如何获取OpenGL 3D模型截面图？

Question

I have load an obj file to render my opengl model using pyopengl and pygame. The 3D model show successfully.

Below is the 3D model i render with obj file, Now i cut my model into ten pieces through y axis , my question is how to get the sectional drawing in each piece?

I'm really very new to openGL, Is there any way can do that?

Answer 1

There are two ways to do this and both use clipping to "slice" the object.

1. In older versions of OpenGL you can use user clip planes to "isolate" the slices you desire. You probably want to rotate the object before you clip it, but it's unclear from your question. You will need to call glClipPlane() and you will need to enable it using glEnable with the argument GL_CLIP_PLANE0 , GL_CLIP_PLANE1 , ...

If you don't understand what a plane equation is you will have to read up on that.

In theory you should check to see how many user clip planes exist on your GPU by calling glGetIntegerv with argument GL_MAX_CLIP_PLANES but all GPUs support at least 6.

2. Since user clip planes are deprecated in modern Core OpenGL you will need to use a shader to get the same effect. See gl_ClipDistance[]

Searching around on Google should get you plenty of examples for either of these.

Sorry not to provide source code but I don't like to post code unless I am 100% sure it works and I don't have the time right now to check it. However I am 100% sure you can easily find some great examples on the internet.

Finally, if you can't make it work with clip planes and some hacks to make the cross sections visible then this may indeed be complicated because creating closed cross sections from an existing model is a hard problem.

Answer 2

You would need to split the object, and then rotate the pieces so that they are seen from the side. (Or move the camera. The two ideas are equivalent. But if you're coding this from scratch, you don't really have the abstraction of a 'camera'.) At that point, you can just render all the slices.

This is complicated to do in raw OpenGL and python, essentially because objects in OpenGL are not solid. I would highly recommend that you slice the object into pieces ahead of time in a modeling program. If you need to drive those operations with scripting, perhaps look into Blender's python scripting system.

Now, to explain why:

When you slice a real-life orange, you expect to get cross sections. You expect to be able to see the flesh of the fruit inside, with all those triangular pieces.

There is nothing inside a standard polygonal 3D model.

Additionally, as the rind of a real orange has thickness, it is possible to view the rind from the side. In contrast, one face of a 3D model is infinitely thin, so when you view it from the side, you will see nothing at all. So if you were to render the slices of this simple model, from the side, each render would be completely blank.

(Well, the bits at the end will have 'caps', like the ends of a loaf a bread, but the middle sections will be totally invisible.)

Without a programming library that has a conception of what a cut is, this will get very complicated, very fast. Simply making the cuts is not enough. You must seal up the holes created by slicing into the original shape, if you want to see the cross-sections. However, filling up the cross sections has to be done intelligently, otherwise you'll wind up with all sorts of weird shading artifacts (fyi: this is caused by n-gons, if you want to go discover more about those issues).

To return to the original statement:

Modeling programs are designed to address problems such as these, so I would suggest you leverage their power if possible. Or at least, you can examine how Blender implements this functionality, as it is open source.

In Blender, you could make these cuts with the knife tool*, and then fill up the holes with the 'make face' command (just hit F). Very simple, even for those who are not great at art. I encourage you to learn a little bit about 3D modeling before doing too much 3D programming. It personally helped me a lot.

*(The loop cut tool may do the job as well, but it's hard to tell without understanding the topology of your model. You probably don't want to get into understanding topology right now, so just use the knife)