# Learning from Scarce Information: Using Synthetic Data to Classify Roman Fine Ware Pottery

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## Abstract

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## 1. Introduction

## 2. Materials and Methods

- The four neural network architectures used in this experiment were modified from their standard and initialised with the ImageNet weights (see Section 2.2).
- Three different sets of simulated pottery vessels were generated so that the impact of (the quality of) simulation on the classifier’s performance could be assessed by comparison. The production of the synthetic datasets is discussed in Section 2.3.
- Training, validating, and testing the different neural networks was done using the smartphone photographs of real terra sigillata vessels from the Museum of London. To make sure these photographs were usable, we created an algorithm which automatically detects the pot, centers it in the photograph and crops out unnecessary surroundings. This process is detailed in Section 2.4.
- To mitigate the impact of small sample size on our performance metrics, we created 20 different training-validation-test partitions, the creation process of which is detailed in Section 2.5.
- We then trained each of the combinations of four networks and four sets of initial weights with these partitions.
- The results of each of 16 combinations of network architectures and pre-training regimes were assessed across the 20 training-validation-test partitions. The definition of the metrics used for this evaluation is discussed in Section 2.6, the results themselves are detailed in Section 3.

#### 2.1. Data Collection

- With the vessel placed upright on its base and assuming the origin of coordinates is located at the center of the pot, photographs were taken from azimuth angles of 0, 45 and 90 degrees and declination angles 0, 45 and 90 degrees. A last photograph with a declination higher than 90 degrees was taken by resting the mobile on the table.
- The vessel was rotated by an azimuth angle of 90 degrees and the process of point 1 repeated.
- The vessel was then turned upside down, thus using the rim to support it, and 4 photographs at azimuth 45 degrees from the declination detailed in point 1 were taken.

#### 2.2. Neural Nets Configurations

- Backbone convolutional neural net base architecture (i.e., the last layers of these architectures were removed until the convolutional structure);
- A global average pooling layer after the convolutional structure;
- A drop out layer with 0.3 exclusion probability;
- A final bottleneck dense layer with softmax activation, that is, a linear dense layer followed by a softmax transformation of the outputs.

#### 2.3. Pot Simulations

- Matplotlib (1000 images per class):To generate this dataset, the Python package Matplotlib [52] was used. The simulated pot was floated in a homogeneous background, no shadow is projected but the pot color is affected by the light. The light source is a point far away so only the angle has been changed. The profiles were softened to avoid their small defects creating circular patterns that could be identified by the neural net. For the same reason, a small amount of random noise was added to the surface mesh points position.
- Blender1 (1400 images per class):This dataset was generated using Blender [53]. We built a background set close to the original photography setting with a uniform color. The lighting conditions were randomly chosen using the different lighting classes available, namely: sun, spot, point. Thanks to the rendering feature Cycles, we could simulate shadow projections as well as changes of illumination in both pot and setting. The profile was softened by distance using the software and no noise was added. The material properties where not changed from the default ones except for the color.
- Blender2 (1300 images per class):The process followed to create this dataset is similar to the one used to create Blender1, however, the material properties were changed to make them more similar to the terra sigillata pots. Thus, some images exhibit simulated pots with a reflective material which creates light-saturated regions in the image of the pot. We also added decoration and motifs to some classes, namely:
- -
- Dr24–25: half of the images show a striped pattern near the top.
- -
- Dr35 & Dr36: half of the images show a pattern of four or six leaves on their rim.
- -
- Dr29 & Dr37: The decoration has a lot of variability in reality. We have simulated it in half of the images through two noise pattern displacements (Blender Musgrave and Magic textures).

No other decoration or characteristic details were reproduced in our simulated data. Finally, we sieved each synthetic dataset, removing images that were taken from too close or show some defects resulting from the simulated breaking procedure.

