11
votes
Why are width and height divided by 2 in the perspective projection matrix?
Because $x_{proj}$ doesn't vary from $0 \to width$, it varies from $-\tfrac{width}{2} \to \tfrac{width}{2}$.
What's important is not the width, but the minimum and maximum values of $x_{proj}$. ...
- 6,880
9
votes
Why does opengl use 4d matrices for everything?
So,
A lot of folks would tell you that a 4x4 matrix is used so you can get a translation component rolled into your linear transformation. Meaning that with a 3x3 matrix you can only compute rotations,...
- 3,484
8
votes
Accepted
How to translate object to origin?
No, he's talking about subtracting the position of the cube from the vertex positions, so that your cube is positioned at the origin.
If you positioned the cube at (10, 30, 15), you subtract that ...
- 7,811
7
votes
Accepted
What does it *mean* to scale in an arbitrary direction?
When you scale along the X-axis, the X-coordinate (parallel to the axis) gets stretched, while the Y-coordinate (perpendicular to the axis) remains the same. You can think of scaling along an ...
- 2,402
7
votes
Accepted
Why do we multiply vertex from left side in vertex shader with matrices?
There is no 100% consensus on what order matrix multiplication should model things, worse the industry is split along this. Some sources use row major and some sources use column major matrices. Great ...
- 8,457
7
votes
Accepted
Render with camera perspective off-center
Yes, you can use an off-axis projection matrix.
This is what I use in my code (note: I shift the centre upwards, not left as you do in your example.)
...
- 280
6
votes
Solving a problem from *Foundations of Computer Graphics*:
Let me try to make my comment into a complete answer.
The general idea is to build a linear system using those 6 point pairs and solve for the desired 12 unknowns. You may find this paper[1] useful ...
- 655
5
votes
How is that possible that matrices can be thought as coordinate systems?
If you have a 3x3 matrix representing some transformation, you will actually have the X,Y,Z vectors of that transformation in the rows or columns (depending on if it's a row major or column major ...
- 7,811
5
votes
Why do we multiply vertex from left side in vertex shader with matrices?
OpenGL uses column-major matrices. For example, the translation values will be in the last row rather than the last column of the matrix. For example when loading matrices into uniforms in glsl, the <...
- 3,441
5
votes
why is translating in 3D space the same as shearing in 4D space?
In a linear transformation system, your origin is always a fixed point, since 0*anything = 0. So imagine you have a cinema screen, and the origin is at the centre of the screen. Using linear ...
- 2,402
5
votes
Accepted
Screen space coordinates to Eye space conversion
Object Space → World Space → Eye Space → Clip Space → Normalized Device Space → Window Space
You get from Object Space to World Space by multiplying by the "World
Matrix" (This may actually be ...
- 410
5
votes
Why does opengl use 4d matrices for everything?
The bottom row allows you to create perspective foreshortening. That is, it makes lines that are getting further away appear to converge. When arranged this way, we call this a perspective projection ...
- 3,441
4
votes
What's a parent vector space or coordinate system?
You have to step outside the world of mathematics for a while and look at what we are trying to achieve. Mathematics in its purest form only tells us what kinds of properties certain constructs have, ...
- 8,457
4
votes
Accepted
How to derive a perspective projection matrix from its components?
The second matrix translates the eye [...]
You don't do that in a projection matrix. You do that with your view matrix:
Model (/Object) Matrix transforms an object into World Space
View Matrix ...
- 1,613
4
votes
Accepted
Inverse matrix order of operation
Yes. If you're compounding operations to make a matrix, then the inverse matrix will be the compound of the inverse operations, in the reverse order. So if $C = AB$ then $C^{-1} = B^{-1}A^{-1}$
Think ...
- 2,402
4
votes
Accepted
Is it possible to make a projection matrix to not project in the center?
It's not possible to cut a hole in the image by altering the projection matrix, no. However, you can mask out rendering in that region by using the depth test or stencil test.
For example, before ...
- 25.3k
4
votes
Applications of the outer product in shaders
You can construct the orthogonal projection matrix onto $u$ as $P= uu^T$ if $u$ is unit length, otherwise it is $P_{u,u^{\perp}}=u(u^Tu)^{-1}u^T = \frac{uu^T}{u^Tu}$.
If you have $P_{u,v^{\perp}}=u(v^...
- 2,561
3
votes
Accepted
Unable to pass custom Matrix4 to GLSL as a uniform
glUniformMatrix4fv(transformLocation, sizeof(transform), GL_FALSE, &transform.m[0][0]);
The second parameter to ...
- 9,997
3
votes
row and column majored rotation matrix pre- or post- multiplied
Okay let us start by pointing out that a colmun major matrix is the same as a transposed row major matrix. So:
$$
{M_{cm}}^{T} = M_{rm} \tag{1}
$$
Then notice that matrixes have following properties....
- 8,457
3
votes
Ladder to DNA using Transformation Matrix
You can't perform this transformation by applying constant transformation matrix to the ladder model since it's not linear transformation like joojaa said in the comments. What you would have to do ...
- 3,686
3
votes
How is that possible that matrices can be thought as coordinate systems?
A matrix can be used to transform a coordinate system into a new one. More specifically, it can be used to transform the basis vectors of a coordinate system. That's how it defines a new coordinate ...
- 1,585
3
votes
Accepted
Detect a lossy matrix decomposition?
You can detect a matrix that can't be decomposed in TRS form by taking its 3×3 upper-left submatrix, interpreting its columns as vectors, and dotting them together in all combinations (1 with 2, 2 ...
- 25.3k
3
votes
Accepted
Do I need a normal matrix if I calculate lighting in an objects space?
By lighting in object space, sure, you could avoid transforming the normals into world space. However, you'd have to transform the light positions/vectors into the object's space. Also, if you want ...
- 25.3k
3
votes
Accepted
Move the camera in orthographic 3D
It seems that some basic computer graphics knowledge is missing.
Normally you have two matrices for the camera (viewMatrix and projectionMatrix), both are 4x4 matrices. The viewMatrix stores the ...
- 1,366
3
votes
Accepted
New QEM, question about degenerate case
When they solve $Av=-b$ the assumption is that $A$ is non-singular. Whenever $A$ is singular the problem can either have no solution, or infinitely many solutions. In their case I believe it is the ...
- 2,561
2
votes
How to translate object to origin?
Normally woth rotating objects in 3d you have a pivot. With rotation matrices it is assumed that the pivot is at (0;0;0), the origin. This means that if the pivot is not at (0;0;0), you have to move ...
- 1,615
2
votes
Correctness of logical steps for create the matrix of viewing transformation
I would like to address an issue in your naming. This issue makes the question a bit odd, but not impossible, as I will try to show.
There is an, often unspoken, implied relationship between the ...
- 8,457
2
votes
Correctness of logical steps for create the matrix of viewing transformation
I'm assuming you mean $A$ is object$\rightarrow$world matrix and $B$ is your camera$\rightarrow$world matrix? If that's the case, to calculate object object$\rightarrow$camera matrix you calculate:
$$...
- 3,686
2
votes
Accepted
Rotation matrix for a 3D object in space
Skimming the math. Starting with a quaternion $Q=w+\left(x,y,z\right)$ then we can rotate $\mathbf{v}$ by:
$$\mathbf{v}' = Q\mathbf{v}Q^{-1}$$
and if $Q$ is unit magnitude this reduces to:
$$\...
- 154
2
votes
What's a parent vector space or coordinate system?
First start to be sure you understand what matrices are (4x4 which are the most common in CG).
1) about your first question. You are taking about a hierarchy of transforms. I don't know if you use ...
- 656
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