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Questions and Answers
What is the dual of the fundamental theorem?
What is the dual of the fundamental theorem?
- A projectivity between the sets of lines on two points in a plane is determined by three concurrent lines and their images. (correct)
- A projectivity between two sets of points on a line is determined by three collinear points and their images.
- A projectivity between two sets of points on a line is determined by two concurrent lines and their images.
- A projectivity between two sets of points on a plane is determined by four collinear points and their images.
What is the center of projectivity?
What is the center of projectivity?
- A line that passes through the intersection of corresponding points.
- A line that is perpendicular to the intersection of corresponding points.
- A point that lies on the joins of the cross intersections of corresponding lines. (correct)
- A point that lies on the intersections of corresponding lines.
What is the purpose of the center of projectivity?
What is the purpose of the center of projectivity?
- To find the axis of projectivity.
- To construct additional pairs of corresponding lines. (correct)
- To find the intersection of corresponding lines.
- To construct additional pairs of corresponding points.
How can the axis of projectivity be determined?
How can the axis of projectivity be determined?
What is the relationship between X and X'?
What is the relationship between X and X'?
What is the purpose of Theorem 4.2.6?
What is the purpose of Theorem 4.2.6?
What is the role of duality in the theorems presented in this section?
What is the role of duality in the theorems presented in this section?
What is the requirement for the lines in Theorem 4.2.5?
What is the requirement for the lines in Theorem 4.2.5?
What is the significance of A'A being a self-corresponding line in a projectivity?
What is the significance of A'A being a self-corresponding line in a projectivity?
What is the common point that the axis of projectivity passes through in Figure 4.4a?
What is the common point that the axis of projectivity passes through in Figure 4.4a?
What is the purpose of the axis of projectivity in a projective transformation?
What is the purpose of the axis of projectivity in a projective transformation?
What is the significance of the cross joins in Figure 4.4a?
What is the significance of the cross joins in Figure 4.4a?
What is the condition for two pairs of points, such as B, B' and D, D', to have the same axis of projectivity?
What is the condition for two pairs of points, such as B, B' and D, D', to have the same axis of projectivity?
What is the consequence of taking another pair of points, such as B, B', as the centers of the projective pencils?
What is the consequence of taking another pair of points, such as B, B', as the centers of the projective pencils?
What can be constructed using the axis of projectivity in a projective transformation?
What can be constructed using the axis of projectivity in a projective transformation?
What is the purpose of the example in Figure 4.4b?
What is the purpose of the example in Figure 4.4b?
What is the primary reason for using homogeneous coordinates in computer graphics?
What is the primary reason for using homogeneous coordinates in computer graphics?
A point with coordinates (75,000, 60,000) can be represented in homogeneous coordinates as:
A point with coordinates (75,000, 60,000) can be represented in homogeneous coordinates as:
What is a characteristic of a 3x3 matrix representing a transformation in homogeneous coordinates?
What is a characteristic of a 3x3 matrix representing a transformation in homogeneous coordinates?
What is the benefit of using homogeneous coordinates when dealing with points with very large coordinates?
What is the benefit of using homogeneous coordinates when dealing with points with very large coordinates?
What can be represented using a 3x3 matrix with homogeneous coordinates?
What can be represented using a 3x3 matrix with homogeneous coordinates?
What is the consequence of using homogeneous coordinates when dealing with points with very large coordinates?
What is the consequence of using homogeneous coordinates when dealing with points with very large coordinates?
What is a characteristic of points represented in homogeneous coordinates?
What is a characteristic of points represented in homogeneous coordinates?
What is the relationship between the point (5, 2, 3) and the point (10, 4, 6) in homogeneous coordinates?
What is the relationship between the point (5, 2, 3) and the point (10, 4, 6) in homogeneous coordinates?
What is the result of the matrix transformation in the example that causes lines parallel to the z-axis to meet at the vanishing point?
What is the result of the matrix transformation in the example that causes lines parallel to the z-axis to meet at the vanishing point?
What is the characteristic of the determinant of the matrix for an axonometric projection?
