Introduction

Direct volume rendering (DVR) is a powerful technique to visualize complex structures within volumetric data. Its main advantage, compared to standard surface rendering, is the ability to concurrently display information about the surface and the interior of objects. This aids the user in conveying spatial relationships of different structures.

In medicine, visualization of volumetric datasets acquired by computed tomography (CT), magnetic resonance imaging (MRI), or ultrasound imaging helps to understand patient's pathological conditions, improves surgical planning, and has an important role in education. However, a typical data size of today's clinical routine is about $512 \times 512 \times 1024$ (12 bit CT data) and will increase in the near future due to technological advances in acquisition devices. Conventional slicing is of limited use for such large datasets due to the enormous amount of slices. However, providing interactive three-dimensional volume visualization of such large datasets is a challenging task.