The decoupling
Every surrogate-matching method couples the synthetic data to a network being trained on it, and that coupling made ImageNet-1K expensive. In 2023 the best ImageNet-1K result tabulated in a TPAMI review was TESLA’s 27.9% at 50 images per class. SRe2L removed the coupling in three stages:
- Squeeze: train one teacher on the real data.
- Recover: synthesize images from noise by inverting the frozen teacher, matching its class predictions and the running mean and variance stored in its batch-normalization layers.
- Relabel: have the teacher label every augmented crop of the synthetic images, and train students on those soft labels.
With no student in the synthesis loop, resolution and evaluation architecture became free. SRe2L reported 46.8% at 50 IPC with ResNet-18 and 60.8% with ResNet-101, and synthesis 16–52× faster than trajectory matching.
Two branches, one diagnosis
Successors agreed on what was wrong: images of one class, recovered independently against the same global statistics, look alike (LPLD measures intra-class cosine similarity 0.84 for recovered images vs 0.70 for real ones). They disagreed on the remedy.
Inversion: add more, or better, teacher signal.
- a global-to-local crop curriculum (CDA)
- several backbones (G-VBSM), later weighted by measured usefulness (CV-DD) or split across architectures so no single one imprints its bias (PRISM)
- directed perturbation of the teacher (DWA)
- staggered optimization lengths within a class (DELT)
- a systematic audit of schedules and batch sizes (EDC)
- residual connections back to real pixels (FADRM)
Nearly all of these moved from noise to real-image initialization.
Realistic selection: stop inverting. RDED composes synthetic images from real patches scored by the teacher, and distills ImageNet-1K at 10 IPC in minutes rather than hours. NRR-DD optimizes only the non-critical regions of selected patches. CIM anchors a feature-gap objective on real images and drops batch-norm matching entirely.
What the evidence says, once the labels are controlled
With teacher relabeling, which is how nearly every paper reports results, tuned inversion methods now edge past RDED. At 10 IPC with ResNet-18 they cluster around 46–50% (EDC 48.6%, CV-DD 49.5%, PRISM 49.4%, CIM 48.7%), and at 50 IPC around 58–61%. These rankings are fragile, for three reasons.
The same method, reported five ways. SRe2L at 10 IPC with ResNet-18 is reported as 21.3% in its own paper, 31.1% when CIM re-runs it, 41.9% when DELT re-runs it with CDA’s configuration, and 43.1% as CV-DD’s tuned baseline. That 20-point spread from recipe alone is larger than most improvements claimed in the family. RD³ unifies batch size and learning-rate schedule across methods and finds the published 27.3-point gap shrinks to 6.7.
Take the labels away and the ranking inverts. Without relabeling (ImageNet-1K, 10 IPC, ResNet-18), SRe2L falls to 1.1% and G-VBSM to 0.8%, while RDED keeps 19.7% and CIM 22.0% (CIM, Table 7). With strict one-hot labels, RDED’s own images drop from 42.0% to 16.3% (NRR-DD). Hard truths about soft labels finds RDED to be the one large-scale method that reliably beats random real images under hard labels, though it can still trail the strongest coreset.
CIM’s explanation. Relabeling works only when synthetic images stay close enough to the real distribution that the teacher’s predictions on them stay meaningful. Inverted images drift; images built from real content do not. Realism is therefore not cosmetic in this family. It decides whether the teacher’s labels are valid.
The label bill
The relabel stage has a cost that images-per-class does not count. Soft labels exceed the images by more than 30× (LPLD), up to roughly 30 GB at 50 IPC. The bill is compressible:
- Batching synthesis within classes raises diversity enough that labels can be pruned at random by 40× (LPLD).
- Static labels on real images plus a learned compensator cut label storage by up to 99% (INFER).
- Two scalars per crop recover about 70% of the soft-vs-hard gap at roughly 500× less storage (NRR-DD).
- A storage-free, label-smoothed hard-label phase between soft phases beats pruned labels by 9 points at equal storage (HALD).
Where the family stands
Decoupled synthesis made ImageNet-scale condensation practical and remains the reference pipeline for large datasets. The evidence from 2025–2026 suggests reading its headline numbers as measurements of the whole teacher-plus-recipe pipeline. The images contribute most clearly when they stay anchored to real data. The open problems are hard-label evaluation, which only two papers in the family report for their own method; honest accounting of label storage; and cross-architecture robustness, where batch-norm inversion methods degrade sharply on vision transformers (RD³). See Evaluation.