Hypercool: Reducing Encoding Cost in Overfitted Codecs with Hypernetworks

Overfitted image codecs like Cool-chic achieve strong compression by tailoring lightweight models to individual images, but their encoding is slow and computationally expensive. To accelerate encoding, Non-Overfitted (N-O) Cool-chic replaces the per-image optimization with a learned inference model, trading compression performance for encoding speed. We introduce HyperCool, a hypernetwork architecture that mitigates this trade-off. Building upon the N-O Cool-chic framework, HyperCool generates content-adaptive parameters for a Cool-chic decoder in a single forward pass, tailoring the decoder to the input image without requiring per-image fine-tuning. Our method achieves a 4.9% rate reduction over N-O Cool-chic with minimal computational overhead. Furthermore, the output of our hypernetwork provides a strong initialization for further optimization, reducing the number of steps needed to approach fully overfitted model performance. With fine-tuning, HEVC-level compression is achieved with 60.4% of the encoding cost of the fully overfitted Cool-chic. This work proposes a practical method to accelerate encoding in overfitted image codecs, improving their viability in scenarios with tight compute budgets.