Sign in to follow this  
Ectara

OpenGL Projection Matrices

Recommended Posts

Ectara    3097
Using a projection matrix to project a point from 3D space to screen coordinates, I get bizarre and invalid values when attempting to project a point outside the viewing frustum. Is this normal behavior, and if so, how do APIs like OpenGL draw lines and polygons that are partially outside the viewing frustum?

Share this post


Link to post
Share on other sites
Ectara    3097
While true, as that link describes, it discards primitives, not vertices. So if half a triangle is visible, it doesn't discard the entire triangle.

Share this post


Link to post
Share on other sites
Ectara    3097
On a 640x480 viewport, I'd get things like -674, -842 and -24235334, 5234523. All seemingly random, yet consistent if I send the same vertices through. Everything in the frustum comes up as expected.

Share this post


Link to post
Share on other sites
szecs    2990
I have a bit similar question:
I use gluProject, and if the the projected point is behind the camera (behind the plane of the screen), the depth values > 1.0
I exploit that thing, so I don't have to check again, if the point is behind the camera.
Can I trust this behavior, or it's not API/lib/whatever independent?

Share this post


Link to post
Share on other sites
Zipster    2365
Without knowing what the actual vertices and matrices are, it's difficult to tell what's going wrong, if anything. Keep in mind that by the time you get to screen coordinates you've already performed a homogeneous divide by 'w', so if that 'w' happened to very large in clip-space you could see very large 'x' and 'y' values before the divide. But 1 million into 50 million is still just 50, so it all depends on what steps you've taken and what data you're looking at.

Share this post


Link to post
Share on other sites
Ectara    3097
I'll take a closer look and see what's going on at each step under the hood, and check during various circumstances. I'll post code and vertices with the current matrices if I can't find it today; no sense bothering you further if I find the bug myself.

Share this post


Link to post
Share on other sites
Ectara    3097
@szecs, my mind is a bit hazy on my installed OpenGL implementation, but unless you see code for each version, I wouldn't rely on it, seeing as one implementation could change it slightly and throw it all off. Try caching the 6 frustum planes(or even just the near plane), and test if it is behind the near plane.

Share this post


Link to post
Share on other sites
Ectara    3097
I did find one thing out. Projecting a point slightly out of the frustum acts accordingly; for instance, a point might be projected to -60, 20 on a 800x600 display, so the projection is working correctly, per se. But I did notice, that when a vertex went around +/-20000, it changed sign, causing the usual havoc. I swapped some of the float variables with integers where possible, in hopes that the float variables had over/underflowed(I have little way of telling what the next value after +/- 20000 was supposed to be).

Wait, wait, wait. When it goes around +/- 20000, something else also happens: the vertices begin to pass the 90 degree rotation mark. Meaning the signs have flipped because z is now past my clamped range of [0,1], being that the vertices are now behind me. A couple quick lines of code, and all is well, except for now if my focal point is behind me, everything goes haywire. But this isn't really an issue, seeing as my focal point should never be behind me in the first place. Thanks to those who offered advice.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Similar Content

    • By Arulbabu Donbosco
      There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window. 
      I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
      anyone, please help me .. how to go further... to create an application like VR CAVE. 
       
      Thanks
    • By cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
       
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      http://www.informit.com/articles/article.aspx?p=2120983
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
      Thanks!
    • By Picpenguin
      Hi
      I'm new to learning OpenGL and still learning C. I'm using SDL2, glew, OpenGL 3.3, linmath and stb_image.
      I started following through learnopengl.com and got through it until I had to load models. The problem is, it uses Assimp for loading models. Assimp is C++ and uses things I don't want in my program (boost for example) and C support doesn't seem that good.
      Things like glVertexAttribPointer and shaders are still confusing to me, but I have to start somewhere right?
      I can't seem to find any good loading/rendering tutorials or source code that is simple to use and easy to understand.
      I have tried this for over a week by myself, searching for solutions but so far no luck. With tinyobjloader-c and project that uses it, FantasyGolfSimulator, I was able to actually load the model with plain color (always the same color no matter what I do) on screen and move it around, but cannot figure out how to use textures or use its multiple textures with it.
      I don't ask much: I just want to load models with textures in them, maybe have lights affect them (directional spotlight etc). Also, some models have multiple parts and multiple textures in them, how can I handle those?
      Are there solutions anywhere?
      Thank you for your time. Sorry if this is a bit confusing, English isn't my native language
  • Popular Now