Projective geometry is closely related to perspective geometry. These types of geometry originated with artists around the 14th century.

learn more… | top users | synonyms

19
votes
4answers
7k views

Finding the Transform matrix from 4 projected points (with Javascript)

I'm working on a project using Chrome - JS and Webkit 3D CSS3 transform matrix. The final goal is to create a tool for artistic projects using projectors and animation - somewhat far away from using ...
2
votes
1answer
214 views

Resolution of Singularities: Base Point

Consider the curve $y^2=4x^3-ax-b$, where $a$ is a fixed constant and $b$ is a free constant. For each value of $b$ we get a family of curves. Part 1: Show that the family of curves intersect at ...
14
votes
1answer
2k views

Plücker Relations

Let $K$ be a field, $1 \leq d \leq n$ integers and $V$ an $n$-dimensional vector space. The Plücker relations are quadratic forms on $\wedge^d V$ whose zero set is exactly the set of decomposable ...
4
votes
1answer
131 views

Cross-ratio relations

The way I define the cross-ratio in projectve geometry: Let $P_0,P_1,P_2,P_3$ being four points on a projective line G, such that $P_0,P_1,P_2$ are pairwise distinct. Let $\pi:\mathbb KP^1\rightarrow ...
1
vote
1answer
36 views

Projective Co-ordinate Geometry

I am learning projective geometry in my computer vision course. So, we represent a co-ordinate point in an image as a homogeneous co-ordinate as $(x,y,1)$. My professor says that if we are given two ...
7
votes
3answers
9k views

The intersection of two parallel lines

My teacher told me that two parallel lines have a point of intersection, it is called Point at infinity. But I I can't understand how it can be true or how was it proved, can someone explain this to ...
5
votes
1answer
383 views

Problem in Deducing Perspective Projection Matrix

I understand the traditional way(use similar triangle and make depth value linear) to deduce the perspective projection matrix. But I want to try another approach after I read this text: Fundamentals ...
10
votes
2answers
1k views

Augmented Reality Transformation Matrix Optimization

i am a software developer, i'm working on an Augmented Reality system. I'd like to receive some advice in order to optimize my math model. My program has to be slim and fast. Here's the situation: ...
7
votes
1answer
646 views

Rectify image from congruent planar shape objects

I am implementing an algorithm to remove projective distortions on the following image. I understand this is possible by applying the following transformation: $$ \begin{matrix} 1 ...
3
votes
3answers
167 views

Problem in proving that $\mathbb{A}^2$ is not homeomorphic to $\mathbb{P}^2$

let $k$ be an algebraic closed field. All the spaces are equipped with the usual zariski topologies. All the proofs of this fact that I've seen rely on the fact that two lines in $\mathbb{P}^2$ ...
0
votes
1answer
35 views

Whitehead's axioms of projective geometry and a vector space over a field

According to Wikipedia(http://en.wikipedia.org/wiki/Projective_geometry#Axioms_of_projective_geometry), the axioms of projective geometry due to A. N. Whitehead are: G1: Every line contains at least ...
0
votes
1answer
197 views

In the affine plane, I am having trouble with these definitions

If the number of points in an affine plane is finite, then if one line of the plane contains $n$ points then: all lines contain $n$ points, every point is contained in $n + 1$ lines, there are $n^2$ ...
0
votes
1answer
98 views

Homography between ellipses

This is a spin-off from a comment on Stack Overflow. How can I find a homography between two ellipses in the plane?
0
votes
0answers
164 views

Sample Code to Generate Points on the Rim of a Randomly Rotated Cone : What's Going On Here?

Related to this question: http://math.stackexchange.com/questions/407897/randomly-generate-point-on-shell-from-3-points-2-angles-with-uniform-angle-dis I'm trying to reverse engineer the ...
27
votes
5answers
4k views

Help understanding Algebraic Geometry

I while ago I started reading Hartshorne's Algebraic Geometry and it almost immediately felt like I hit a brick wall. I have some experience with category theory and abstract algebra but not with ...
10
votes
1answer
401 views

A modern textbook on affine and projective spaces

Requirements: Scalar fields other than $\mathbb{R}$ and $\mathbb{C}$. Precise. Visual explanations are good, but they must complement definitions and proofs, not replace them. No repetition of text. ...
14
votes
1answer
428 views

topologies of spaces in escher games

There have been a couple of games released (or in development) in the past couple of years which do some weird topological tricks: Echochrome (video), Crush (video), and Fez (video). Do the spaces ...
9
votes
1answer
437 views

Learning projective geometry

My ultimate goal is to learn some algebraic geometry with the more concrete immediate goal of understanding things like how $\mathbb R\mathrm P^2$ is embedded in $\mathbb C\mathrm P^2$ or how $\mathbb ...
10
votes
2answers
318 views

Is a divisor in the hyperplane class necessarily a hyperplane divisor?

