Improved Diffusion-based Image Colorization
via Piggybacked Models

1The Chinese University of Hong Kong, 2Caritas Institute of Higher Education


Image colorization has been attracting the research interests of the community for decades. However, existing methods still struggle to provide satisfactory colorized results given grayscale images due to a lack of human-like global understanding of colors. Recently, large-scale Text-to-Image (T2I) models have been exploited to transfer the semantic information from the text prompts to the image domain, where text provides a global control for semantic objects in the image. In this work, we introduce a colorization model piggybacking on the existing powerful T2I diffusion model. Our key idea is to exploit the color prior knowledge in the pre-trained T2I diffusion model for realistic and diverse colorization. A diffusion guider is designed to incorporate the pre-trained weights of the latent diffusion model to output a latent color prior that conforms to the visual semantics of the grayscale input. A lightness-aware VQVAE will then generate the colorized result with pixel-perfect alignment to the given grayscale image. Our model can also achieve conditional colorization with additional inputs (e.g. user hints and texts). Extensive experiments show that our method achieves state-of-the-art performance in terms of perceptual quality.


Method overview. Our framework consists of two components: a latent diffusion guider model and a lightness-aware VQVAE model. When the grayscale image \(I_g\) and the optional textual descriptions \(t\) or hint points \(\{h\}\), then the latent diffusion guider model guides the pretrained Stable Diffusion model to generate a "colorized" latent \(z_c\) through the denoising diffusion process. The lightness-aware VQVAE model then uses \(z_c\) as the latent color prior and incorporates the grayscale information of \(I_g\) to produce a pixel-aligned colorization \(I_c\).


Unconditional image colorization

Unconditional image colorization with old photos


      title={Improved Diffusion-based Image Colorization via Piggybacked Models}, 
      author={Hanyuan Liu and Jinbo Xing and Minshan Xie and Chengze Li and Tien-Tsin Wong},