Converting Shape to Mesh in python?

[qnzhou]

Hi there,

I am trying to write a custom texture plugin in python, and I am wondering how to access the mesh vertices and triangles in the Texture.eval method?

Here is the context:
The Texture.eval method has an argument of type SurfaceInteraction which has a shape property of type Shape. I have checked that shape.is_mesh is true. However, the necessary methods to access vertices and triangles are defined in the Mesh class. In C++, a simple static_cast will do the job, but is there a way to convert a Shape object to a Mesh object in python? Any pointers are greatly appreciated!

[rtabbara]

Hi @qnzhou,

One approach is to leverage dr.dispatch to achieve what you're after:

def custom_eval(self, other_args):
    if self.is_mesh():
        verts = mi.traverse(self)['vertex_positions']
        # perform computation here
        return final_output
    return mi.Float(0)

dr.dispatch(si.shape, custom_eval, other_args)

where other_args could be other input you need to compute your evaluation.

It's not ideal to repeatedly call mi.traverse so you could alternatively cache the mapping of shapes to vertices/faces

[qnzhou]

Thanks @rtabbara ! This works! However, I realized that a texture plugin in python is probably not a good idea given the large number of Texture.eval calls. Performance is super slow even with very simple eval function.

[zzyyyy12138]

In Python, you can't directly cast from a base class (in this case Shape) to a derived class (in this case Mesh). Instead, you have to use some form of type checking and conditional statement to ensure that the Shape object you're working with is indeed a Mesh. Here's an example of how to do that:

python复制代码

  | def my_texture_plugin(interaction):
-- | --
  | shape = interaction.shape
  |  
  | if isinstance(shape, Mesh):
  | # Access vertices and triangles
  | vertices = shape.vertices
  | triangles = shape.triangles
  |  
  | # Do something with vertices and triangles...
  | pass
  | else:
  | raise ValueError('The shape is not a Mesh')

This code checks if the shape object is an instance of Mesh, and if it is, it retrieves the vertices and triangles. If the shape is not a Mesh, it raises a ValueError.

This is a common pattern in Python when you need to work with derived classes through a base class reference. Note that this assumes that the Shape and Mesh classes have been implemented in Python, and that the Mesh class has vertices and triangles attributes. The exact implementation will depend on the libraries you're using