  Recently, diffusion models have shown remarkable results in image synthesis
by gradually removing noise and amplifying signals. Although the simple
generative process surprisingly works well, is this the best way to generate
image data? For instance, despite the fact that human perception is more
sensitive to the low frequencies of an image, diffusion models themselves do
not consider any relative importance of each frequency component. Therefore, to
incorporate the inductive bias for image data, we propose a novel generative
process that synthesizes images in a coarse-to-fine manner. First, we
generalize the standard diffusion models by enabling diffusion in a rotated
coordinate system with different velocities for each component of the vector.
We further propose a blur diffusion as a special case, where each frequency
component of an image is diffused at different speeds. Specifically, the
proposed blur diffusion consists of a forward process that blurs an image and
adds noise gradually, after which a corresponding reverse process deblurs an
image and removes noise progressively. Experiments show that the proposed model
outperforms the previous method in FID on LSUN bedroom and church datasets.
Code is available at https://github.com/sangyun884/blur-diffusion.