Let $V$ be a smooth irreducible projective curve over an algebraically closed field $k$, embedded in some projective space $\mathbb{P}^n$, and let $[H]$ be the induced hyperplane divisor class on $V$. ...
2
votes
0answers
142 views

Projection and Pseudocontraction on Hilbert space

Let $H$ be a real Hilbert space with inner product $\langle\cdot, \cdot \rangle: H \times H \rightarrow \mathbb{R}$, and induced norm $\left\| \cdot \right\|: H \rightarrow \mathbb{R}_{\geq 0}$. Let ...
8
votes
0answers
499 views

Prerequisite of Projective Geometry for Algebraic Geometry

I studied Euclidean Geometry in high-school, and I have not studied anything relates to geometry since I started studying in university. I am now intending to study Algebraic Geometry, however, I ...
7
votes
2answers
346 views

Perspective problem - trapezium turned square

True or false: If you draw a trapezium on the ground, there always exists a point above (but not necessarily directly above) the trapezium such that the trapezium looks like a square from that point. ...
5
votes
2answers
439 views

Analytically flavored book in projective geometry

I am looking for a book in projective geometry, using the apparatus of linear algebra, complex analysis, and, perhaps, modern algebra, in full. The counterexample is the Hartshorne's book on ...
4
votes
1answer
78 views

Description of varieties in $\mathbb{P}^2\times \mathbb{P}^1$

If $[x:y]$ are coordinates of $\mathbb{P}^1$ and $[X:Y:Z]$ are coordinates of $\mathbb{P}^2$, what do the following varieties look like? $V(x^2X+y^2Y+xyZ)\subset \mathbb{P}^2\times \mathbb{P}^1$ ...
4
votes
3answers
667 views

How do you create projective plane out of a finite field?

I have heard and read unclear mentions of links between projective planes and finite fields. Is it possible to construct a projective plane (or a Steiner system) starting out with a field? Could you, ...
3
votes
1answer
1k views

homeomorphism between the real projective line and a circle

I'm currently following an introductory course in geometry and it was mentioned that the real projective line is homeomorphic to a circle. Could someone please state the topologies on both the real ...
3
votes
1answer
2k views

Equation for non-orthogonal projection of a point onto two vectors representing the isometric axis?

Suppose I have two vectors that are not orthogonal (let's say, an isometric grid) representing the new axis. Suppose I want to project a point onto these two vectors, how would I do it? Dot product ...
2
votes
1answer
90 views

Desargues Theorem of two triangles in perspective has symmetric order 120(why?)

10 points and 10 lines construct Desargues' theorem, but since the order of the symmetric group is 120 this means we are permuting 5 elements. But I am confused to what these elements are. In total ...
1
vote
1answer
4k views

2D Coordinates of Projection of 3D Vector onto 2D Plane

The plane $P$ is passing through the origin and has normal $n$. $u$ is a 3D vector and $u'$ its projection onto $P$: $u' = u - \langle u,n \rangle n$ (assuming $n$ has unit length). $e'_1$ and ...
10
votes
4answers
1k views

Textbook for Projective Geometry

So while not actually a specific problem I'm struggling with, I was hoping for some of your insight! For a course, I'm currently reading Stillwell's Four Pillars of Geometry. While it does a nice ...
5
votes
1answer
113 views

Distinguished open sets in $\mathbb{P}^n$

I have given a homogenous polynomial $F\in\mathbb{C}[x_0,\ldots,x_n]$ of degree $d>0$ and consider the open set $U_F=\{p\in\mathbb{P}^n; F(p)\neq 0\}$. I'd like to prove that $U_F$ is isomorphic to ...
5
votes
1answer
53 views

Finding the inverse of a map from $CP^1$ to $S^2$

Given the map: $$f:CP^1 \to S^2\ ,\ f[z:w] = \left(\frac{2\mbox{Re}(w\bar{z})}{|w|^2+|z|^2},\frac{2\mbox{Im}(w\bar{z})}{|w|^2+|z|^2}, \frac{|w|^2-|z|^2}{|w|^2+|z|^2}\right)$$ How would I go about ...
3
votes
1answer
56 views

How do you work with the space of circles on the sphere considered as the projective line?

I'm trying to prove some things about the action of the Möbius group on the "circlines" in the extended complex plane, ie. circles on $\mathbb{C}P^1$. I find that while I have a good grip on Möbius ...
3
votes
1answer
73 views

Is $\langle(26543),(34)(56),(12)(3654)\rangle $ isomorphic to $A_6$?

My question is the one in the title. If you want to understand the context of the problem, please read further. I reduced a problem to proving the question. Background is: Valentiner group ...
3
votes
1answer
69 views

Is the universal hyperplane section the blowup of the baselocus?

