Augmented reality (AR) is a promising technology where the surgeon can see the medical abnormality in the context of the patient. It makes the anatomy of interest visible to the surgeon which otherwise is not visible. It can result in better surgical precision and therefore, potentially better surgical outcomes and faster recovery times. Despite these benefits, the current AR systems suffer from two major challenges; first, incorrect depth perception and, second, the lack of suitable evaluation systems. Therefore, in the current paper we addressed both of these problems. We proposed a color depth encoding (CDE) technique to estimate the distance between the tumor and the tissue surface using a surgical instrument. We mapped the distance between the tumor and the tissue surface to the blue-red color spectrum. For evaluation and interaction with our AR technique, we propose to use a virtual surgical instrument method using the CAD model of the instrument. The users were asked to reach the judged distance in the surgical field using the virtual tool. Realistic tool movement was simulated by collecting the forward kinematics joint encoder data. The results showed significant improvement in depth estimation, time for task completion and confidence, using our CDE technique with and without stereo versus other two cases, that are, Stereo-No CDE and No Stereo-No CDE.

Incorrect depth perception and lack of good evaluation systems are major barriers in clinical translation of augmented and mixed reality AR/MR. Thus, a systematic study of depth cues is necessary. Therefore, in the current paper we present a method to introduce the quantitative depth cue Motion Parallax (MP) in surgical scenes and study its effect on depth perception when combined with binocular disparity. In addition to this we present an innovative virtual tool method to evaluate depth. To introduce MP, we reconstructed the tissue surface using structure from motion technique. Then to get accurate absolute scale of the reconstructed surface stereo-triangulation was used. The simulated tumor was rendered beneath the reconstructed point-cloud by rendering a hole for `X-ray' like vision. The MP was introduced by rotating the entire scene from side-to-side with a tumor-surface-point as pivot for maximum impact. Finally for evaluation, we used a virtual surgical tool rendered using real-time da Vinci surgical API's forward kinematics data. In total, 12 subjects participated in a within-subjects-experiment design to study four cases, i.e., Stereo + MP ( S+MP), Mono + MP ( M+MP), Stereo + No MP (S+N-MP) and Mono + No MP (M+N-MP). The subjects significantly overestimated Judged Percentage of True Distance in M+MP when compared to M+N-MP (probability (p)=0.000, Number of Samples (N))=120) and S+N-MP cases ( p=0.001, N = 120). Furthermore, the observed VariableError was less in S+MP and S+N-MP cases when compared to M+MP and M+N-MP cases. The use of Motion Parallax in console interfaces for surgical robotics showed overestimation of judged distance. But to our knowledge it is the first work studying the effect of motion parallax and stereo in the surgical context. Therefore, its further study is warranted.