# OpenGL Approximate frustum culling

This topic is 4223 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

## Recommended Posts

Hi! I am developing a game with a pretty big heightmap terrain and I'm an a tight schedule. I really need to get frustum culling implemented fast. I have piece of heigthmap terrain terrain, that I've divided into square shaped pieces which are in separate display lists. Now I'd need a quick way to check if to draw one of those pieces or not. Basic frustum culling. However, my system is very simple, the camera moves only along the x-axis, so it's always looking in the same direction. Also, there's nothing behind the camera. I figured that the easiest way to do "close enough" frustum culling is to check if one of the square-piece's corners is inside an infinite cone drawn around frustum. I don't have to worry about near and far planes. Now I'd need to know how to get the FOV of the cone. I am using gluPerspective, so I have fovy (constant) and aspect ratio (changing because of different aspect splitscreens). So, how to (easily) calculate the FOV from the center of the screen (the "front" vector) to the corner of the frustum? Any math, code or other form of help is welcome! Thanks in advance, -Riku P.S. Is someone heard of or using a similar culling method? The guys at #opengl@freenode said they hadn't even thought about it.

##### Share on other sites
gluPerspective() takes the vertical field of view as an argument, but you'll most likely need the horizontal fov for culling. Here's a function to convert from the former to the latter:
template < typename T >T yfov_to_xfov(T yfov, T aspect) {    return T(2.0 * std::atan(std::tan(yfov * T(.5)) * double(aspect)));}
The angle from the camera forward vector to a side plane of the frustum will then be half this value. (Remember to convert between radians and degrees as appropriate.)

That may not address all your questions, but perhaps it will help.

##### Share on other sites
deavik,
Thanks for the URL, a nice tutorial. I really do not need real frustum culling. There are plenty of tutorials on how to do that. I'll reconsider the need if it ends up harder to do a cone-frustum culling I want to do.

jyk,
I had already figured that out. I need the FOV from the center to the corner. I know a hard way to calculate it. Perhaps I'll plug that into Mathematica and simplify it or maybe I'll just use the traditional pen and paper way =).

What I've got is this:

diagonal_fov = 2.0 * atan( sqrt( tan(0.5 * fovx)^2 + tan(0.5 * fovy)^2 ) );

Does anyone know a simplified form of that? My trig is not that good.

Thanks guys!
-Riku

EDIT: diagonal_fov formula had a mistake, fixed

[Edited by - RichardoX on July 30, 2006 3:24:01 AM]

• 10
• 12
• 10
• 10
• 11
• ### Similar Content

• Good Evening,
I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
I am really stucked right now because of the fundamental question:
Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on.
In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
Should I treat those debug objects as entities/components?
For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
Regards,
LifeArtist
• By QQemka
Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
Let's go:
Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
There were several more but i forgot/solved them at time of writing
• By RenanRR
Hi All,
I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
#version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);
So, some doubts:
- Why use it like that?
- Is it okay to manipulate the camera that way?
-in this way, are not the vertex's positions that changes instead of the camera?
- I need to pass MVP to all shaders of object in my scenes ?

What it seems, is that the camera stands still and the scenery that changes...
it's right?

Thank you

• Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

int rgbValue = int(textureSample.w);//4 bytes of data packed as color
// algorithm might not be correct and endianness might need switching.
vec3 extractedData = vec3(  rgbValue & 0xFF000000,  (rgbValue << 8) & 0xFF000000, (rgbValue << 16) & 0xFF000000);
extractedData /= 255.0f;

• While writing a simple renderer using OpenGL, I faced an issue with the glGetUniformLocation function. For some reason, the location is coming to be -1.
Anyone has any idea .. what should I do?