cv-multilabel-rare-class-image-oversampling

Overview

Multi-label image data has a long-tail label distribution but each image has multiple labels at once, so per-row oversampling is ambiguous: which class drives the multiplier? The HPA-winning trick is to define a per-class multiplier vector (e.g. 1 for common classes, 2-4 for rare), and for each image take the max multiplier across its labels. Common-only images stay at 1 copy; an image carrying any rare class gets multiplied; images with multiple rare classes still get only one bump (you don't double-multiply). This shifts the rare-class effective frequency upward without bloating common-class samples or distorting label correlations the way per-class image generation would.

Quick Start

import pandas as pd

class Oversampling:
    def __init__(self, csv_path, multi):
        df = pd.read_csv(csv_path).set_index('Id')
        df['Target'] = [[int(i) for i in s.split()] for s in df['Target']]
        self.labels = df
        self.multi = multi  # per-class duplication factor

    def get(self, image_id):
        labels = self.labels.loc[image_id, 'Target']
        return max((self.multi[l] for l in labels), default=1)

# 28-class HPA example: rare classes get 4x, common stay at 1x
multi = [1,1,1,1,1,1,1,1, 4,4,4,1,1,1,1,4,
         1,1,1,1,2,1,1,1, 1,1,1,4]
sampler = Oversampling(LABELS_CSV, multi)
train_ids = [iid for iid in train_ids for _ in range(sampler.get(iid))]

Workflow

1. Compute per-class positive counts in the training set
2. Set multi[c] inversely proportional to count, but cap at ~4-8 (more inflates training time without lift)
3. For each image, lookup its labels and take the max multiplier
4. Materialize the oversampled ID list once at epoch start (or use a WeightedRandomSampler with the same ratios)
5. Shuffle the expanded list every epoch
6. Validate that the rare-class effective frequency moved closer to the head — but never to parity (full balance overfits the rare classes hard)

Key Decisions

• Max not sum across labels: summing double-counts images with multiple rare classes and inflates them disproportionately.
• Cap the multiplier at ~4-8: 10x oversampling causes per-image overfitting (model memorizes rare-class images).
• Hand-tune the multiplier vector: pure inverse-frequency over-corrects; eyeball the vector and check that the effective per-class loss is close to flat.
• Per-image, not per-pixel-augmentation: this is sample replication, not heavier augmentation; combine the two for best results.
• Don't apply at validation time: oversampled validation gives misleading metrics — keep validation at natural distribution.
• Equivalent via WeightedRandomSampler: PyTorch users can pass the per-image multipliers as weights instead of materializing duplicates.

References

• Pretrained ResNet34 with RGBY (0.460 public LB)