Sign in to follow this  
Bruno

OpenGL Oblique fustrum clipping

Recommended Posts

Bruno    155
I have been tryng to use the method displayed here : http://www.terathon.com/code/oblique.html But i just can't get it to work, it's like its clipping any geometry that i render clipPlane[0] = 0; clipPlane[1] = -1; clipPlane[2] = 0; clipPlane[3] = 12; With that, if i supply it to a standard clip plane, glClipPlane(GL_CLIP_PLANE0, clipPlane); It works just fine, however if i supply it to the oblique culling function, it just culls everything.. What i do is this :

void ModifyProjectionMatrix(const float clipPlane[4])
{
	float	matrix[16];
	float	q[4];

	glGetFloatv(GL_PROJECTION_MATRIX, matrix);
	

	q[0] = (sgn(clipPlane[0]) + matrix[8]) / matrix[0];
	q[1] = (sgn(clipPlane[1]) + matrix[9]) / matrix[5];
	q[2] = -1.0F;
	q[3] = (1.0F + matrix[10]) / matrix[14];
	
	// Calculate the scaled plane vector
	float c[4];
	c[0] = clipPlane[0] * (2.0F / (clipPlane[0] * q[0]));
	c[1] = clipPlane[1] * (2.0F / (clipPlane[1] * q[1]));
	c[2] = clipPlane[2] * (2.0F / (clipPlane[2] * q[2]));
	c[3] = clipPlane[3] * (2.0F / (clipPlane[3] * q[3]));
	
	// Replace the third row of the projection matrix
	matrix[2] = c[0];
	matrix[6] = c[1];
	matrix[10] = c[2] + 1.0F;
	matrix[14] = c[3];

	// Load it back into OpenGL
	glMatrixMode(GL_PROJECTION);
	glLoadMatrixd(matrix);
         glMatrixMode(GL_MODELVIEW);
}



	 glPushMatrix();
    	  ModifyProjectionMatrix(clipPlane);
           render_stuff();
          glPopMatrix();
What could be wrong here ? thanks, Bruno [Edited by - Bruno on July 17, 2005 5:23:54 PM]

Share this post


Link to post
Share on other sites
Eric Lengyel    3460
Hi --

The problem is that in the following line of the original code,

Vector4D c = clipPlane * (2.0F / (clipPlane * q));

the clipPlane * q is a 4D dot product, but you've translated it as a componentwise product. I'll update my code to make it more explicit. If you change your code to the following, it ought to work. (Also remember that planes specified with glClipPlane are transformed by the inverse of the modelview matrix.)


void ModifyProjectionMatrix(const float clipPlane[4])
{
float matrix[16];
float q[4];

glGetFloatv(GL_PROJECTION_MATRIX, matrix);

q[0] = (sgn(clipPlane[0]) + matrix[8]) / matrix[0];
q[1] = (sgn(clipPlane[1]) + matrix[9]) / matrix[5];
q[2] = -1.0F;
q[3] = (1.0F + matrix[10]) / matrix[14];

// Calculate the scaled plane vector
float d = 2.0F / (clipPlane[0] * q[0] + clipPlane[1] * q[1]
+ clipPlane[2] * q[2] + clipPlane[3] * q[3]);

float c[4];

c[0] = clipPlane[0] * d;
c[1] = clipPlane[1] * d;
c[2] = clipPlane[2] * d;
c[3] = clipPlane[3] * d;

// Replace the third row of the projection matrix

matrix[2] = c[0];
matrix[6] = c[1];
matrix[10] = c[2] + 1.0F;
matrix[14] = c[3];

// Load it back into OpenGL
glMatrixMode(GL_PROJECTION);
glLoadMatrixd(matrix);
glMatrixMode(GL_MODELVIEW);
}


Share this post


Link to post
Share on other sites
Bruno    155
Hi Eric,

Thanks a lot, that was the problem indeed, i made a incorrect translation.
However, there's seems to be another problem with this.

I'm using the to "cut" the water level in a reflection\refraction shader.
When i look at the ground the clipplane seems to move with the mouse and clips away the ground, if i look at the horizont is looks ok, if i look up, the clipplane moves again and "unclips" everything above water level.., Seems to me the near clip plane is moving with the mouse or something like that.
I tried adjusting the near clip plane \ far clip plane but did't had much look.
How is this fixable ?

thanks,

Bruno

Share this post


Link to post
Share on other sites
Bruno    155
Hi again,

I messed around with this, but it just doens't want to work.. :(
Anyways, i googled around, and found a sample in nvidia, right here :

http://download.developer.nvidia.com/developer/SDK/Individual_Samples/DEMOS/OpenGL/oblique_frustum.zip

I converted their messy code, i replaced my clipplane with this, and it worked at the first try..,
I would still rather use your code Eric, seems more simple\faster and cleaner than nvidia, but what could be wrong here ?

thanks,
Bruno

Share this post


Link to post
Share on other sites
Eric Lengyel    3460
Hi --

Don't use the Nvidia sample -- it doesn't consider the effect on the far plane, and your depth buffer precision will be horrible.