#### 2.4. Pot Detector: Cropping and Centering the Images

Algorithm 1: Locate and crop pot images |

Input: Image Output: Image_output. Cropped and centered pot image. scaled_image = scale input Image to $30\times 30$;scaled_image = change color basis from RGB to HSV;dataset_pixels = create dataset where each pixel in scaled_image have features: [Saturation, Value, ${\mathrm{position}}_{x}$, ${\mathrm{position}}_{y}$]; dataset_pixels = linearly normalize dataset_pixels features into [0,1] interval;pixel_group = assign cluster to each pixel using DBSCAN (Eps = 0.1, MinPts = 5) algorithm;dataset_pixels = dataset_pixels remove pixels of groups with less than 25 pixels;group_distance = for all groups compute mean(${({\mathrm{position}}_{x}-0.5)}^{2}$+${({\mathrm{position}}_{y}-0.5)}^{2}$ );pot_group = argmin(group_distance);dataset_pixels = dataset_pixels remove pixels not in pot_group;${L}_{x}$ = max(${\mathrm{position}}_{x}$)) − min(${\mathrm{position}}_{x}$));${L}_{y}$ = max(${\mathrm{position}}_{y}$)) − min(${\mathrm{position}}_{y}$));scale ${L}_{x}$ and ${L}_{y}$ to the original size;L = $1.5\phantom{\rule{0.277778em}{0ex}}\xb7$ max(${L}_{x}$, ${L}_{y}$);L_half = integer_part(L/2);${\mathrm{center}}_{x}$ = mean(${\mathrm{position}}_{x}$);${\mathrm{center}}_{y}$ = mean(${\mathrm{position}}_{y}$);Image_output = Image[ from ${\mathrm{center}}_{x}$ − L_half to ${\mathrm{center}}_{x}$ + L_half, from ${\mathrm{center}}_{y}$ − L_half to ${\mathrm{center}}_{y}$ + L_half]; |

#### 2.5. Training-Validation-Test Partitions

#### 2.6. Performance Metrics

- Uniform prior: all classes have the same probability and, thus, are equally weighted in the final metrics.$$\begin{array}{c}\hfill {P}_{\mathrm{test}}\left(G\right)=\frac{1}{\mathrm{card}\left(\mathcal{G}\right)}=\frac{1}{9}.\end{array}$$
- MoL prior: We assume that the MoL pot classes distribution is a good estimator of ${P}_{\mathrm{test}}\left(G\right)$. Let ${\mathcal{D}}^{\mathrm{dataset}}$ be the full dataset of our collection, the probability will be given by the number of pots of class i over the total number of pots:$$\begin{array}{c}\hfill {P}_{\mathrm{test}}(G=i)=\frac{\mathrm{card}\left(\{v\in \mathcal{V}|\exists (x,v,g)\in {\mathcal{D}}^{\mathrm{dataset}};g=i\}\right)}{\mathrm{card}\left(\{v\in \mathcal{V}|\exists (x,v,g)\in {\mathcal{D}}^{\mathrm{dataset}}\}\right)}.\end{array}$$

## 3. Results

#### 3.1. Accuracy

#### 3.2. Confusion Matrix

#### 3.3. Effects of Damage

#### 3.4. Effects of Viewpoint

#### 3.5. Reality Gap

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Example of plain variant of the Dragendorff 18 form (

**left**) and a rouletted variant of Dragendorff 18-31 (18-31R;

**right**). Both are part of our class Dr18. Photos taken by Arch-I-Scan with permission from the Museum of London.

**Figure 2.**Bar chart and table detailing the number of different pots per class in our dataset. Classes are labelled starting with Dr followed by the number of the Dragendorff form they are based on [19]. The table gives the exact number of vessels per class.

**Figure 3.**Daniël van Helden photographing a near complete terra sigillata vessel, with a complete profile from rim to base, in the antiquarian collection of the Museum of London (photo taken by Victoria Szafara).

**Figure 4.**Example of the three different perspectives (standard, zenith, flipped; seen in the three left-most images) and the damaged condition (right-most image) using photos from class Dr18. Note that images have already been through the automatic cropping procedure detailed in Section 2.4. Photos taken by Arch-I-Scan with permission from the Museum of London.

**Figure 5.**Diagram of the neural nets considered in the article. Four different base architectures are considered: Mobilenet v2, Inception v3, Resnet50 v2 and VGG19. The last layers of these architectures were removed up to the convolutional structure and substituted by a Global Average Pooling layer followed by a Drop Out layer with $0.3$ drop probability and a final dense layer with softmax activation and 9 nodes (one for each of the 9 pot classes).

