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Kuzushiji Recognition

https://www.kaggle.com/c/kuzushiji-recognition/

Install

Use Python 3.6.

Install:

pip install -r requirements.txt
python setup.py develop

Install libjpeg-turbo for your OS as documented at https://github.com/ajkxyz/jpeg4py

Install apex as documented at https://github.com/NVIDIA/apex (python-only is good enough)

Run

All steps required for submission are in heavily commented run-all.sh and level2.sh. Note that run-all.sh was never run in one step (it would take too much time), so may contain errors. Most model parameters are in the _runs folder (they come from actual models).

One extra model was used to create pseudolabels which is not used in the final solution (resnext101_32x16d_wsl on fold0 with batch size 10, instead of resnext101_32x8d_wsl on the same fold), but it's contribution is extremely minor and quality is very similar to resnext101_32x8d_wsl.

All run logs and configs (for classification models) are in _runs folder.

Overview

Note: a slighly more complete description is at https://www.kaggle.com/c/kuzushiji-recognition/discussion/112712

General approach is as follows:

  • Dataset is split into 5 folds by book.
  • Class-agnostic bounding boxes are predicted for all characters using an object detection network (with resnet152 backbone pretrained on ImageNet). Out-of-fold predictions are obtained for all 5 folds. This is done in kuzushiji.segment module.
  • A "classification" model is trained using OOF detection predictions. An extra class seg_fp (segmentation false-positive) is added for bounding boxes which have low overlap with ground truth boxes, so classification model can correct errors of segmentation model. Classification model is trained on all folds. This is done in kuzushiji.classify module (knn, blend and language modeling are not used). Models with resnet152 and resnext101_32x8d_wsl backbones are used, they are trained on large crops containing multiple symbols, using FPN and roi align with a classification head.
  • Pseudolabelling is performed, in OCR terms this is similar to "writer adaptation", although here it is applied to the whole test for simplicity.
  • A second level model is trained on classification predictions, which creates the final submission.

Why such approach was chosen? There are two other candidate approaches:

  • End-to-end model which does detection and classification (e.g. Faster-RCNN). This may be possible with some effort, but here it seems that segmentation is quite easy, while classification is hard, and it's more convenient to tune a classification model alone without worrying about detection, also pipeline is easier and more flexible.
  • A separate detection model, and then a classifier on single-character crops. This is probably the easiest approach to get a reasonable result, and makes it very easy to improve a classification model. Still I felt that using larger crops as inputs should provide better context for the model, so that it can see nearby symbols and would not suffer from not ideal crops. But it could be that classification on character crops can be better.

Next come more details on each stage.

Segmentation

Segmentation into characters is done with a Faster-RCNN model with resnet152 backbone trained with torchvision. Only one class is used, so it does not try to predict the character class. This model trains very fast and gives high quality boxes. Competition F1 metric (assuming perfect prediction for the classes) was around ~0.99 on validation.

Some details:

  • torchvision detection pipeline was adapted,
  • resnet152 backbone worked a bit better than default resnet50 (even though it was not pre-trained on COCO, doing this would offer another small boost),
  • pipeline was modified to accept empty crops (crops without ground truth objects) to reduce amount of false positives,
  • it was trained on 512x384 crops, with page height around 1500 px, and full pages were used for inference,
  • augmentations used: scale, minor color augmentations (hue/saturation/value), Albumentations library was used.

Overall many more improvements are possible here: using mmdetection, better models, pre-training on COCO, blending predictions from different folds for submission, TTA, separate model to discard out-of-page symbols, etc. Still it seemed that classification was more important.

See kuzushiji.segment, which is based on torchvision detection reference, and kuzushiji.segment.dataset for the dataset.

Classification

Classification is performed by a model which gets as input a large crop from the image (512x768 from 2500-3000px high image) which contains multiple characters. It also recieves as input boudning boxes predicted by segmentation model (these are out-of-fold predictions). This is similar to multi-class detection, but with frozen bounding boxes. ResNet base is used, layer4 is discarded, features are extracted for each bounding box with roi_align from layer2 and layer3 and concatenated, and then passed into a classification head.

Some details:

  • surprisingly, details such as architecture, backbone and learning regime made a lot of difference, much more than usual.
  • head with two fully-connected layers and two 0.5 dropout layers was used, and all details were important: features from roi pooling were very high-dimentional (more than 13k), first layer reduced this to 1024, and second layer performed final classification. Addng more layers or removing intermediate bottleneck reduced quality.
  • bigger backbones made a big difference, best model was the largest that could fit into 2080ti with a reasonable batch size: resnext101_32x8d_wsl from https://github.com/facebookresearch/WSL-Images
  • in order to train resnext101_32x8d_wsl on 2080ti, mixed precision training was required along with freezing first convolution and whole layer1 (as I learned from Arthur Kuzin, this is a trick used in mmdetection: https://github.com/open-mmlab/mmdetection/blob/6668bf0368b7ec6e88bc01aebdc281d2f79ef0cb/mmdet/models/backbones/resnet.py#L460)
  • another trick for reducing memory usage and making it train faster with cudnn.benchmark was limiting and bucketing number of targets in one batch.
  • model was very sensitive to hyperparameters such as crop size and shape and batch size (and I had a bug in gradient accumulation).
  • SGD with momentum performed significantly better than Adam, cosine schedule was used, weight decay was also quite important.
  • quite large scale and color augmentations were used: hue/saturation/value, random brighness, contrast and gamma, all from Albumentations library.
  • TTA (test-time-augmentation) of 4 different scales was used.
  • resnext101_32x8d_wsl took around 15 hours to train on one 2080ti.

