# Category: Today’s paper

## About Laplacian Mesh Optimization

Today I’ve read the paper “Laplacian Mesh Optimization” by Andrew Nealen, Takeo Igarashi, Olga Sorkine, Marc Alexa.

## Mesh optimization and Smoothing

Mesh optimization is the reduction of the number of vertices in an initial mesh of triangles without change in the shape. And smoothing is to smooth surface by averaging or removing vertices that is significantly different from other vertices in mesh. Smoothing is performed by subdivision, but the number of vertices is kept in this study.

## Novelty of the study

This study introduce a framework for triangle shape optimization and feature preserving smoothing of triangular meshes that is guided by the vertex Laplacians.

## Method

### Laplacian matrix and Laplacian of vertex

Mesh Graph(V is vertices, E is edges): $$G =\{V, E\}$$

vertices: $$V =[v_1^T, v_2^T,…, v_n^T]^T$$ $$v_i =[v_{ix}, v_{iy},v_{iz}]$$

x, y, z element of vertices: $$V_d =[v_{1d}, v_{2d},…, v_{nd}]^T$$ $$d \in \{x, y, z\}$$

Laplacian of vertex: $$\delta_i =\sum_{\{i,j\} \in E}w_{ij}(v_i, v_j)$$

Weight w is defined below: $$\sum_{\{i,j\} \in E}w_{ij} = 1$$ $$w_{ij} =\frac{\omega_{ij}}{\sum_{\{i,j\} \in E}\omega_{ij}}$$ example for ω: $$\omega_{ij} = 1…(1)$$ $$\omega_{ij} = \cot \alpha + \cot \beta…(2)$$

(1) is uniform weight and (2) is cotangent weight. the angle α and β are defined as follows.

The element of n×n Laplacian Matrix L is define as follows:

x, y, z element of Laplacian: $$\Delta_d =[\delta_{1d}, \delta_{2d},…, \delta_{2d}]^T = LV_d$$ $$d \in \{x, y, z\}$$

the discrete mean curvature normal: $$\overline{\kappa_i}n_i = \delta_{i, c\overline{\kappa_i}}$$ the discrete mean curvature: $$\overline{\kappa_i}$$ the unit length surface normal $$n_i$$

### Optimization algorithm

Displaced vertices: $$V’ =[{v’}_1^T, {v’}_2^T,…, {v’}_n^T]^T$$ $$v’_i =[v’_{ix}, v’_{iy},v’_{iz}]$$

x, y, z element of V’: $$V’_d =[v’_{1d}, v’_{2d},…, v’_{nd}]^T$$ $$d \in \{x, y, z\}$$

energy to minimize:

To solve the above problem, an equations of the form AV’_d = b is introduced.

1, Least squares meshes.

2, Detail preserving triangle shape optimization.

The displaced vertices are calculated by the following formula. $$V’_d =(A^TA)^{−1}A^Tb$$

### Global triangle shape optimization

In this work, new general 2n×n system AV’_d = b is defined as follows.

W_p is positional weight that constraints vertex position. Larger weights in
W_p preserve the original geometry.

W_L: Laplacian weight that enforce regular triangle shapes and surface smoothness.

L is Laplacian matrix(uniform, cotangent or laplacian with discrete mean curvature ). And f is the corresponding right-hand side with the diagonal matrix W_L.

### Application

Check the original paper to see examples of Shape optimization and Smoothing.

## A non-hierarchical procedure for re-synthesis of complex textures

I’ve read “A non-hierarchical procedure for re-synthesis of complex textures“. REFERENCE: Image Texture Tools: Texture Synthesis, Texture Transfer, and Plausible Restoration.

## Texture re-synthesis

In this paper, texture re-synthesis means the name of the technique to generate output texture with a similar pattern to the one of input image.

## Novelty of the study

This work propose a method for synthesizing an image with the same texture as a given input image. In this method, a plausible value for dot in output image is selected from input image. Important point is the order to select dot to transfers complex features of the input image to the output image.

## Method

### overview

Select pixels in output image and set a value from pixels in input image. Pixel value estimation depends on entropy, amounts of information, of pixels in the input image. To measure how closely a patch from the input image matched one from the output image, distance function below is used. In other word, the distance function indicates the plausibility of the selected pixel.

1. Find empty location in the output image with the highest priority.
2. Choose a pixel from the input image to place in the location.
3. Update priorities of empty neighboring location based on the new pixel value

### Analysis of pixel interrelationships in the input image

To select important pixel with large amount of information, define the number of bits of information G(s, u). The prioritization weighting and the order of pixel addition can be decided by using this G(s, u). Refer the original paper to derive G(s, u) and weightings from RGB data of the input image.