**Figure 6.**Diagram of the pot simulation. Starting from a digitized profile drawing the profile is extracted as an ordered list of points. With that profile we generated simulated photographs using the package Matplotlib or the 3D modelling tool Blender.

**Figure 7.**2D diagram of the breaking procedure. Mesh points that fall within radius R (purple circle) from P (purple point) are susceptible to being removed. Points $\left\{p\right\}$ (green points) are inside the crown of radii ${r}_{\mathrm{min}}$ and ${r}_{\mathrm{max}}$ (green circles), the lines that separate mesh points closer to $\left\{p\right\}$ than P are indicated as a dashed cyan line. Black points in the pot mesh are not susceptible to be removed; orange ones are susceptible, but have been pardoned because they are closer to a green point $\left\{p\right\}$ than to P (purple); and lastly, red ones have been removed.

**Figure 8.**Examples of all simulated datasets and real photograph of our 9 different Dr classes. From left to right: Matplotlib, Blender1, Blender2, real photo. Photos taken by Arch-I-Scan with permission from the Museum of London.

**Figure 9.**Two plots showing accuracy results for the four architectures considered: Inception v3 (green), Resnet50 v2 (blue), Mobilenet v2 (orange) and VGG19 (red). The figure on the left shows accuracy using the uniform prior, the one on the right using the MoL prior. The different pre-training regimes considered are displayed along the x-axis. Each point indicates the value of $\widehat{\mathrm{acc}}$ with its two sigma error band draw as a line. The transparent band shows $\pm {\widehat{\sigma}}_{\mathrm{acc}}$.

**Figure 10.**Four plots showing the accuracy results of the pre-training regimes relative to each other, using the uniform prior, for the four networks: Inception v3 (

**superior left**), Resnet50 v2 (

**superior right**), Mobilenet v2 (

**inferior left**) and VGG19 (

**inferior right**). Each point represents a single training-validation-test partition with the system pre-trained with the Blender dataset (blue) and Matplotlib dataset (red) where their positions are given as the relative percentage with respect to the uniform prior accuracy in the same partition for Blender2 pre-training (Y axis) and ImageNet pre-training (X axis). As can be seen, most of the points are located above $0\%$ with respect to ImageNet accuracy and below for Blender2.

**Figure 11.**Plot detailing the accuracy of the Inception v3 model across the different viewpoints and comparing performance with Blender2 and ImageNet pre-training. Each point represents the accuracy for each pot class and viewpoint: standard (s, green triangle), zenith (t, orange cross), flipped (f, blue point). The dashed grey line shows the point where the accuracies of both pre-training regimes are the same. The fact that most of the points are well above these lines shows how pre-training with Blender2 improves the performance in general. Notice also that the standard view shows generally a better performance in both models in comparison with the zenith and flipped views.

**Figure 12.**Plot showing the variation in the accuracy of the Inception v3 model pre-trained with Blender2 across viewpoints and different pot classes.

**Figure 13.**Plot of the Inception v3 model accuracy for each pre-training for the different pot classes. The horizontal dashed grey line points out the random assignment accuracy $1/9$.

**Table 1.**Mobile phones used to take pictures of pots, in the MoL storerooms, and relevant properties of their cameras.

Phone Model | Camera Resolution | Aperture | Focal Length | Pixel Size |
---|---|---|---|---|

Samsung Galaxy A20e | 13 MP | f/1.9 | 28 mm | 1.4 $\mathsf{\mu}$m |

Apple iPhone 6 | 8 MP | f/2.2 | 29 mm | 1.5 $\mathsf{\mu}$m |

Motorola Moto E Play 5th gen. | 8 MP | f/2.0 | 28.3 mm | 1.12 $\mathsf{\mu}$m |

Samsung Galaxy A70 | 32 MP | f/1.7 | 26 mm | 0.8 $\mathsf{\mu}$m |

**Table 2.**Summary of accuracy results using the uniform prior. The different statistics are computed over the 20 test results for each partition (numbers are rounded up to two significant digits after zeros): $\overline{\mathrm{acc}}$ is the average accuracy with its two sigma bootstrap error, ${\sigma}_{\mathrm{acc}}$ is the variance estimation, ${\mathrm{min}}_{\mathrm{acc}}$ is the minimum result and ${\mathrm{max}}_{\mathrm{acc}}$ is the maximum. Each statistic is computed for the four architectures (model) and all the different initial weights considered (pre-train).

