无穷远直线被影像为该平面的消影线l = ( l 1 , l 2 , l 3 ) T \textit{\textbf{l}}=(\textit{\textbf{l}}_1,\textit{\textbf{l}}_2,\textit{\textbf{l}}_3)^T l = ( l 1 , l 2 , l 3 ) T ,由平行线交点计算,无穷远点由直线上的距离比确定,使消影线映射到规范位置达到仿射矫正H − T ( l 1 , l 2 , l 3 ) T = ( 0 , 0 , 1 ) T = l ∞ \textbf{H}^{-T}(\textit{\textbf{l}}_1,\textit{\textbf{l}}_2,\textit{\textbf{l}}_3)^T=(0,0,1)^T=\textit{\textbf{l}}_\infty H − T ( l 1 , l 2 , l 3 ) T = ( 0 , 0 , 1 ) T = l ∞ ;
1.7.3 the circular points and their dual
Result 7:
在射影变换的H \textbf{H} H 下,虚圆点I \textbf{I} I 和J \textbf{J} J 为不动点的充要条件是H \textbf{H} H 是相似变换,任何圆都交l ∞ \textit{\textbf{l}}_\infty l ∞ 于虚圆点I = ( 1 , i , 0 ) T \textbf{I}=(1,i,0)^T I = ( 1 , i , 0 ) T ,J = ( 1 , − i , 0 ) T \textbf{J}=(1,-i,0)^T J = ( 1 , − i , 0 ) T ;I = H S I = [ s cos θ − s sin θ t x s sin θ s cos θ t y 0 0 1 ] [ 1 i 0 ] = se − i θ [ 1 i 0 ] = I
\textbf{I}=
\textbf{H}_S\textbf{I}=
\left[
\begin{array}{ccc}
\textit{s}\cos\theta & -\textit{s}\sin\theta & \textit{\textbf{t}}_x\\
\textit{s}\sin\theta & \textit{s}\cos\theta & \textit{\textbf{t}}_y\\
0 & 0 & 1
\end{array}
\right]
\left[
\begin{array}{c}
1\\
i\\
0
\end{array}
\right]=
\textit{se}^{-i\theta}
\left[
\begin{array}{c}
1\\
i\\
0
\end{array}
\right]=
\textbf{I}
I = H S I = ⎣ ⎡ s cos θ s sin θ 0 − s sin θ s cos θ 0 t x t y 1 ⎦ ⎤ ⎣ ⎡ 1 i 0 ⎦ ⎤ = se − i θ ⎣ ⎡ 1 i 0 ⎦ ⎤ = I
与虚圆点对偶的退化二次曲线(d e t C ∞ ∗ = 0 det\textbf{C}^*_\infty=0 d e t C ∞ ∗ = 0 ):C ∞ ∗ = IJ T + JI T = [ 1 0 0 0 1 0 0 0 0 ]
\textbf{C}^*_\infty=\textbf{IJ}^T+\textbf{JI}^T=
\left[
\begin{array}{ccc}
1 & 0 & 0\\
0 & 1 & 0\\
0 & 0 & 0
\end{array}
\right]
C ∞ ∗ = IJ T + JI T = ⎣ ⎡ 1 0 0 0 1 0 0 0 0 ⎦ ⎤
1.7.4 angles on the projective plane
cos θ = l 1 m 1 + l 2 m 2 ( l 1 2 + l 2 2 ) ( m 1 2 + m 2 2 ) → cos θ = l T C ∞ ∗ m ( l T C ∞ ∗ l ) ( m T C ∞ ∗ m )
\cos\theta=\frac{\textit{l}_1\textit{m}_1+\textit{l}_2\textit{m}_2}{\sqrt{(\textit{l}_1^2+\textit{l}_2^2)(\textit{m}_1^2+\textit{m}_2^2)}}
\rightarrow
\cos\theta=\frac{\textit{\textbf{l}}^T\textbf{C}^*_\infty\textit{\textbf{m}}}{\sqrt{(\textit{\textbf{l}}^T\textbf{C}^*_\infty\textit{\textbf{l}})(\textit{\textbf{m}}^T\textbf{C}^*_\infty\textit{\textbf{m}}})}
cos θ = ( l 1 2 + l 2 2 ) ( m 1 2 + m 2 2 ) l 1 m 1 + l 2 m 2 → cos θ = ( l T C ∞ ∗ l ) ( m T C ∞ ∗ m ) l T C ∞ ∗ m Result 8:
一旦二次曲线C ∞ ∗ \textbf{C}^*_\infty C ∞ ∗ 在射影平面上被辨认,那么欧氏角和长度比可以用来测量,l T C ∞ ∗ m \textit{\textbf{l}}^T\textbf{C}^*_\infty\textit{\textbf{m}} l T C ∞ ∗ m 在射影框架下不变,l T C ∞ ∗ m = 0 \textit{\textbf{l}}^T\textbf{C}^*_\infty\textit{\textbf{m}}=0 l T C ∞ ∗ m = 0 ,则直线l \textit{\textbf{l}} l 和m \textit{\textbf{m}} m (共轭)正交;
1.7.5 recovery of metric properties from images
