From U-Net to Diffusion Transformer — The Architecture Underneath the Denoiser
For the first years of diffusion, "the model" meant a U-Net — a convolutional hourglass borrowed from medical image segmentation. Then the same migration that had already run through NLP and vision ran through generation: replace the handcrafted spatial architecture with a plain transformer over patch tokens, and let scale do the work. The Diffusion Transformer (DiT) is now the backbone of the frontier image and video systems — and the reasons why are a case study in what architectures are actually for.
The denoiser was a U-Net — convolutional hourglass, skip connections, interleaved cross-attention — until the Diffusion Transformer re-answered the same requirements generically: patchify the latent, run a standard transformer, condition via adaptive layer norm. The real win is economic: LLM-style scaling curves, inherited infrastructure, and spacetime patches that set up video.