Points and Vectors
A point represents a position, while a vector represents a direction, or a change in position. Typically, both are written vertically. $$ x = (10, 5) $$ $$ \vec{v} = (2, 1) $$ Both, of course, have component-based addition and subtraction. Vectors also have some special properties.
Matrix Notation
A matrix is an $n \times m$ grid of values. $$ \begin{bmatrix} 1 & 3 \ 4 & 12 \ 1 & 5 \end{bmatrix} $$ It has the following functions:
- Transpose: the matrix flipped across its diagonal
- Determinant: the area/volume of the shape(2D parallelogram or 3D parallelopiped) made by the vectors
- Matrix Multiplication: new matrix where $(i, j)$th value is the $i$th row of the first matrix dot the $j$th column of the second matrix. Imagine moving the first matrix down.
Dot Product
A dot product can be thought of as the length of the projection of one vector onto the other vector. This means that dot product is a really nice way of telling the similarity of two vectors. $$ \vec{v_1} \cdot \vec{v_2} = |\vec{v_1}||\vec{v_2}|cos(\theta) = v_1(x)v_2(x) + v_1(y)v_2(y) $$ You’ll find that the dot product of two vectors facing directly away from each other is $-1$, two that are perpendicular to each other is $0$, and two that are parallel is $1$.
Cross Product
The cross product is an operation defined on only 3 dimensional vectors. You’ll see it used on 2D vectors, but in this case, the third dimension is assumed to be $0$. $$ \vec{v_1} \cdot \vec{v_2} = |\vec{v_1}||\vec{v_2}|sin(\theta) = \begin{bmatrix} y_1z_2 - y_2z_1 \ z_1x_2 - z_2x_1 \ x_1y_2 - x_2y_1 \end{bmatrix} $$ The cross product creates a vector that is orthogonal to both input vectors. The length of this vector is the area of the parallelogram created by the two input vectors. When vectors are parallel or antiparallel(facing away from each other), the cross product is 0.
Transformation Matrices
In graphics, the three general transformations, translation, rotation, and scale(dilation), are represented through transformation matrices.
Scaling
Given a vector $( x, y, z )$, if you wanted to scale it by $( s_x, s_y, s_z )$, then you would multiply it by the following matrix: $$ \begin{bmatrix} s_x & 0 \ 0 & s_y \end{bmatrix} \begin{bmatrix} x \ y \end{bmatrix} = \begin{bmatrix} s_x \times x \ s_y \times y \end{bmatrix} $$ Note that the initial positions of vertices matter, as these transformations scale from $(0, 0)$. So, a point at $(5, 0)$ would never change position if scaled along the y component, even by a million. A million times zero is still zero.
Rotation in 2D
Rotation of a 2D vector by $\theta$ counter clockwise looks like such: $$ \begin{bmatrix} cos(\theta) & -sin(\theta) \ sin(\theta) & cos(\theta) \end{bmatrix} $$ This is relatively straightforward, but if you’d like you can test it on $0\degree$, $90\degree$, and other common rotation values to see how it affects vectors.
Translation
If you think about it, you could just add a vector to a type to translate it from its current point. However, we actually don’t do this in graphics for two very good reasons:
- GPUs are optimized to do floating point matrix multiplication over anything else
- When combining transformation matrices by multiplying them, it is impossible to do so if all the types are not matrices. If you throw a vector in there, you cannot consolidate transformations in order. You would have to save every transformation separately
Therefore, we must use a matrix for translation. However, it turns out that this doesn’t work. It is impossible to represent a translation as a 2D transformation matrix. We have to use another tool.
Homogeneous Coordinates
To do this, we add an extra coordinate to every point: $$ \begin{bmatrix} x \ y \ w \end{bmatrix} \to \begin{bmatrix} x/w \ y/w \end{bmatrix} $$ A regular coordinate of $x, y$ would look like this: $$ \begin{bmatrix} x \ y \end{bmatrix} \to \begin{bmatrix} x \ y \ 1 \end{bmatrix} $$ Now, we can represent a translation of $t_x, t_y$ like this: $$ \begin{bmatrix} 1 & 0 & t_x \ 0 & 1 & t_y \ 0 & 0 & 1 \ \end{bmatrix} \begin{bmatrix} x \ y \ 1 \end{bmatrix} = \begin{bmatrix} t_x + x \ t_y + y \ 1 \end{bmatrix} $$ If you try making the homogeneous coordinate of the vector 0, you’ll find that the transformation actually doesn’t apply. This sort of acts as an indicator for whether a type is a point or vector, because vectors should not be translated(as that would be a translation of a plane). Therefore, a vector should have a homogeneous value of 0, while a point should have a homogeneous value of 1.
3D Vectors and Matrices
3D vectors and matrices generally follow from 2D vectors and matrices. Homogeneous vectors now have 4 coordinates. Scaling is trivial. Rotation has two new axes, however, represented as a rotation by $\alpha, \beta, \gamma$. There is a very complex formula for it which can be found here.