## Result

In this paper seven neighborhood of pixels was used. It takes 4.5 minutes and 6.5MB of memory to produce on a 8000Mhz Pentium2.

You can see more result and application in the original paper.

## Temporally Coherent Sculpture of Composite Objects

Today I’ve read the paper “Temporally Coherent Sculpture of Composite Objects” by Sampaio , Artur P.; Chaine , Raphaëlle; A.Vidal, Creto; Cavalcante-Neto, Joaquim B.

## Interactive virtual sculpting of composite shapes

This work propose the method of deforming an agglomeration of component elements (named CompEls) in real time as below.

There are some exiting techniques to represent a surface composed small 3D objects, for example, stipples and texture bombing. However, they are time consuming and not intended to be used in real time.

## Novelty of the study

Realtime sculpting and visualize CompEls using techniques below.

### Empirical description by uniform meshes

Because the physically accurate interaction of components is too complex to compute in realtime, this method uses empirical description. The system focus on the outer layers of the shape represented as the quasi-uniform meshes. The system detect that the surface expand by the emergence of new sampling anchors.

quasi-uniform meshes is defined by threshold d and t below.
d: the maximum distance between two sampling point
t: the minimum distance below which the edge collapse
Check “Freestyle: Sculpting meshed with self-adaptive topology” for details.

CompEls are placed according to samples on the triangle of the mesh. Each one has 3parameters, the barycentric coordinate, depth level and orientation relative to the triangle.

### Temporal coherence and continuity by controlling depth level

In this work, visual continuity is maintained. CompEls newly added are generated in deep layers and then appear to the surface from inside. And extra CompEls are first pushed toward the deeper layer before disappearing.

### CompEl movement accompanying edge modification

#### Edge split

If the length of edge become longer than threshold d, the edge will be split. the barycenters of CompEls don’t move but be assigned new face.

#### Edge collapse

If the length of edge become shorter than threshold t, the edge will collapse and all faces involved are reshaped. And each barycenters of CompEls are projected new faces.

#### Edge flip

Edge flip is one of the ways of maintaining the distance of vertices without splitting or deleting edges.

#### Change in topological genus

Change in topological genus is to reconnect vertices to maintain distance of correctly without moving the coordinates of them.

### Techniques in rendering

#### Variability

CompEls are displayed using instance based modeling. Visual variability of each instance is acquired by using different texture and using low frequency noise.

#### Back CompEl Culling

If there is no back face culling function in graphic card, the system replaces CompEls on back face to the out of the view by comparing the eye direction to the normal vector of the quasi-uniform mesh which is closest to the CompEl’s barycenter.

#### CompEl squish

CompEl squish is the way of visual intersection prevention. By squeezing CompLes at a constant scale in the direction perpendicular to the canvas, CompEls are prevented to overlap in appearance.

## Result

Please refer to the original paper to see some resulting images.

## Discrete Differential-Geometry Operators for Triangulated 2-Manifolds

Today I’ve read the paper “Discrete Differential-Geometry Operators
for Triangulated 2-Manifolds
” by Mark Meyer, Mathieu Desbrun, Peter Schröder, Alan H. Barr.

## Novelty of the study

Deriving first and second order operators at the vertices of a mesh using the 1-ring(star neighborhood). In other words, providing the way to extend the definition of curvature from continuous surfaces to discrete meshes.

## Definitions of the curvature in continuous surface

The normal curvature and related notions are defined as follows.

## Approximation

The average calculation is restricted to be within the immediately neighboring triangles referred as the 1-ring.

The voronoi region is used as small area around point xi.

However, the formula for the Voronoi finite-volume area does not hold in the presence of obtuse angles. So mixed area is used in the actual calculation.

The Laplace-Beltrami operator K is computed.
The Laplace-Beltrami operator (the mean curvature normal operator) is a generalization of the Laplacian from flat spaces to manifolds.

## Plausible vegetation synthesis from a single photo

Today I’ve read the paper “Single-picture reconstruction and rendering of trees for plausible vegetation synthesis” by Argudo, Oscar; Chica, Antonio; Andujar, Carlos.

## Vegetation synthesis in DTM

Vegetation is often seen in Digital Terrain Model(DTM), but it takes too much effort for artist to model. There are some techniques to improve the efficiency in vegetation synthesis, L-systems, the space colonization algorithm and so on. However they require point cloud, multiple photographs or various parameter setting.

## Novelty of the study

This work proposes a pipeline to reconstruct and render plausible trees from a single photograph.

## Method of crown reconstruction from the photo.

1. Envelope mesh synthesis
2. Color synthesis
3. Relief synthesis
4. Radial distance map
5. Branches and leaves generation

### 1, Envelope mesh synthesis

Get tree silhouette pixels S using 8-connectivity from the photo.

