The key highlights and insights of the content are:
The authors introduce "posterior flows" - generalizations of "probability flows" to a broader class of stochastic processes not necessarily diffusion processes. They propose a method to transform the posterior flow of a "linear" stochastic process into a straight constant-speed (SC) flow, which facilitates fast sampling along the original posterior flow without training a new model.
The authors show that the posterior flow of any linear stochastic process can be transformed into a straight flow and then into a straight constant-speed flow. This transformation involves "variable scaling" and "time adjustment" or "variable shifting" operations, which allow the authors to easily compute the velocity of the SC flow from the velocity of the original (posterior) flow.
The authors demonstrate that DDIM can be viewed as a special case of their method, which transforms the probability flow associated with VP SDE / DDPM into a straight constant-speed counterpart.
The authors explore the use of higher-order numerical ODE solvers, such as Runge-Kutta and linear multi-step methods, on the transformed SC flows to further enhance sample quality and reduce the number of sampling steps.
Comprehensive theoretical analysis and extensive experiments on a 2D toy dataset and large-scale image generation tasks validate the correctness and effectiveness of the proposed transformation method and high-order numerical solvers.
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arxiv.org
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