Allow refine to do negative values and custom edge lengths for remeshing about manifold

[elalish]

Splitting edges is much easier than collapsing them, but we do already have an internal edge collapse method. Right now it's only for removing degenerates, but it may be modifiable to general decimation.

Making Refine take a maximum edge length as input should be quite a bit simpler, or even a curvature tolerance.

[fire]

@elalish Can I persuade you to merge this first before SDFs?

[elalish]

Well, I'm already in the middle of SDFs. Really what you should be asking is when can I dedicate more time to Manifold? I'm trying to work something out, hopefully I'll have more soon.

Can I have some more detail about how you plan to use this feature and the use cases? That'll help me with building a good API.

[fire]

We've hammered out a workflow using mesh optimizer. https://github.com/zeux/meshoptimizer So I'd like to match the api.

@JFonS

meshoptimizer gives you a relative error and a way to convert it to an absolute value

That's because it internally rescales the mesh, making the largest AABB axis be 1.0

static SimplifyFunc simplify_func;
typedef size_t (*SimplifyWithAttribFunc)(unsigned int *destination, const unsigned int *indices, size_t index_count, const float *vertex_data, size_t vertex_count, size_t vertex_stride, size_t target_index_count, float target_error, float *result_error, const float *attributes, const float *attribute_weights, size_t attribute_count);
static SimplifyScaleFunc simplify_scale_func;
typedef float (*SimplifyScaleFunc)(const float *vertex_positions, size_t vertex_count, size_t vertex_positions_stride);

https://github.com/godotengine/godot/blob/d6e7dcbea9c297253e05a75691d5707aa0a749b3/scene/resources/importer_mesh.cpp#L257

See also https://github.com/godotengine/godot/blob/ba1024f42d9ba279b13eee0cbe3286089dc39ffb/scene/resources/surface_tool.h#L77

[fire]

I was hoping to use this for constructing a csg edit tree and then sampling the sdfs to make a mesh. (citation Sony Media Molecule Dreams).

The idea is once constructed the resulting mesh is a mesh like any other lod'd mesh.

reference

https://www.youtube.com/watch?v=Pi3r18ld-S8