Sign in to follow this  
metalcrusader

OpenGL zFail & infinite view frustum PROBLEM!

Recommended Posts

metalcrusader    122
Hello! I have problems implementing Stencil Shadow Volumes with zFail. To prevent that the volume is clipped by the far plane, I have to use an „infinite view frustum“. But how can I make that in OpenGL ( not the theory and need the OpenGL-Call!). And why do I need that infinite view frustum when I limit the volume to for example 10 and the far clipping plane is at 100000? I ask because in my implementation I get a second „ghost shadow“ and i don´t know how to remove it. I know that the volume must be closed so I use the reversed backfaces as the front cap and the extruded backfaces as back faces but the „ghost shadow“ remains. Please help me!!!

Share this post


Link to post
Share on other sites
godmodder    828
Closing the volumes only isn't enough. The farplane should always be removed. This is accomplished by setting it to infinity like this:

gluPerspective(fov, aspect, 1.0f, -1.0f);

As you can see, I've set the farplane to -1.0f. This should solve your problem. Remember, for nvidia cards you can recover the 1% depth-precision loss by using the GL_NV_depth_clamp extension, but you won't notice any significant improvement with a 24-bit zbuffer.

Share this post


Link to post
Share on other sites
metalcrusader    122
Thank you very much for answering! It seems that only a very few people have experiences with zFail!
I tried my program with the infinite far plane, but I still get the same second "ghost shadow". Is there anything else I have to consider for using this infinite far plane?

Another question: I found an extension called "EXT_depth_bounds_test". Can this extension solve by problem and how can I use it?

Share this post


Link to post
Share on other sites
godmodder    828
There are a few things you need for zfail to work:

- an infinite farplane
- a nearplane of 1.0f (depth precision)
- be sure to set 8 bits of stencil
- be sure to use glClearStencil(0) at init
- draw front faces with:
glStencilFunc(GL_ALWAYS, 0, ~0);
glStencilOp(GL_KEEP, GL_INCR_WRAP_EXT, GL_KEEP);
- draw back faces with:
glStencilFunc(GL_ALWAYS, 0, ~0);
glStencilOp(GL_KEEP, GL_DECR_WRAP_EXT, GL_KEEP);
- be sure to use the wrapping extension in the above operations!
- your volume geometry must face outwards (with the normals)
- you have to draw front and back caps
- draw your volume geometry (last 2 verts of the quads) AND your backward caps at infinity (use w=0.0f or draw at a VERY LARGE offset (100000 or so))
- be sure to use some polygon offset like: glPolygonOffset(1.0f, 4.0f)

When you get these 11 requirements straight, I think it'll work. It works for me!

Share this post


Link to post
Share on other sites
metalcrusader    122
Thank you again!
I did everything you advised me to do - but the "ghost shadow" didn't want to disapear. Then I compared my code with an old zPass implementation and I noticed that I have set the mask parameter of glStencilFunc to 0 instead of 1. That was all! The ghost shadow disappers and is fine!

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 pseudomarvin
      I assumed that if a shader is computationally expensive then the execution is just slower. But running the following GLSL FS instead just crashes
      void main() { float x = 0; float y = 0; int sum = 0; for (float x = 0; x < 10; x += 0.00005) { for (float y = 0; y < 10; y += 0.00005) { sum++; } } fragColor = vec4(1, 1, 1 , 1.0); } with unhandled exception in nvoglv32.dll. Are there any hard limits on the number of steps/time that a shader can take before it is shut down? I was thinking about implementing some time intensive computation in shaders where it would take on the order of seconds to compute a frame, is that possible? Thanks.
    • 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!
  • Popular Now