Fix the silhouette points on the z = 0 plane and maintain their tangents of the point (i, j, 0) to be f(i, j). Then get crown mesh C as height map. To realize this constraint, define the outer silhouette S_o as the set of pixels outside C but adjacent to a silhouette pixel.

Crown C is determined by minimizing the thin-plate energy defined by a biharmonic equation inside the crown C. In actual calculation, each of unknown(x, y, z) are figured out using equation below. The matrix M is bilaplacian filter.

### 2, Color synthesis

Create color map from the photo by using the texture synthesis algorithm of Image texture tools(P. Harrison, 2005). Then create cube map from the　synthesized texture.

### 3, Relief synthesis

The next step is estimation of relief (depth) from luminance of the photo. At first, the luminance is normalized in the [0,1] range. The system computes a blur pyramid of the luminance. Lower levels (low-frequencies) will have larger weights, while higher levels are used to add small fine details to the relief.

Cube map of the relief is generated from the cube map of color in this way.

### 4, Radial distance map

Create radial distance map(RDM) that stores distance from the center of the crown to the surface, and set values by projection of the cube map of relief.

### 5, Branches and leaves generation

Branches are created by the Space Colonization Algorithm in the crown. And leaves are represented as set of billboards for simplicity.

## Rendering

There are 2 ways of rendering the tree.

1. Fragment-based relief mapping which is for distant trees
2. Billboard cloud with an RGBA texture drawing leaves and branches which is for close trees

Check the output trees generated by this method in the original paper.

## PointNet, deep neural network that consumes point cloud.

Today I’ve read the paper “PointNet: Deep Learning on Point Sets for 3D Classification and Segmentation” by Charles R. Qi, Hao Su, Kaichun Mo, Leonidas J. Guibas.

## What is PointNet?

PointNet is a neural network that directly consumes point cloud, unordered point set. While the architecture is simple, it provides an approach to object classification, part segmentation and semantic segmentation with a good performance.

## Novelty of the study

While typical CNN requires volume data, like voxel, or a collection of images, these representations cause a lack of detail while the sampling process. So PointNet take point clouds directly as an input. The input is (x, y, z) coordinates of N points, which are given in no particular order.

## Applications of PointNet

PointNet can perform well in 3 tasks below.

• 3D Object Classification
• 3D Object Part Segmentation
• Semantic Segmentation in Scene

## Architecture

### overall

Each one of input points is input to the same mlp(Multilayer perceptron), and features are extracted.

### Symmetric function

As a strategy to make a model invariant to input permutation, a symmetric function is used. A symmetric function is the function whose value is the same no matter the order of the given n arguments, n points in this case. In this model, the symmetric function is mlp network and max-pooling which aggregates point features.

### Local and global information aggregation

Both the local and global information is required for point segmentation. PointNet concatenate the global feature with point features and extract new per point features.

### Invariance to the transformation

The semantic labeling need to be invariant to geometric transformations of shapes. PointNet predicts an affine transformation matrix by a mini-network (T-Net). The mini-network is composed by basic modules of feature extraction, max pooling and fully connected layers.

## Visualization

They visualize critical point sets and upper-bound shapes in this paper. It enables us to summarize an input point cloud by a sparse set of key points.

## Weakly-Supervised Component Suggestions for Assembly Modeling

Today, I’ve read a paper “ComplementMe: Weakly-Supervised Component Suggestions for 3D Modeling” by Sung, Minhyuk; Su, Hao; Kim, Vladimir; Chaudhuri, Siddhartha; Guibas, Leonidas.

## What is assembly-based modeling?

It is one of the methods of modeling in which user selects and arrange components into the completed object. For example, a chair model is composed of components of back, seat, foot and handrail.

In incremental interactive assembly modeling, user chooses a complementary component one by one to add it into the partial assembly.
It is desirable for such modeling systems to suggest applicable parts as complementary components so that they require two key technical components, retrieval and placement.

The problem is that it takes too much time and effort to label components and create dataset for component suggestion.

This work proposes a method of weakly-supervised component suggestions for 3D modeling.

## Novelty of the study

This system has 3 neural net, Retrieval Network, Embedding Network and Placement network.

Retrieval Network is trained to take partial assembly X and output a probability distribution as a weighted gaussian distribution over the embedded space for each complementary component.
Embedding Network is trained to take part geometry Y and maps it to low-dimensional vector.
Retrieval Network and Embedding Network are trained jointly.
Placement network is trained to take X and Y and predict 3D coordinates, strictly translation of each complementary component.

Each architecture contains PointNet which takes 3D unordered points as input and classify into k class as output.

φ is weight, μ is mean and σ is standard deviation of Gaussian in the figure below.

## Future work

Because this method randomly sample points over the surface of the input shape, larger components sometimes have bigger influences than small essential components.