I think I've heard this statement before but I'd like to make sure it's true. Let $X$ be a variety and $L$ a line bundle on it. Take $S < P\left(H^0(X,L)\right)$ to be a linear subspace of the ...
3
votes
1answer
251 views

The Process of Choosing Projective Axes to Put an Elliptic Curve into Weierstrass Normal Form

I'm reading the book "Rational Points on Elliptic Curves" and on page 23 the author takes an arbitrary (non-singular) elliptic curve in the projective plane and finds a rational point $O$, referring ...
2
votes
2answers
83 views

Module over a ring which satisfies Whitehead's axioms of projective geometry

I asked this question(Characterization of a vector space over an associative division ring). It was pointed out that the answer was negative. So I reconsidered the problem and have come up with the ...
2
votes
1answer
87 views

Blow-up in Projective Space: Choosing the Appropriate Chart

Consider $x^2+y+\alpha=0$ (made-up example), where $(x,y)\in\mathbb{C}^2$ and $\alpha\in\mathbb C$ is a free parameter. The equation defines a family of curves. The curves have no common points in ...
2
votes
1answer
150 views

Projective plane division by non-concurrent lines

How many regions does n non-concurrent lines divide a Projective Plane into? (probably a standard problem, but I am having conflicting answers)
1
vote
1answer
44 views

Show that three pairwise non-intersecting lines in $\mathbb{R}\mathbb{P}^{3}$ have a transversal.

Let $\mathbb{P}(U1)$ and $\mathbb{P}(U2)$ be two non-intersecting lines in the 3-dimensional projective space $\mathbb{R}\mathbb{P}^{3}$ = $\mathbb{P}(\mathbb{R}^{4})$. Show that $\mathbb{R}^{4}$ ...
1
vote
1answer
125 views

Birational isomorphism $\mathbb{P}^n\times \mathbb{P}^m\to \mathbb{P}^{n+m}$

One can show that $\mathbb{P}^n\times \mathbb{P}^m$ is birational to $\mathbb{P}^{n+m}$ by making note of the Zariski topologies and the canonical isomorphism between affine spaces $\mathbb{A}^n\times ...
1
vote
1answer
105 views

projective geometry hyperplane

For $j=0,\ldots,n$ consider the affine hyperplane $A_j:=e_j+\langle e_0,\ldots,e_{j-1},e_{j+1},\ldots,e_n\rangle$ in $\mathbb K^{n+1}$ and the associated embedding $\tau_j:\mathbb ...
1
vote
1answer
119 views

Find the line closest to 2 2D points passing through a 3rd point

Given 3 homogeneous 2D points, $p=[p_x,p_y,1]$, $q=[q_x,q_y,1]$, and $v=[v_x,v_y,v_z]$, with $p$ and $q$ finite (last coordinate is $1$), but $v_z$ maybe either $1$ - finite or $0$ - at infinity: ...
1
vote
1answer
692 views

Finding a 3D transformation matrix based on the 2D coordinates

I have a square with the length of the sides being 1. This square is transformed by an unknown transformation matrix in the 3D space and then projected back to the plane (the projection is known). I ...
1
vote
1answer
126 views

Rank of a degenerate conic

This question comes from projective geometry. A degenerate conic $C$ is defined as $$C=lm^T+ml^T,$$ where $l$ and $m$ are different lines. It can be easily shown, that all points on $l$ and m lie on ...
0
votes
1answer
103 views

Chirality of a Möbius band without boundary?

In this answer it is remarked that the real projective plane minus one point is homeomorphic to the Möbius strip without boundary. A normal Möbius strip is topologically equivalent to a real ...
0
votes
0answers
35 views

What Projections preserve Pseudocontractiveness?

Let $f: \mathbb{R}^n \rightarrow \mathbb{R}^n$ be a pseudocontraction, i.e., $$ \left\| f(x) - f(y) \right\|^2 \leq \left\| x-y\right\|^2 + \left\| f(x) - f(y) - (x-y) \right\|^2 $$ for all $x,y \in ...
0
votes
1answer
43 views

In projective geometry and curved lines

For example, Fano's Geometry exemplifies a triangle with a circle within it. How is this possible? Are lines not defined to be straight? Is this geometry projective geometry(or is projective geometry ...
0
votes
2answers
208 views

Projective transformation by mapping fixed points to ideal circular points

I am trying to compute a projective transformation out of four pairs of points following the procedure described in this post, namely: Two circular points. A similarity rotation center. An arbitrary ...
0
votes
1answer
66 views

Find the line closest (SSD) to 2 2D points passing through a 3rd point

Given 3 homogeneous 2D points, $p=[p_x,p_y,1]$, $q=[q_x,q_y,1]$, and $v=[v_x,v_y,v_z]$, with $p$ and $q$ finite (last coordinate is $1$), but $v_z$ maybe either $1$ - finite or $0$ - at infinity: ...