Make sure that your clipping plane is specified in camera-space coordinates, and that its normal points into the view frustum. Note that unless your modelview matrix is the identity, this will not be the same plane as you would pass to glClipPlane().

-- Eric

Share this post


Link to post
Share on other sites
Eric Lengyel    3460
I just noticed that you're using the glLoadMatrixd() function to load the new projection matrix. You're loading garbage with that because it's reading doubles where you have floats. See what happens when you change it to glLoadMatrixf(). I'm not even sure how that compiled, unless you're using a straight C compiler with all sensible error checking turned off.

Share this post


Link to post
Share on other sites
Bruno    155
you are right, glLoadMatrixd() is wrong there, i pasted it here with it in, but its not in the actual code, i forgot it there when i was tryng it with doubles for extra precision.

I believe the clipPlane i supply to your code, is in the same space as the camera, but about the modelview matrix being the identity matrix, if i reset it to the identity matrix i will loose my camera view, or am i missing something ?
I will have to try that, but just calling a glLoadIdentity to the modelview before calling your function, might to the job ?

Anyways, regarding the nvidia example, i don't have any precision artifacts, at least in this card ( nvidia gf6800 gt ), with a near plane of 0.03 and a far plane of 200.
Maybe the artifacts show up in bigger rations on near plane\far plane ? or maybe in other video cards ?

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 povilaslt2
      Hello. I'm Programmer who is in search of 2D game project who preferably uses OpenGL and C++. You can see my projects in GitHub. Project genre doesn't matter (except MMO's :D).
    • By ZeldaFan555
      Hello, My name is Matt. I am a programmer. I mostly use Java, but can use C++ and various other languages. I'm looking for someone to partner up with for random projects, preferably using OpenGL, though I'd be open to just about anything. If you're interested you can contact me on Skype or on here, thank you!
      Skype: Mangodoor408
    • By tyhender
      Hello, my name is Mark. I'm hobby programmer. 
      So recently,I thought that it's good idea to find people to create a full 3D engine. I'm looking for people experienced in scripting 3D shaders and implementing physics into engine(game)(we are going to use the React physics engine). 
      And,ye,no money =D I'm just looking for hobbyists that will be proud of their work. If engine(or game) will have financial succes,well,then maybe =D
      Sorry for late replies.
      I mostly give more information when people PM me,but this post is REALLY short,even for me =D
      So here's few more points:
      Engine will use openGL and SDL for graphics. It will use React3D physics library for physics simulation. Engine(most probably,atleast for the first part) won't have graphical fron-end,it will be a framework . I think final engine should be enough to set up an FPS in a couple of minutes. A bit about my self:
      I've been programming for 7 years total. I learned very slowly it as "secondary interesting thing" for like 3 years, but then began to script more seriously.  My primary language is C++,which we are going to use for the engine. Yes,I did 3D graphics with physics simulation before. No, my portfolio isn't very impressive. I'm working on that No,I wasn't employed officially. If anybody need to know more PM me. 
       
    • By Zaphyk
      I am developing my engine using the OpenGL 3.3 compatibility profile. It runs as expected on my NVIDIA card and on my Intel Card however when I tried it on an AMD setup it ran 3 times worse than on the other setups. Could this be a AMD driver thing or is this probably a problem with my OGL code? Could a different code standard create such bad performance?
    • By Kjell Andersson
      I'm trying to get some legacy OpenGL code to run with a shader pipeline,
      The legacy code uses glVertexPointer(), glColorPointer(), glNormalPointer() and glTexCoordPointer() to supply the vertex information.
      I know that it should be using setVertexAttribPointer() etc to clearly define the layout but that is not an option right now since the legacy code can't be modified to that extent.
      I've got a version 330 vertex shader to somewhat work:
      #version 330 uniform mat4 osg_ModelViewProjectionMatrix; uniform mat4 osg_ModelViewMatrix; layout(location = 0) in vec4 Vertex; layout(location = 2) in vec4 Normal; // Velocity layout(location = 3) in vec3 TexCoord; // TODO: is this the right layout location? out VertexData { vec4 color; vec3 velocity; float size; } VertexOut; void main(void) { vec4 p0 = Vertex; vec4 p1 = Vertex + vec4(Normal.x, Normal.y, Normal.z, 0.0f); vec3 velocity = (osg_ModelViewProjectionMatrix * p1 - osg_ModelViewProjectionMatrix * p0).xyz; VertexOut.velocity = velocity; VertexOut.size = TexCoord.y; gl_Position = osg_ModelViewMatrix * Vertex; } What works is the Vertex and Normal information that the legacy C++ OpenGL code seem to provide in layout location 0 and 2. This is fine.
      What I'm not getting to work is the TexCoord information that is supplied by a glTexCoordPointer() call in C++.
      Question:
      What layout location is the old standard pipeline using for glTexCoordPointer()? Or is this undefined?
       
      Side note: I'm trying to get an OpenSceneGraph 3.4.0 particle system to use custom vertex, geometry and fragment shaders for rendering the particles.
  • Popular Now