Model | Pretrain | $\widehat{\overline{\mathbf{acc}}}$ | ${\widehat{\mathit{\sigma}}}_{\mathbf{acc}}$ | ${\mathbf{min}}_{\widehat{\mathbf{acc}}}$ | ${\mathbf{max}}_{\widehat{\mathbf{acc}}}$ |
---|---|---|---|---|---|

Inception v3 | Blender2 | 0.818 ± 0.008 | 0.021 | 0.78 | 0.87 |

Inception v3 | Blender1 | 0.809 ± 0.011 | 0.025 | 0.76 | 0.84 |

Inception v3 | Matplotlib | 0.771 ± 0.014 | 0.033 | 0.72 | 0.85 |

Inception v3 | ImageNet | 0.719 ± 0.020 | 0.047 | 0.61 | 0.82 |

Resnet50 v2 | Blender2 | 0.779 ± 0.010 | 0.023 | 0.74 | 0.82 |

Resnet50 v2 | Blender1 | 0.765 ± 0.013 | 0.030 | 0.72 | 0.82 |

Resnet50 v2 | Matplotlib | 0.726 ± 0.014 | 0.034 | 0.67 | 0.82 |

Resnet50 v2 | ImageNet | 0.667 ± 0.020 | 0.047 | 0.55 | 0.77 |

Mobilenet v2 | Blender2 | 0.769 ± 0.014 | 0.032 | 0.72 | 0.84 |

Mobilenet v2 | Blender1 | 0.752 ± 0.012 | 0.028 | 0.70 | 0.82 |

Mobilenet v2 | Matplotlib | 0.705 ± 0.017 | 0.039 | 0.65 | 0.78 |

Mobilenet v2 | ImageNet | 0.655 ± 0.021 | 0.049 | 0.56 | 0.76 |

VGG19 | Blender2 | 0.735 ± 0.010 | 0.025 | 0.69 | 0.77 |

VGG19 | Blender1 | 0.727 ± 0.010 | 0.024 | 0.69 | 0.78 |

VGG19 | Matplotlib | 0.671 ± 0.017 | 0.039 | 0.60 | 0.74 |

VGG19 | ImageNet | 0.552± 0.024 | 0.057 | 0.47 | 0.65 |

**Table 3.**Summary of accuracy results using the MoL prior. The different statistics are computed over the 20 test results for each partition: $\overline{\mathrm{acc}}$ is the average accuracy with its two sigma bootstrap error, ${\sigma}_{\mathrm{acc}}$ is the variance estimation, ${\mathrm{min}}_{\mathrm{acc}}$ is the minimum result and ${\mathrm{max}}_{\mathrm{acc}}$ is the maximum. Each statistic is computed for the four architectures (model) and all the different initial weights considered (pre-train).

Model | Pretrain | $\widehat{\overline{\mathbf{acc}}}$ | ${\widehat{\mathit{\sigma}}}_{\mathbf{acc}}$ | ${\mathbf{min}}_{\widehat{\mathbf{acc}}}$ | ${\mathbf{max}}_{\widehat{\mathbf{acc}}}$ |
---|---|---|---|---|---|

