# Tag Info

18

Assuming a perspective projection and a view point external to the sphere, then the 'boundary' formed by the view point and the circle on the sphere which forms the horizon WRT the view point, will be a cone. Doing a perspective projection (onto a plane) is then equivalent to intersecting this cone with the plane which thus produces a conic section. FYI ...

14

This is more like a long comment to @SimonF's answer that I'm trying to make somewhat self contained. All cuts of cone are possible, hyperbola, parabola and ovals. This is easy to test by drawing images in a 3D engine by a extremely wide angle camera. Rotate the camera to say in 30 degree angle so the object is not in the middle of your focus. Then ...

13

Projection systems are used to convert a 3D shape to a planar (2D) shape. According to the type of projection system, different results and shapes like rectangles, pies, ellipses, circles, ... can be produced out of a sphere. Projection systems can be classified by the characteristics of the result they generate. To continue, I would like to use a very ...

11

SimonF's reasoning basically convinced me, but I decided to do a sanity check. I loaded up a UE4 level that happens to have some spheres, like this one: I set the camera FOV up to 160 degrees to give lots of perspective distortion, and positioned it so the sphere was near the corner of the image: Then I took this into Inkscape and used the ellipse tool to ...

8

Identifying your axes in both figures and adding the camera position to your first figure would help you understand what's going on. You could also have a single variables for all your points, generating a 2D matrix with the rows as each point and columns as the components $x$, $y$ and $z$. That way, you could handle the projection using a simple matrix ...

8

Orthographic projections are parallel projections. Each line that is originally parallel will be parallel after this transformation. The orthographic projection can be represented by a affine transformation. In contrast a perspective projection is not a parallel projection and originally parallel lines will no longer be parallel after this operation. Thus ...

7

The near and far planes of a viewing frustum aren't needed for simple 3D→2D projection. What the near and far planes actually do, in a typical rasterizer setup, is define the range of values for the depth buffer. Depths in the [near, far] range will be mapped into [0, 1] to be stored in the depth buffer. However, the depths aren't simply linearly rescaled. ...

6

You are on the right track but what you need to do is to calculate u/w and v/w, and also 1/w for each vertex, which you interpolate linearly in screen space in your rasterizer. Then for every pixel you divide the interpolated u/w and v/w coordinates with the interpolated 1/w to get perspective correct uv-coordinates for the pixel. The same applies to all ...

5

It might help to think of it this way: Both the near plane and the far plane are the size that will fit onto the screen you are viewing on. The further away something is, the bigger it can be and still fit on the screen. Close up, a playing card can fill the whole screen, but in the distance, an entire building can fit on the screen. Consider the example ...

5

The ratio is with a quick and dirty visual measurement $665:501$ which is approximately $5:4$. You can measure it by taking the ratio of the vanishing angles $\alpha/\beta$ (see picture 1) because we are so close to the center. Image 1: Ratio of the inbound angles We can check the situation visually by drawing a 2 point perspective grid. For this we need ...

5

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.) void camera_setAspectRatio(float aspect, float zNear, float zFar, bool offaxis) { // create a projection matrix const float f = 1.0f / tanf(fovy_radians/2.0f); fovx_radians = 2.0f * ...

4

In this case, the geometry of similar triangles ABC and ADE is used to determine the height of D via the solution of DE. It is obvious that if the near plane is at 0 (AE=0), then a division by 0 occurs -- hence, why the near plane cannot be located at position 0. This is not why the nearZ plane cannot be zero. The goal of perspective math is not to project ...

4

Instead of subdividing until reaching certain edge length on screen, you should look into GPU Gems 2 article "Adaptive Tessellation of Subdivision Surfaces with Displacement Mapping" by Michael Bunnell. It uses edge flatness test to adaptively tessellate the geometry which results in better quality with the same number of triangles. With your current ...

4

Projective transformations (represented by 4×4 projection matrices) are invertible. You can go from NDC coordinates back to view space using the inverse of the projection matrix, in the same way that you go from view space to NDC. That is: take your NDC $x, y, z$ coordinates, append $w = 1$ to make a 4D vector, transform by the inverse projection matrix, ...

