  We introduce a novel diffusion transformer, LazyDiffusion, that generates
partial image updates efficiently. Our approach targets interactive image
editing applications in which, starting from a blank canvas or an image, a user
specifies a sequence of localized image modifications using binary masks and
text prompts. Our generator operates in two phases. First, a context encoder
processes the current canvas and user mask to produce a compact global context
tailored to the region to generate. Second, conditioned on this context, a
diffusion-based transformer decoder synthesizes the masked pixels in a "lazy"
fashion, i.e., it only generates the masked region. This contrasts with
previous works that either regenerate the full canvas, wasting time and
computation, or confine processing to a tight rectangular crop around the mask,
ignoring the global image context altogether. Our decoder's runtime scales with
the mask size, which is typically small, while our encoder introduces
negligible overhead. We demonstrate that our approach is competitive with
state-of-the-art inpainting methods in terms of quality and fidelity while
providing a 10x speedup for typical user interactions, where the editing mask
represents 10% of the image.
