# Affine geometry

Affine geometry, a fundamental branch of mathematics, explores the intriguing concepts of points, lines, and planes while preserving the properties of parallelism. This pivotal area bridges algebra and geometry, allowing for the profound investigation of transformations and their impact on geometric figures. Grasping the basics of affine geometry is essential for advancing in mathematical understanding and unlocking the mysteries of space and shape relationships.

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## What is Affine Geometry?

Affine geometry is an essential branch of mathematics that explores the relationships and properties that remain constant when geometric figures are transformed through scaling, rotation, and translation. It focuses on the concepts of parallelism and the division of figures into proportional segments without considering distances or angles. This field of study allows for a deeper understanding of the geometric principles that are invariant under such transformations, making it a fundamental area of study in mathematics and its applications.

### Understanding the Basics of Affine Geometry

At its core, affine geometry investigates how objects are related to each other through transformations that preserve points, straight lines, and planes. Essentially, affine transformations include any manipulation of a geometric figure that does not alter the collinearity of points, meaning that points lying on a straight line before transformation will still lie on a straight line after transformation.

There are several key operations in affine geometry, including:

• Translations, where every point of a figure moves in the same direction and by the same distance.
• Scaling, which changes the size of a figure but not its shape.
• Rotation around a point, without changing the figure's size or shape.
• Shear transformation, which shifts a figure in a direction parallel to one of its sides, effectively changing its shape but not its area.

These transformations highlight the flexibility and broad applicability of affine geometry in solving real-world problems and understanding spatial relationships.

Example: Consider a piece of paper on which a triangle is drawn. If the paper is translated (moved in any direction without rotation), scaled (enlarged or reduced), or sheared (the top is moved sideways while the bottom remains fixed), the basic shape of the triangle remains the same. These changes can be described and predicted using affine geometry, illustrating how this branch of mathematics can be applied to understand and model transformations.

### History and Development of Affine Geometry

The origins of affine geometry can be traced back to the work of ancient mathematicians, but it was formally developed and named in the 17th century by Leonhard Euler and August Ferdinand Möbius. Euler's introduction of the concept of a function and his work on the geometry of location provided foundational ideas for affine geometry. Möbius, on the other hand, is renowned for his development of the Möbius strip, which plays a significant role in the advancement of topology, an area closely related to affine geometry.

The development of affine geometry paralleled the evolution of projective geometry, as both fields stem from the study of perspective and visual representation. Over the centuries, affine geometry has expanded, incorporating concepts such as affine spaces, affine maps, and affine transformations, further enriching the mathematical toolkit for analyzing and understanding the geometric world.

Central to the history of affine geometry is the concept of affine space, which generalizes the notions of Euclidean spaces while discarding the metric structure (that is, notions of distance and angle). This abstraction allows for the study of geometry in a broader sense, making it possible to explore properties of figures that are independent of the classical constraints of size and shape. Affine spaces provide a framework for understanding how geometric transformations can be applied, offering a more flexible approach to geometry that has applications in areas ranging from computer graphics to theoretical physics.

## Fundamental Theorem of Affine Geometry

The Fundamental Theorem of Affine Geometry is a cornerstone concept that underpins the structure and applications of affine geometry. It offers a framework for understanding how transformations within an affine space maintain relationships between geometric entities. This theorem not only deepens the comprehension of affine transformations but also links affine geometry with other mathematical areas, providing a robust foundation for various applications in science and engineering.

### Key Principles of the Fundamental Theorem

At the heart of the Fundamental Theorem of Affine Geometry are several key principles that denote the invariance under affine transformations. These principles focus on the preservation of points, straight lines, and planes, along with the crucial concept of parallelism and ratios of segments on parallel lines. The theorem asserts that an affine transformation preserves:

• Collinearity of points: Points that lie on a straight line remain on a straight line after transformation.
• Parallelism: If two lines are parallel before the transformation, they remain parallel afterward.
• Ratios of segments: The division of segments on parallel lines in a particular ratio remains unchanged.

Mathematically, if $$T$$ is an affine transformation, then for any points $$A$$, $$B$$, and $$C$$ lying on a straight line and any scalar $$k$$, the ratio $$\frac{\overline{AB}}{\overline{BC}}$$ is preserved under $$T$$.

Example: Consider two points $$A$$ and $$B$$ on a plane, and their images $$A'$$ and $$B'$$ under an affine transformation. If the original points $$A$$ and $$B$$ are connected by a line, the points $$A'$$ and $$B'$$ will also be connected by a line, demonstrating the principle of collinearity preservation. Furthermore, if there is a third point $$C$$ such that $$C$$ divides the segment $$AB$$ in a ratio of 2:1, then its image $$C'$$ under the same affine transformation will divide the segment $$A'B'$$ in the same ratio, illustrating the conservation of segment ratios.