3

What you are missing is, that in OpenGL's NDC space (i.e. clip space after division by w) all 3 coordinates are in the range $[-1,1]$. So ndc_depth is in the range $[-1,1]$, while your computation assumes a range of $[0,1]$. So let's take your computation but add an initial step to map $[-1,1]$ to $[0,1]$: float ndc_depth = Pclip.z / Pclip.w; float ...

3

The projection matrix distorts the view frustum (the volume the camera can see) into a unit cube. So everything with all coordinates in the range -1 to 1 after projection is potentially visible, and everything else can be clipped. To make things further away look smaller, we need to divide by their Z distance. We can't put this value directly into the ...

3

Here's a hint to get you started: Parallel lines include the line through the camera. So really all you need is the direction from the camera to the vanishing point on the view plane. Then create lines parallel to that line.

3

It is bigger because it fills the same view and it ts further away. It would be smaller if it wouldn't fill the same view but then it wouldn't fill the camera view and it wouldnt work. So the inverse relation applies. If you want something to stay constant in size in the view then it needs to grow as it gets further. Your just comparing apples with ...

3

Copying this from another thread where i posted this as the answer but as Wyck suggested, the correct answer is the first one. There is the whole derivation of it but I'll be discussing a brief overview. This is for the perspective projection where the line joining the eye and the center of the projection/image plane is perpendicular to it. Like here As ...

3

Jim Blinn's book Jim Blinn's Corner: Notation, Notation, Notation has a couple of chapters which go through this in detail with all of the edge cases. This book is a collection of essays from his column in IEEE Computer Graphics and Applications. Highly recommended.

2

Perspective correct interpolation of normals works just as it does any color or coordinate or other linearly varying attribute. Each component of each varying vector is interpolated independently as if they were scalars, using exactly the same equation.

2

It should be possible to do this properly by using a modified projection matrix for your entire scene that has its far plane at infinity, as detailed here. That should allow you to properly render points infinitely far away from the camera. The presentation says that such a point is represented by a direction (a vector with .w = 0), so it should be as simple ...

2

FYI every set of parallel lines will have their own vanishing point. The reason why there is usually only a single vanishing point (the most obvious one) talked about in an image is due to aesthetic choice. A perspective projection can be seen as a point projection of a 3D scene onto a 2D plane. The way to determine where a point in the 3D scene ends up on ...

2

There is the whole derivation of it but I'll be discussing a brief overview. This is for the perspective projection where the line joining the eye and the center of the projection/image plane is perpendicular to it. Like here As we can easily see, by similar triangles $\triangle ABC$ and $\triangle AEF$ we have $Y_p / Y = D/-Z$ where $Y_p$ is the ...

2

"A not so simple approach". I may have messed a little bit too much with grouping the terms, do forgive my elementary math skills, it's a side effect of using tools like wolfram and mathematica too much. Without loss of generality we can assume that the near plane is given as $z=1$ (that is having normal $(0,0,1)$ and a point on it $(0,0,1)$) and the camera ...

1

You definitely need to know distances and angle of view of the camera. For example, if you add lines that are slightly further apart, they will be at different angles. Like a multi-lane highway.

1

Eye position, projection reference point PRP, camera center, or projection center are all just different words for the same thing. gluLookAt() applied to the GL_MODELVIEW matrix works for both, parallel and perspective projection. You can choose between parallel and perspective projection by setting up the GL_PROJECTION matrix. For perspective projection ...

1

Vanishing points are "points at infinity" in projective geometry, which are represented by $w = 0$ in homogeneous coordinates. You can construct the vanishing point of a ray or line by taking its $xyz$ direction vector and adjoining $w = 0$. (For a line, use the direction vector and its negation to get vanishing points in both directions.) Then you can ...

1

Fortunately your scenario is rather simple, your camera is on a line directly perpendicular above and centered on the quad you're concerned about. So what you want is for the quad to fill the whole screen, I assume (more or less) exactly the whole screen, i.e. the size of your projected quad in window space is your entire viewport. The mathematics of this ...

1

For completeness (and in addition to Nathan Reed's answer), I explicitly add the inverse projection matrices for perspective and orthographic cameras. Perspective Camera \begin{align} \mathrm{T}_{\mathrm{view \rightarrow projection}} &= \begin{bmatrix} \mathrm{T}_{00} &0 &0 &0 \\ 0 &\mathrm{T}_{11} &0 &0 \\ 0 &0 &\...

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