  We present iFusion, a novel 3D object reconstruction framework that requires
only two views with unknown camera poses. While single-view reconstruction
yields visually appealing results, it can deviate significantly from the actual
object, especially on unseen sides. Additional views improve reconstruction
fidelity but necessitate known camera poses. However, assuming the availability
of pose may be unrealistic, and existing pose estimators fail in sparse view
scenarios. To address this, we harness a pre-trained novel view synthesis
diffusion model, which embeds implicit knowledge about the geometry and
appearance of diverse objects. Our strategy unfolds in three steps: (1) We
invert the diffusion model for camera pose estimation instead of synthesizing
novel views. (2) The diffusion model is fine-tuned using provided views and
estimated poses, turned into a novel view synthesizer tailored for the target
object. (3) Leveraging registered views and the fine-tuned diffusion model, we
reconstruct the 3D object. Experiments demonstrate strong performance in both
pose estimation and novel view synthesis. Moreover, iFusion seamlessly
integrates with various reconstruction methods and enhances them.
