# Supervised 3D Mesh Reconstruction

## Problem Description:

We are given an initial source shape (sphere in this case) and we want to deform this source shape to fit target shape (dolphin in this case). For this demonstration, we will be using Triangle Mesh for the representation of source and target shape.

Triangle Mesh has two main components, vertices and faces. Deformation of source shape to fit target shape can be achieved by offsetting source's vertices to fit target surface. Also, the number of vertices and faces is not equal in source and target shape.

Note

For visualization purpose we will require to install Makie and compatible backend (GLMakie or WGLMakie). To install it simply run ] add Makie in the julia prompt.

using Flux3D, Zygote, Flux, FileIO, Statistics, Plots
using Makie, GLMakie

Flux3D.Makie.inline!(true)
Flux3D.Makie.set_theme!(show_axis = false)

download("https://github.com/nirmalsuthar/public_files/raw/master/dolphin.obj",
"dolphin.obj")
"sphere.obj")

Triangle Mesh is handled by TriMesh in Flux3D. TriMesh also supports batched format, namely padded, packed and list, which allow us to use fast batched operations. We can load TriMesh with load_trimesh function (supports objstlplyoff and 2DM)

dolphin = load_trimesh("dolphin.obj")
src = load_trimesh("sphere.obj")

## Preprocessing data

Preprocessing tgt (dolphin), such that its mean is zero and also scale it according to the bounding box of src (sphere), So that src can converge at greater speed.

tgt = deepcopy(dolphin)
verts = get_verts_packed(tgt)
center = mean(verts, dims=2)
verts = verts .- center
scale = maximum(abs.(verts))
verts = verts ./ scale
tgt._verts_packed = verts

## Visualizing TriMesh

We will use visualize function for visualizing TriMesh. This function uses Makie for plotting. In fact, we can also visualize PointCloud using this function, which makes this function handy dealing with different 3D format.

Flux3D.Makie.vbox(visualize(src), visualize(tgt))

## Defining loss objective

Starting from the src mesh, we will deform src mesh by offsetting its vertices (by offset array), such that new deformed mesh is close to target mesh. Therefore, our loss function will optimize the offset array. We will be using the following metrics to define loss objective:

• chamfer_distance - the distance between the deformed mesh and target mesh, which is calculated by taking randomly 5000 points from the surface of each mesh and calculating chamfer_distance between these two pointcloud.
• laplacian_loss - also known as Laplacian smoothing will act as a regularizer.
• edge_loss - this will minimize edges length in deformed mesh, also act as a regularizer.
function loss_dolphin(x::AbstractArray, src::TriMesh, tgt::TriMesh)
src = Flux3D.offset(src, x)
loss1 = chamfer_distance(src, tgt, 5000)
loss2 = laplacian_loss(src)
loss3 = edge_loss(src)
return loss1 + 0.1*loss2 + loss3
end

## Defining learning rate and optimizer

lr = 1.0
opt = Flux.Optimise.Momentum(lr, 0.9)
_offset = zeros(Float32, size(get_verts_packed(src))...)

## Using GPU for fast training [Optional]

We can convert the TriMesh structure to GPU or CPU usinggpu and cpu function which is exactly the same syntax as Flux.

tgt = tgt |> gpu
src = src |> gpu
_offset = _offset |> gpu

## Optimizing the offset array

We first initialize offset array as zeros, hence deformed mesh is equivalent to src mesh (sphere). Next, we calculate loss using this offset array and we compute derivatives wrt. offset array and finally optimize the array.

@info("Training...")
θ = Zygote.Params([_offset])
for itr in 1:2001
loss_dolphin(_offset, src, tgt)
end
Flux.update!(opt, _offset, gs[_offset])
if (itr%10 == 1)
loss = loss_dolphin(_offset, src, tgt)
@show itr, loss
save("src_$(itr).png", visualize(Flux3D.offset(src, _offset))) end end anim = @animate for i ∈ 1:8 Plots.plot(load("src_$(1+250*(i-1)).png"), showaxis=false)
end
gif(anim, "src_deform.gif", fps = 2)

## Postprocessing the predicted mesh

We create a new TriMesh by offsetting src by final offset array and scale up the finalmesh by the same scaling factor we scale down tgt, such that finalmesh has similar bounding box as dolphin mesh.

final_mesh = Flux3D.offset(src, _offset)
final_mesh = Flux3D.scale!(final_mesh, scale)

## Saving the final_mesh

Flux3D provide IO function save_trimesh to save TriMesh (supports objstlplyoff and 2DM)

save_trimesh("results/final_mesh.off", final_mesh)
save("results/final_mesh.png", visualize(final_mesh))
Flux3D.Makie.vbox(visualize(final_mesh), visualize(dolphin))

## Finally..

• Look into the other examples in examples/
• Read more the TriMesh in TriMesh section in documentation and

Metrics/Transforms section for manipulating TriMesh and computing standard metrics.