What is the characteristic of the determinant of the matrix for an axonometric projection?
What is the result of the matrix transformation in the example that projects onto the plane z = 2?
What is the result of the matrix transformation in the example that projects onto the plane z = 2?
What type of transformation is represented by the matrix for a 90° rotation about the x-axis?
What type of transformation is represented by the matrix for a 90° rotation about the x-axis?
What is the purpose of axonometric projections in engineering?
What is the purpose of axonometric projections in engineering?
What is the result of the product of two matrices in an axonometric projection?
What is the result of the product of two matrices in an axonometric projection?
What is the viewing plane in an axonometric projection?
What is the viewing plane in an axonometric projection?
What is the characteristic of the matrix for an axonometric projection?
What is the characteristic of the matrix for an axonometric projection?
What is the necessary condition for a projectivity between two lines in a plane?
What is the necessary condition for a projectivity between two lines in a plane?
What does 'uniquely determined' mean in the context of the fundamental theorem of projective geometry?
What does 'uniquely determined' mean in the context of the fundamental theorem of projective geometry?
Why does the assumption that X' and X'' are distinct contradict the axiom in the proof of the fundamental theorem?
Why does the assumption that X' and X'' are distinct contradict the axiom in the proof of the fundamental theorem?
What is the purpose of the fundamental theorem of projective geometry from a theoretical point of view?
What is the purpose of the fundamental theorem of projective geometry from a theoretical point of view?
What is the minimum number of perspectivities required to project three distinct points on one line into any three distinct points on a second line?
What is the minimum number of perspectivities required to project three distinct points on one line into any three distinct points on a second line?
What is the role of the center of perspectivity S in the proof of Theorem 4.2.3?
What is the role of the center of perspectivity S in the proof of Theorem 4.2.3?
What is the relationship between the points A, B, C, A', B', and C' in Theorem 4.2.3?
What is the relationship between the points A, B, C, A', B', and C' in Theorem 4.2.3?
What is the consequence of the fundamental theorem of projective geometry in terms of determining additional pairs of corresponding points?
What is the consequence of the fundamental theorem of projective geometry in terms of determining additional pairs of corresponding points?
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Study Notes
Homogeneous Coordinates
- Can represent translations as matrices, which is not possible with Cartesian coordinates
- Each 2x2 matrix from Chapter 2 corresponds to a 3x3 matrix with the same effect for homogeneous coordinates, with a 1 in the lower right-hand corner
- Useful for manipulating points with very large coordinates, as microcomputers cannot store integers larger than 32,767
- Enables representation of points at infinity and transformation of 3D drawings into perspective drawings to create an illusion of depth
Fundamental Theorem of Projective Geometry
- A projectivity between the sets of points on two lines in a plane is uniquely determined by three collinear points and their images
- Proof involves assuming two different transformations mapping the three points to their images, and showing that this leads to a contradiction
Theorem 4.2.3
- Three distinct points A, B, and C on one line can be projected into any three distinct points A', B', and C' on a second line by means of a sequence of at most two perspectivities
- Proof involves choosing a center of perspectivity and constructing a sequence of perspectivities to map the points
Theorem 4.2.5 (Dual of the Fundamental Theorem)
- A projectivity between the sets of lines on two points in a plane is determined by three concurrent lines and their images
Theorem 4.2.6 (Dual of Theorem 4.2.4)
- A projectivity between two sets of lines on two distinct points determines a third point, called the center of projectivity or center of homology, that lies on the joins of the cross intersections of corresponding lines
Matrices in Three-Dimensional Projective Geometry
- Can be used to transform a box into a figure with pairs of edges meeting at one or more vanishing points
- Example: the matrix causes lines parallel to the z-axis to meet at the vanishing point (0, 0, 1/3, 1)
- Axonometric projections are used in engineering to produce two-dimensional views of a three-dimensional object
- Matrices for axonometric projections can be considered the product of two other matrices: a transformation used to rotate or translate, and a projection of the entire figure onto a particular plane, called a viewing plane
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