Inception v3 | Blender2 | 0.821 ± 0.009 | 0.020 | 0.78 | 0.86 |

Inception v3 | Blender1 | 0.814 ± 0.008 | 0.020 | 0.78 | 0.85 |

Inception v3 | Matplotlib | 0.763 ± 0.015 | 0.035 | 0.71 | 0.82 |

Inception v3 | ImageNet | 0.698 ± 0.020 | 0.047 | 0.62 | 0.80 |

Resnet50 v2 | Blender2 | 0.778 ± 0.008 | 0.018 | 0.74 | 0.81 |

Resnet50 v2 | Blender1 | 0.772 ± 0.011 | 0.025 | 0.73 | 0.81 |

Resnet50 v2 | Matplotlib | 0.719 ± 0.012 | 0.029 | 0.66 | 0.79 |

Resnet50 v2 | ImageNet | 0.651 ± 0.018 | 0.042 | 0.54 | 0.74 |

Mobilenet v2 | Blender2 | 0.760 ± 0.012 | 0.028 | 0.71 | 0.81 |

Mobilenet v2 | Blender1 | 0.750 ± 0.014 | 0.032 | 0.69 | 0.81 |

Mobilenet v2 | Matplotlib | 0.699 ± 0.020 | 0.046 | 0.59 | 0.77 |

Mobilenet v2 | ImageNet | 0.663 ± 0.022 | 0.052 | 0.55 | 0.74 |

VGG19 | Blender2 | 0.732 ± 0.010 | 0.024 | 0.68 | 0.77 |

VGG19 | Blender1 | 0.719 ± 0.011 | 0.026 | 0.67 | 0.77 |

VGG19 | Matplotlib | 0.656 ± 0.014 | 0.034 | 0.58 | 0.72 |

VGG19 | ImageNet | 0.532 ± 0.021 | 0.048 | 0.46 | 0.61 |

**Table 4.**Normalized confusion matrix of Equation (8) for Inception v3 pre-trained with the ImageNet dataset. Diagonal elements are highlighted in green, whereas major instances of confusion are marked in red. Major instances of confusion are those off-diagonal elements which exceed double the expected probability if the incorrect identifications would have been uniformly distributed across the classes, see Equation (9).

Predicted Class | |||||||||
---|---|---|---|---|---|---|---|---|---|

Real Class | Dr18 | Dr24–25 | Dr27 | Dr29 | Dr33 | Dr35 | Dr36 | Dr37 | Dr38 |

Dr18 | 63 | 4 | 2 | 2 | 6 | 2 | 14 | 3 | 3 |

Dr24-25 | 5 | 76 | 5 | 1 | 3 | 3 | 2 | 1 | 4 |

Dr27 | 2 | 4 | 74 | 2 | 2 | 9 | 2 | 2 | 4 |

Dr29 | 2 | 2 | 3 | 74 | 2 | 1 | 4 | 7 | 5 |

Dr33 | 3 | 2 | 2 | 1 | 89 | 2 | 0 | 1 | 1 |

Dr35 | 1 | 4 | 11 | 1 | 1 | 67 | 11 | 1 | 3 |

Dr36 | 8 | 4 | 2 | 4 | 1 | 12 | 60 | 2 | 8 |

Dr37 | 2 | 9 | 1 | 11 | 3 | 1 | 1 | 68 | 4 |

Dr38 | 1 | 6 | 2 | 3 | 2 | 2 | 7 | 1 | 76 |

**Table 5.**Normalized confusion matrix of Equation (8) for Inception v3 pre-trained with the Blender2 dataset. Diagonal elements are highlighted in green, whereas major instances of confusion are marked in red. Major instances of confusion are those off-diagonal elements which exceed double the expected probability if the incorrect identifications would have been uniformly distributed across the classes, see Equation (9).

Predicted Class | |||||||||
---|---|---|---|---|---|---|---|---|---|

Real Class | Dr18 | Dr24–25 | Dr27 | Dr29 | Dr33 | Dr35 | Dr36 | Dr37 | Dr38 |

Dr18 | 81 | 2 | 1 | 3 | 2 | 1 | 7 | 2 | 1 |

Dr24-25 | 3 | 80 | 1 | 2 | 2 | 3 | 2 | 2 | 6 |

Dr27 | 1 | 2 | 87 | 1 | 3 | 3 | 1 | 1 | 1 |

Dr29 | 2 | 2 | 1 | 81 | 2 | 1 | 2 | 7 | 3 |

Dr33 | 0 | 1 | 0 | 1 | 95 | 1 | 0 | 0 | 0 |

Dr35 | 1 | 0 | 4 | 1 | 1 | 80 | 10 | 0 | 2 |

Dr36 | 6 | 2 | 0 | 1 | 1 | 11 | 72 | 1 | 6 |

Dr37 | 1 | 6 | 1 | 7 | 1 | 0 | 0 | 82 | 3 |

Dr38 | 2 | 3 | 2 | 1 | 1 | 3 | 8 | 1 | 79 |

**Table 6.**Table showing accuracy results of the Inception v3 model for vessels labelled damaged. $\overline{{n}^{\mathrm{dam}}}$ indicates the average number of damaged pots in each class at a split in the test set, thus lower numbers are less reliable. Notice that the results for damaged vessels of class Dr35 could not be computed as there are no damaged samples.

