Mechanisms of Projective Composition of Diffusion Models
This work addresses a fundamental gap in theoretical understanding for researchers in generative modeling, though it appears incremental as it builds on prior empirical observations.
This paper tackles the problem of understanding how and why composition works in diffusion models, particularly for out-of-distribution extrapolation and length-generalization, by theoretically analyzing when linear score combinations achieve projective composition and proposing a heuristic to predict composition success.
We study the theoretical foundations of composition in diffusion models, with a particular focus on out-of-distribution extrapolation and length-generalization. Prior work has shown that composing distributions via linear score combination can achieve promising results, including length-generalization in some cases (Du et al., 2023; Liu et al., 2022). However, our theoretical understanding of how and why such compositions work remains incomplete. In fact, it is not even entirely clear what it means for composition to "work". This paper starts to address these fundamental gaps. We begin by precisely defining one possible desired result of composition, which we call projective composition. Then, we investigate: (1) when linear score combinations provably achieve projective composition, (2) whether reverse-diffusion sampling can generate the desired composition, and (3) the conditions under which composition fails. We connect our theoretical analysis to prior empirical observations where composition has either worked or failed, for reasons that were unclear at the time. Finally, we propose a simple heuristic to help predict the success or failure of new compositions.