Hatch Textures for Virtual Endoscopy

In contrast to conventional interventions, minimal-invasive surgery can drastically reduce operation trauma and decrease healing time of the patient. In this intervention, the surgeon assesses abnormal changes in the patient’s organs with an endoscope, inserted into the patient’s body through a small hole.

With virtual endoscopy a visual inspection of the respective organs of the patient is possible without any surgery at all. A virtual endoscopic scene, through which the surgeon can navigate on a computer screen, is constructed from patient-specific 3D models of the intervention area, which are reconstructed from patient-specific medical imaging data. Additional to diagnosis, virtual endoscopy is also suited for screening, access planning and post-operative evaluation, depending on the clinical application area.
However, the projection of the 3D models to the screen leads to a reduction of depth cues in the virtual scene. This hinders the perception of distances in the virtual scene as well as the perception of 3D shape of the virtual objects.
The most important depth cue in virtual endoscopy is the relative movement of objects due to shifts in perspective (motion parallax). But also, appropriate texturing, like with hatch textures, provides further depth cues and therefore eases the perception of distance and 3D shape. This is especially important in still scenes, where no information from motion parallax is present.  

Based on the work of Kim et al. hatching follows the curvature of the 3D objects in our virtual endoscopy system. It adds two more depth cues to the virtual scene. First, there is “shape from texture”. Hatching lines appear denser the farther away the objects are from the virtual camera. The human visual system therefore can deduce distance from the arrangement of the hatching. On the other hand, hatch textures add perspective to the scene. The texture patterns consist of parallel lines, which converge towards a vanishing point and enable the human to deduce the relative distance of two objects in space.

Starting with two textures, that have been developed and evaluated by Kim and colleagues, we integrated different hatch textures to our system and applied a texturing technique called mipmaps, to account for aliasing effects. For the design of the textures stroke regularity, lightness, and contrast have a huge influence on the perception of the virtual scene. Figure 1 shows a virtual scene with different texturing styles.