Clipping a 3D model ( maybe using a clipplane?)

[McSwan]

Hi,

Just says I have a 3d mesh, and I want to create a clipplane to cut, so the top doesn't render (ie anything above the plane doesn't render) and people can see inside of the model.
ie bad ascii drawn model
*
***
******


_________ <- clipping plane makes the top of the model not render, so you can see inside it.
***
******

It is ok to have the clipplane clip all models, although might be nice to be able to clip against just 1 model if it's possible and not too hard.

Any suggestion from the experts on the best way of doing it ? Maybe adding an additional Clip plane to the camera would be the go ?

[nullsquared]

You might use a user-clippane (check RenderSystem), but it's best to just use a shader that manually clips the object.

Basically, just pass the plane to the shader (using a custom renderable parameter, for example), and check the dot product of the plane's normal with the direction of the current fragment from the plane's origin. If it's negative, disard; the texel. Or just use clip(-1); or more simply, clip(dot(planeNormal, directionFromPlaneOriginToFragment));

[nikki]

If the plane is always local-Y-axis aligned, you could do a simple Y-greater check in the shader with the interpolated position.

[psquare]

I concur with nullsquared.

Using a shader is the easiest and most flexible approach. In fact, you can clip with any geometry that can be represented by a simple equation (e.g Sphere)

[McSwan]

OK,

I got confirmation that all I need to do is create a clip plane that is horizontal and moves up and down, and affects all objects. It's just for clipping building so you can see inside them.

My knowledge on shaders is really bad atm. I'm slowly reading up. I'd need to use a ?global clipping shader? to do what you suggest ? Obviously I can't apply the same shader clipping material to all objects in my scene, which is my current level of understanding shaders.

I thought all was good when I found Rendersystem::addclipPlane. According to Sinbad, I can't use it? Is this still the case or is there something similar I can use?

http://www.ogre3d.org/forums/viewtopic.php?f=2&t=38943

[McSwan]

Thought I'd have a crack at using rendersystem::addplane anyway, but as far as I can tell it doesn't work.

Made a floor plane and clip underneath, but doesn't clip.

Code: Select all

    Entity *ent;
    // Define a floor plane mesh
    Plane p;
    p.normal = Vector3::UNIT_Y;
    p.d = 200;
    Root::getSingleton().getRenderSystem()->addClipPlane(p);
    MeshManager::getSingleton().createPlane("FloorPlane",
        ResourceGroupManager::DEFAULT_RESOURCE_GROUP_NAME, 
        p,2000,2000,1,1,true,1,5,5,Vector3::UNIT_Z);
         // Create an entity (the floor)
    ent = mSceneMgr->createEntity("floor", "FloorPlane");
    //ent->setMaterialName("Examples/RustySteel");

    mSceneMgr->getRootSceneNode()->attachObject(ent);

    // add clip plane
    Plane clipPlane;
    clipPlane.normal = Vector3::UNIT_Y;
    clipPlane.d = 201;
    Root::getSingleton().getRenderSystem()->addClipPlane(clipPlane);

[madmarx]

Beware user ClipPlanes from rendersystem, they are used to clip lights & other things internally in ogre.
It is possible, but maybe not ultra easy. If you don't use shader, maybe you can have a try with clipplanes by using listeners and changing things in the renderqueue (I have never tried, so it is maybe a stupid idea). I think to make a direct call either to render system, either to opengl (in my case I am better at opengl..).


Something like :

1/ put a plane with material (colour_write off, depth_write on) in render queue main -1
2/ lets you culled object after...

should visually work if you are able to manage correctly the traditionnal render queue.