Damaged | |||||
---|---|---|---|---|---|

Class | ImageNet | Matplotlib | Blender1 | Blender2 | $\overline{{\mathit{n}}^{\mathrm{dam}}}$ |

Dr18 | 0.48 | 0.52 | 0.55 | 0.60 | 13.60 |

Dr24–25 | 0.31 | 0.56 | 0.41 | 0.36 | 0.20 |

Dr27 | 0.53 | 0.60 | 0.69 | 0.69 | 4.75 |

Dr29 | 0.69 | 0.73 | 0.77 | 0.72 | 5.55 |

Dr33 | 0.81 | 0.85 | 0.91 | 0.93 | 1.95 |

Dr35 | - | - | - | - | 0.00 |

Dr36 | 0.53 | 0.62 | 0.60 | 0.63 | 3.05 |

Dr37 | 0.56 | 0.33 | 0.69 | 0.76 | 0.40 |

Dr38 | 0.79 | 0.74 | 0.78 | 0.78 | 1.05 |

**Table 7.**Table showing accuracy results of the Inception v3 model for non-damaged vessels. $\overline{{n}^{\mathrm{no}}\phantom{\rule{4.pt}{0ex}}\mathrm{dam}}$ indicates the average number of non-damaged pots in each class at a split in the test set, thus lower numbers are less reliable.

Non-Damaged | |||||
---|---|---|---|---|---|

Class | ImageNet | Matplotlib | Blender1 | Blender2 | $\overline{{\mathit{n}}^{\mathrm{no}}\phantom{\rule{4.pt}{0ex}}\mathrm{dam}}$ |

Dr18 | 0.70 | 0.81 | 0.90 | 0.90 | 31.40 |

Dr24–25 | 0.86 | 0.89 | 0.85 | 0.89 | 1.80 |

Dr27 | 0.8 | 0.90 | 0.92 | 0.92 | 17.25 |

Dr29 | 0.79 | 0.82 | 0.89 | 0.88 | 6.45 |

Dr33 | 0.92 | 0.91 | 0.95 | 0.96 | 5.05 |

Dr35 | 0.67 | 0.69 | 0.78 | 0.80 | 6.00 |

Dr36 | 0.64 | 0.72 | 0.79 | 0.77 | 5.95 |

Dr37 | 0.64 | 0.86 | 0.79 | 0.82 | 2.60 |

Dr38 | 0.75 | 0.81 | 0.77 | 0.82 | 0.95 |

**Table 8.**Table showing summary statistics for the performance of the Inception v3 based model trained only with the simulated images of datasets: Matplotlib, Blender1, Blender2. The mean corresponds to an estimation of the uniform prior accuracy defined in Section 3.1. We can see the improvement in Blender datasets compared with Matplotlib dataset.

Matplotlib | Blender1 | Blender2 | ||||
---|---|---|---|---|---|---|

Model | Mean | Std | Mean | Std | Mean | Std |

Inception v3 | 0.20 | 0.28 | 0.35 | 0.21 | 0.33 | 0.24 |

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**MDPI and ACS Style**

Núñez Jareño, S.J.; van Helden, D.P.; Mirkes, E.M.; Tyukin, I.Y.; Allison, P.M.
Learning from Scarce Information: Using Synthetic Data to Classify Roman Fine Ware Pottery. *Entropy* **2021**, *23*, 1140.
https://doi.org/10.3390/e23091140

**AMA Style**

Núñez Jareño SJ, van Helden DP, Mirkes EM, Tyukin IY, Allison PM.
Learning from Scarce Information: Using Synthetic Data to Classify Roman Fine Ware Pottery. *Entropy*. 2021; 23(9):1140.
https://doi.org/10.3390/e23091140

**Chicago/Turabian Style**

Núñez Jareño, Santos J., Daniël P. van Helden, Evgeny M. Mirkes, Ivan Y. Tyukin, and Penelope M. Allison.
2021. "Learning from Scarce Information: Using Synthetic Data to Classify Roman Fine Ware Pottery" *Entropy* 23, no. 9: 1140.
https://doi.org/10.3390/e23091140