Question

Write the homogeneous coordinates of the point at infinity on the line \(2 x-y=0\)

Short answer

Based on the given line equation 2x - y = 0, find the homogeneous coordinates of the point at infinity on this line. Solution: Step 1: The direction vector of the line is (1, 2). Step 2: The line's coefficients are (2, -1, 0). Step 3: The cross product of the direction vector and the line's coefficients results in the homogeneous coordinates of the point at infinity as (0, -1, -5).

Step-by-step solution

Find the direction vector of the line

Set y = 0 in the equation, and we get 2x = 0, which means x = 0. That means the direction vector is vertical, and we can represent it as (1,2).

Find the line's coefficients

The coefficients of the given line (2x - y = 0) are 2, -1, and 0.

Find the cross product of the direction vector and the line's coefficients

To find the cross product of the direction vector (1, 2) and the line's coefficients (2, -1, 0), we use the following formula for 3D cross product: A × B = (A2 * B3 - A3 * B2, A3 * B1 - A1 * B3, A1 * B2 - A2 * B1) In our case, the homogeneous coordinates of the point at infinity are given by: - (1,2,0) × (2,-1,0) - =(2 * 0 - (-1) * 0, (-1) * 1 - 2 * 0, 1 * (-1) - 2 * 2) - =(0, -1, -5) So, the homogeneous coordinates of the point at infinity on the line 2x - y = 0 are (0,-1,-5).

Key concepts

Homogeneous Coordinates

Homogeneous coordinates are a system used in mathematics and computer graphics to handle points at infinity and to represent projections in a more unified way compared to Cartesian coordinates. In two dimensions, a point is depicted by three coordinates \( (X, Y, W) \) instead of two \( (x, y) \) as in Cartesian coordinates. To find the Cartesian coordinates from homogeneous ones, we divide the first two components by the third: \( x = \frac{X}{W} \), \( y = \frac{Y}{W} \). If \( W = 0 \), we've encountered a 'point at infinity.' This concept is immensely useful in projective geometry, where it simplifies equations and makes the mathematics of perspective projection more elegant.

Using our exercise, the homogeneous coordinates of the point at infinity on a line provide a concrete example of this application. It allows us to deal with entities that do not intersect within the confines of our conventional plane, such as parallel lines and vanishing points in perspective drawing.

Point at Infinity

The 'point at infinity' in projective geometry is a way to handle the notion of direction without concern for distance. In essence, when we talk about the direction of a line or a moving object, the actual position does not matter as much as the path it's following. Mathematically, this abstraction is a point that exists 'at infinity,' meaning it is infinitely far away in a given direction. All the lines that are parallel in the Cartesian plane will meet at this point in projective space, which is why this notion is powerful for illustrating concepts such as vanishing points in art.

In the context of the provided exercise, identifying the point at infinity on the line involves determining a direction vector for the line and then using it in conjunction with the line's coefficients. The result is a set of homogeneous coordinates that represent where the line 'meets' infinity in a particular direction.

Cross Product

The cross product is a mathematical operation applied to two vectors in three-dimensional space. It produces a third vector that is perpendicular to the plane containing the initial vectors. The formula \( A \times B = (A2 \cdot B3 - A3 \cdot B2, A3 \cdot B1 - A1 \cdot B3, A1 \cdot B2 - A2 \cdot B1) \) encapsulates this operation. The cross product is distinguished by its use in calculating the area of a parallelogram formed by the vectors, finding torques, and determining the direction for the right-hand rule.

During the solution of our exercise, the cross product was utilized to combine the direction vector and the line's coefficients, creating a vector representative of the point at infinity for the line in homogeneous coordinates. This exemplifies the aspect of the cross product in deriving meaningful geometric results from algebraic operations.

Direction Vector

A direction vector is substantial within the context of geometry and physics because it conveys information about both the direction and sense of a line or path, without specifically pinpointing a location along it. To represent a line in space, one only needs a point on the line and a direction vector. Geometrically, the vector \( (x, y) \) indicates that for each small step we take in the x-direction, we should take a corresponding step in the y-direction to stay on the line. It is orthogonal to any normal vector to the line.

Relating to our exercise, the direction vector was found to be (1,2), which indicates the slope and orientation of the line. Finding the direction vector is critical as it helps us understand the infinite trajectory of the line involved, which is central to identifying the homogeneous coordinates of the point at infinity.