### Implications of the Fundamental Theorem in Mathematical Concepts

The implications of the Fundamental Theorem of Affine Geometry extend far beyond the immediate realm of affine transformations. This theorem serves as a bridge connecting affine geometry with other significant mathematical concepts, playing a crucial role in the development of linear algebra, projective geometry, and computer graphics. Specifically, it underlines the importance of affine spaces in understanding the geometric aspects of vector spaces.

Moreover, the theorem’s emphasis on invariance principles aids in the study of geometric properties that are not dependent on the traditional Euclidean measurements of distance and angle. This has opened up new methodologies for approaching problems in computer vision, where affine invariance is crucial for recognizing objects under different conditions of scale, rotation, and perspective.

In the context of linear algebra, the Fundamental Theorem of Affine Geometry underscores the concept of linear maps and their extensions to affine maps. An affine space, in this sense, can be viewed as a vector space that has 'forgotten' its origin, allowing for more general types of transformations. This abstraction facilitates the creation of models in computer graphics where objects must be manipulated in ways that are not restricted by fixed origins or scales. As such, the principles enshrined in the theorem help define the operations and transformations that are possible within virtual environments, highlighting the theorem's vast implications for theoretical and applied mathematics.

Remember, the key difference between linear transformations and affine transformations is that affine transformations include translations, while linear transformations do not. This distinction is critical for understanding the applications of the Fundamental Theorem of Affine Geometry in various mathematical fields.

## Linear Transformations in Affine Geometry

Affine geometry, a branch of mathematics focusing on the properties and relationships between figures that remain invariant under certain transformations, is deeply intertwined with the concept of linear transformations. These transformations play a pivotal role in understanding how geometric figures are manipulated within affine spaces.

### Defining Linear Transformations

Linear Transformation: A linear transformation is a function between two vector spaces that preserves the operations of vector addition and scalar multiplication. Mathematically, a transformation $$T$$ from vector space $$V$$ to vector space $$W$$ is linear if for any vectors $$v_1, v_2 \in V$$ and any scalar $$c$$, the following conditions hold:

• $T(v_1 + v_2) = T(v_1) + T(v_2)$
• $T(cv_1) = cT(v_1)$

Example: Consider $$T : \mathbb{R}^2 \rightarrow \mathbb{R}^2$$ given by $$T(x, y) = (3x, 2y)$$. To check if $$T$$ is a linear transformation, let's verify the two conditions for linearity with $$v_1 = (x_1, y_1)$$ and $$v_2 = (x_2, y_2)$$:

• $T((x_1, y_1) + (x_2, y_2)) = T(x_1 + x_2, y_1 + y_2) = (3(x_1 + x_2), 2(y_1 + y_2)) = (3x_1, 2y_1) + (3x_2, 2y_2) = T(x_1, y_1) + T(x_2, y_2)$
• $T(c(x_1, y_1)) = T(cx_1, cy_1) = (3cx_1, 2cy_1) = c(3x_1, 2y_1) = cT(x_1, y_1)$

This example validates that $$T$$ is indeed a linear transformation considering it satisfies both conditions of linearity.

### Role of Linear Transformations in Affine Geometry

In the realm of affine geometry, linear transformations are crucial for understanding how geometric operations such as rotation, scaling, and translation affect figures within an affine space. These transformations facilitate the analysis and visualisation of geometric patterns and relationships in a more abstract and generalised manner, moving beyond the confines of Euclidean geometry.

Linear transformations are particularly significant in affine geometry because they allow for the manipulation of geometric objects while preserving essential properties like collinearity and parallelism. This aspect is crucial for applications ranging from computer graphics to theoretical physics, where maintaining the fundamental characteristics of geometric figures under transformation is vital.

Among the many roles of linear transformations in affine geometry, one of the most profound is their application in the composition of transformations. A sequence of linear transformations can be combined into a single transformation, simplifying complex operations into more manageable forms. This property is utilised in computer graphics, where multiple transformations applied to an object can be computed efficiently as one composite transformation, drastically reducing computational overhead and simplifying the mathematical modelling of visual effects.

While linear transformations are foundational in affine geometry, it's important to distinguish that all affine transformations are not strictly linear; they may also include translations, which do not originate from the origin of the vector space.

## Affine Geometry Examples and Applications

Affine geometry, a pivotal branch of mathematics, concerns itself with the study of properties that remain unchanged through affine transformations such as rotation, translation, and scaling. It finds applications across various fields, notably in computer graphics, physics, and robotics, offering insights into spatial relationships and transformations. Through practical examples, comparisons with related geometries, and exploration of its algebraic forms, one can gain a comprehensive understanding of its significance and utility.

### Practical Affine Geometry Example

One of the most relatable instances of affine geometry in action is in the manipulation of digital images. When an image is rotated, resized, or skewed on a computer, affine geometry principles are applied to calculate the new positions of each pixel. These transformations, despite altering the image's appearance, preserve the collinearity of points and parallelism of lines, fundamental properties of affine geometry.