C ∞ ∗ ′ = ( H P H A H S ) C ∞ ∗ ( H P H A H S ) T = ( H P H A ) ( H S C ∞ ∗ H S ) ( H A T H P T ) = ( H P H A ) ( C ∞ ∗ ) ( H A T H P T ) = [ KK T KK T v v T KK T v T KK T v ]
{\textbf{C}^*_\infty}'=(\textbf{H}_P\textbf{H}_A\textbf{H}_S)\textbf{C}^*_\infty(\textbf{H}_P\textbf{H}_A\textbf{H}_S)^T
\\=(\textbf{H}_P\textbf{H}_A)(\textbf{H}_S\textbf{C}^*_\infty\textbf{H}_S)(\textbf{H}_A^T\textbf{H}_P^T)
\\=(\textbf{H}_P\textbf{H}_A)(\textbf{C}^*_\infty)(\textbf{H}_A^T\textbf{H}_P^T)
\\=
\left[
\begin{array}{cc}
\textbf{KK}^T & \textbf{KK}^T\textbf{\textit{v}}\\
\textbf{\textit{v}}^T\textbf{KK}^T & \textbf{\textit{v}}^T\textbf{KK}^T\textbf{\textit{v}}
\end{array}
\right]
C ∞ ∗ ′ = ( H P H A H S ) C ∞ ∗ ( H P H A H S ) T = ( H P H A ) ( H S C ∞ ∗ H S ) ( H A T H P T ) = ( H P H A ) ( C ∞ ∗ ) ( H A T H P T ) = [ KK T v T KK T KK T v v T KK T v ] Result 9:
在射影平面上,一旦C ∞ ∗ \textbf{C}^*_\infty C ∞ ∗ 被辨认,那么射影失真可以矫正到相差一个相似变换,C ∞ ∗ \textbf{C}^*_\infty C ∞ ∗ 可由两个直角、椭圆、两个已知长度比例(l ∞ \textit{\textbf{l}}_\infty l ∞ 已知)确定,或五个直角直接确定;
1.8 二次曲线的其它性质(More Properties of Conics)
1.8.1 the pole-polar relationship
极点x \textit{\textbf{x}} x 关于二次曲线C \textbf{C} C 的极线l = C x \textit{\textbf{l}}=\textbf{C}\textit{\textbf{x}} l = C x 与C \textbf{C} C 交于两点,C \textbf{C} C 过这两点的两条切线相交于x = C − 1 l \textit{\textbf{x}}=\textbf{C}^{-1}\textit{\textbf{l}} x = C − 1 l ;点x \textit{\textbf{x}} x 在C \textbf{C} C 上,则极线就是过x \textit{\textbf{x}} x 点的切线;y T l = y T C x = 0 \textit{\textbf{y}}^T\textit{\textbf{l}}=\textit{\textbf{y}}^T\textbf{C}\textit{\textbf{x}}=0 y T l = y T C x = 0 ,x \textit{\textbf{x}} x 与y \textit{\textbf{y}} y 共轭(互在彼此的极线上);
1.8.2 classification of conics
C = U T DU → C ′ = U − T C U − 1 = D
\textbf{C}=\textbf{U}^T\textbf{D}\textbf{U}
\rightarrow
\textbf{C}'=\textbf{U}^{-T}\textbf{C}\textbf{U}^{-1}=\textbf{D}
C = U T D U → C ′ = U − T C U − 1 = D D = d i a g ( ε 1 d 1 , ε 2 d 2 , ε 3 d 3 ) = d i a g ( s 1 , s 2 , s 3 ) T d i a g ( ε 1 , ε 2 , ε 3 ) d i a g ( s 1 , s 2 , s 3 ) s i 2 = d i , ε i = ± 1 o r 0
\textbf{D}=diag(\varepsilon_1d_1,\varepsilon_2d_2,\varepsilon_3d_3)=
diag(s_1,s_2,s_3)^T
diag(\varepsilon_1,\varepsilon_2,\varepsilon_3)
diag(s_1,s_2,s_3)
\\s^2_i=d_i,\varepsilon_i=\pm1\ or\ 0
D = d i a g ( ε 1 d 1 , ε 2 d 2 , ε 3 d 3 ) = d i a g ( s 1 , s 2 , s 3 ) T d i a g ( ε 1 , ε 2 , ε 3 ) d i a g ( s 1 , s 2 , s 3 ) s i 2 = d i , ε i = ± 1 o r 0
点二次曲线的仿射分类:
1.9 不动点与直线(Points and Lines)
H e = λ e \textbf{H}\textit{\textbf{e}}=\lambda\textit{\textbf{e}} H e = λ e ,特征矢量对应不动点,H − T e = λ e \textbf{H}^{-T}\textit{\textbf{e}}=\lambda\textit{\textbf{e}} H − T e = λ e ,特征矢量对应不动直线,不动直线不是点点不动,不动点才映射为自身;
1.10 总结
引入齐次坐标表示2D 射影平面
定义了理想点和无穷远直线
通过二次曲线介绍对偶及退化二次曲线概念
射影变换的四个变换层次(欧氏、相似、仿射、射影)及*度和不变量
消影线确定可以实现仿射矫正,虚圆点对偶二次曲线确定可以实现相似矫正
极点与极线概念对二次曲线分类
以射影矩阵的特征矢量描述不动点与不动直线