  Enabling Large Language Models (LLMs) to interact with 3D environments is
challenging. Existing approaches extract point clouds either from ground truth
(GT) geometry or 3D scenes reconstructed by auxiliary models. Text-image
aligned 2D features from CLIP are then lifted to point clouds, which serve as
inputs for LLMs. However, this solution lacks the establishment of 3D
point-to-point connections, leading to a deficiency of spatial structure
information. Concurrently, the absence of integration and unification between
the geometric and semantic representations of the scene culminates in a
diminished level of 3D scene understanding. In this paper, we demonstrate the
importance of having a unified scene representation and reconstruction
framework, which is essential for LLMs in 3D scenes. Specifically, we introduce
Uni3DR^2 extracts 3D geometric and semantic aware representation features via
the frozen pre-trained 2D foundation models (e.g., CLIP and SAM) and a
multi-scale aggregate 3D decoder. Our learned 3D representations not only
contribute to the reconstruction process but also provide valuable knowledge
for LLMs. Experimental results validate that our Uni3DR^2 yields convincing
gains over the baseline on the 3D reconstruction dataset ScanNet (increasing
F-Score by +1.8\%). When applied to LLMs, our Uni3DR^2-LLM exhibits superior
performance over the baseline on the 3D vision-language understanding dataset
ScanQA (increasing BLEU-1 by +4.0\% and +4.2\% on the val set and test set,
respectively). Furthermore, it outperforms the state-of-the-art method that
uses additional GT point clouds on both ScanQA and 3DMV-VQA.