Best single model without pseudolabelling obtained public LB score of 0.935, although score varied quite a lot between folds, most folds were in 0.925 - 0.930 range. A blend of resnet152 and resnext101_32x8d_wsl models across all folds scored 0.941 on the public LB.

Overall, many improvement are possible here, from just using bigger models and freezing less layers, to more work on training schedule, augmentations, etc.

See kuzushiji.classify.main for the training script, kuzushiji.classify.models for the models, and kuzushiji.classify.dataset for the dataset and augmentations.

Pseudolabelling

Pseudolabelling is a technique where we take confident predictions of our model on test data, and add this to train dataset. Even though the model is already confident in such predictions, they are still useful and improve quality, because they allow the model to adapt better to different domain, as each book has it's own character and paper style, each author has different writing, etc.

Here the simplest approach was chosen: most confident predictions were used for all test set, instead of splitting it by book. Top 80% most confident predictions from the blend were used, having accuracy >99% according to validation. Next, two kinds of models were trained (all based on resnext101_32x8d_wsl):

  • models from previous step fine-tuned for 5 epochs (compared to 50 epochs for training from scratch) with starting learning rate 10x smaller than initial learning rate.
  • models trained from scratch with default settings.

In both cases, models used both train and test data for training. Best fine-tuned model scored 0.938 on the public LB. From-scratch models were not submitted separately but from their contribution to the ensemble, they could be even better.

See kuzushiji.classify.pseudolabel for creation of test targets.

Second level model

A simple blend worked already quite well, giving 0.943 public LB (without pseudolabelled from-scratch models). Adjusting coefficients of the models didn't improve the validation score, even though resnext101_32x8d_wsl models were noticeably better.

Since all models were trained across all folds, it was possible to train a second level model, a blend of LightGBM and XGBoost. This model was inspired by Pavel Ostyakov's solution to Cdiscount’s Image Classification Challenge, which was a classification problem with 5k classes: https://www.kaggle.com/c/cdiscount-image-classification-challenge/discussion/45733

Each of 4 model kinds from classification contributed classes and scores of top-3 predictions as features. Also max overlap with other bboxes was added. Then for each of all classes in top-3 predictions, and for a seg_fp class, we created one row with an extra feature candidate, which had a class as a value, and the target is binary: whether this candidate class was a true class which should be predicted. Then for each top-3 class, we added an extra binary feature which tells whether this class is a candidate class.

Here is a simplified example with 1 model and top-2 predictions, all rows created for one character prediction (seg_fp was encoded as -1, top0_s means top0_score, top0_is_c means top0_is_candidate):

top0_cls  top1_cls  top0_s  top1_s  candidate  top0_is_c  top1_is_c  y
83        258       15.202  7.1246  83         True       False      True
83        258       15.202  7.1246  258        False      True       False
83        258       15.202  7.1246  -1         False      False      False

XGBoost and LighGBM models were trained across all folds, and then blended. It was better to first apply models to fold predictions on test and then blend them.

Such blend gives 0.949 on public LB.

I'm extremely bad at tuning such models, so there may be more improvements possible. Adjusting seg_fp ratio was tried and provided some boost on validation but didn't work on public LB.

See kuzushiji.classify.level2_features where main features are created, and kuzushiji.classify.level2 where model are trained.

Discarded ideas

  • language model: a simple bi-LSTM language model was trained, but it achieved log loss of only ~4.5, while image-base model was at ~0.5, so it seemed that it would provide very little benefit.
  • kNN/metric learning: it's possible to use activations before the last layer as features, extract them from train and test, and then at inference time look closest (by cosine distance) example from train. This gave a minor boost over classification for single models, but inference time was quite high even with all optimizations, blending was less clear, so this was discarded.

Hardware and libraries

Almost all models were trained on my home server with one 2080ti. resnet152 classification models were trained on GCP with P100 GPUs as they required 16 GB of memory and I had some GCP credits. A few models towards the end were trained on vast.ai.

All models are written with pytorch, detection models are based on torchvision. Apex is used for mixed precision training, and Albumentations for augmentations.

License

License is MIT. Files under detection are taken from torchvision with minor modifications, which is licensed under BSD-3-Clause. Also files in kuzushiji.segment are based on detection reference from torchvision under the same license.