Zippo: RGB-Alpha Joint Modeling With a Unified Diffusion Model

Recent advances in generative models have sparked growing interest in moving beyond pure image generation toward transparent image generation, i.e., joint generation of image and its alpha mask. However, most existing approaches adopt a two-stage pipeline, where a diffusion-based model first generates an RGB image and a subsequent matting head predicts the alpha mask. This separation not only leads to error accumulation and inaccurate predictions but also overlooks the intrinsic correlation between the cross-modal data. In this work, we introduce Zippo, a unified diffusion framework, zipping color and transparency distributions into a single diffusion model, by learning joint distribution of RGB image and alpha mask. Zippo not only generates high-fidelity images but also produces plausible and sharp alpha masks. In practice, Zippo inflates the latent space into a unified representation that encodes cross-modal data, and builds upon it with a modality-aware diffusion process that flexibly switches between RGB and alpha domains. In this process, conditioning on one modality while denoising the other allows the model to generate RGB images from alpha masks and predict transparency from input images. In addition to single-modality prediction, we further design a modality-aware noise reassignment strategy to empower Zippo with the joint generation capability of RGB images and their corresponding alpha masks under text guidance. With these techniques, Zippo supports a wide range of transparent image generation tasks, including image-alpha joint generation, image matting, and alpha mask conditioned image generation. Extensive experiments demonstrate that Zippo not only delivers superior visual fidelity but also achieves competitive performance in visual downstream prediction, highlighting joint image-alpha modeling as a powerful alternative to traditional paradigms.