Example: If you apply a skew transformation to an image of a square to make it a parallelogram, despite the change in shape, the opposite sides of the parallelogram remain parallel, and the corner points, which were collinear along the sides of the square, remain so along the sides of the parallelogram. Mathematically, if the original coordinates of the square's corners are $$A$$, $$B$$, $$C$$, and $$D$$, after an affine transformation $$T$$, their new positions $$A'$$, $$B'$$, $$C'$$, and $$D'$$ can be calculated using $$T$$, showcasing how affine geometry governs their spatial relationships.

### Affine and Projective Geometry: A Comparison

Affine and projective geometries are closely related yet distinct in their focus and applications. While both study properties invariant under certain transformations, their foundational transformations differ. Affine geometry concentrates on transformations preserving parallelism, whereas projective geometry extends these concepts to include points at infinity, effectively treating all lines as if they eventually meet.

Exploring the differences between affine and projective geometries highlights the unique perspective each brings to the understanding of space and transformation. Projective geometry, by considering points at infinity, allows for a unified treatment of cases that affine geometry handles separately. This is particularly evident in the analysis of vanishing points in art, where parallel lines in the three-dimensional world converge to a single point in a two-dimensional drawing. Meanwhile, affine geometry provides the tools necessary for many practical applications in computer vision and robotics, where the maintenance of parallelism and the manipulation of objects without the concern for distance is crucial.

Transformations in affine geometry, such as translations and rotations, are easily observed in everyday life, from architectural designs to the way objects move and interact. Projective transformations, on the other hand, are a bit more abstract, often requiring a conceptual leap to points at infinity.

### Exploring Affine Algebraic Geometry

Affine algebraic geometry, an extension of affine geometry, delves into the study of algebraic varieties and sets defined by polynomial equations within an affine space. This field combines the geometric insights of affine transformations with the algebraic structure of polynomials, shedding light on complex geometric structures and their properties.

Example: Consider the set of all points $$x, y$$ in the plane that satisfy the equation $$y = x^2$$. This set forms a parabola, which is an example of an affine algebraic curve. Analyzing this curve through the lens of affine algebraic geometry involves studying its properties under various affine transformations, such as translations and rotations, and understanding how these operations affect the polynomial equation representing the curve.

Affine algebraic geometry's strength lies in its ability to classify and understand complex geometric shapes through their algebraic representations. This field provides powerful tools for solving problems in number theory, topology, and mathematical physics, demonstrating the profound interconnectedness of geometry and algebra. By examining the algebraic properties of geometric figures, affine algebraic geometry opens the door to a deeper understanding of the structure and behaviour of spaces beyond the intuitive notions of shape and distance.

## Affine geometry - Key takeaways

• Affine geometry is a branch of mathematics focusing on properties constant under transformations like scaling, rotation, and translation, and does not consider distances or angles.
• The Fundamental Theorem of Affine Geometry asserts the preservation of collinearity, parallelism, and segment ratios in affine transformations.
• Linear transformations are functions that preserve vector addition and scalar multiplication and are integral to affine geometry.
• Affine transformation examples include manipulation of digital images, preserving collinearity and parallelism while changing appearance.
• Affine algebraic geometry combines geometric insights with algebra, studying properties of shapes defined by polynomial equations.
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What are the basic principles of affine geometry?
Affine geometry studies the properties of geometric figures that are invariant under affine transformations, such as parallelism, midpoints, and ratios of distances on parallel lines. It disregards properties related to angles, distances, and circles, focusing on linear transformations and affine spaces.
What is the difference between affine geometry and Euclidean geometry?
Affine geometry studies the properties of geometric figures that remain invariant under affine transformations, such as parallelism and division ratios, without considering angles and distances. Euclidean geometry, on the other hand, deals with shapes and spaces based on distance and angle measurements, including the notions of perpendicularity and circle properties.
How do transformations work in affine geometry?
In affine geometry, transformations work through operations that preserve points, straight lines, and planes. These transformations include translations, rotations, scalings, and shears, enabling objects to be moved, rotated, or resized, while maintaining parallelism between lines.
How do affine mappings or transformations affect coordinates and shapes?
Affine mappings or transformations alter coordinates and shapes by applying scaling, rotation, translation, and shearing, maintaining points, straight lines, and planes. Colinear points remain colinear, and ratios of distances along parallel lines stay constant. Consequently, while shapes may change in size, skew, or position, their fundamental geometric properties are preserved.
What are the applications of affine geometry in computer graphics and image processing?
In computer graphics and image processing, affine geometry is utilised for modelling and transforming two-dimensional and three-dimensional objects, enabling operations such as rotation, scaling, translation, and shearing. It's key for simulating realistic movements and perspectives, morphing images, and conducting geometric corrections in photographs and video frames.

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