Volume and Surface Rendering

For volume processing, it is often useful to see "inside" a volume — that is, to construct 3D representations of internal structures inside that volume. The term 3D rendering refers to the computer graphics process of converting 3D models into 2D images for display on a 2D computer screen. This process considers the positioning of the objects in a 3D scene and creates a 2D image based on this perspective. Any change in the view, such as rotating the scene, generates a new render to reflect the new perspective of the 3D scene, as shown below.

In 3D graphics, the Viewing frustrum is the computed view from a virtual camera, represented here as a truncated pyramid lying on its side. Notice the smaller end of the pyramid is closest to the camera, while the large end is further way, representing foreshortening. Everything between the near and far clip planes is the viewing area, and will be rendered. Here, the "near clip plane" represents what is rendered in the 3D Scene. If you move the position of the camera, you change the position of the viewing frustrum and render a different image. For example, if you rotate the camera to the right, the red cylinder would appear in front of the yellow sphere.

There are two main types of rendering that we deal with in this course: Volume and Surface Rendering.

Volume Rendering

With Volume Rendering, you visualize 3D structures by adjusting the transparencies of the voxels of a 3D volume. Usually, the outer edge and background voxels are made completely transparent — to allow viewing inside the volume. Voxels that compose structures of interest are set to varying levels of opacity to reveal (or render) these structures. In effect, each voxel in volume has an opacity setting based on its intensity value. These opacities are set using a lookup table called an alphamap, which maps opacity to intensity.

This is often used as a quick and easy way to explore the internal structures of a volume. By simply adjusting the Alphamap, you can visualize different structures in the volume.

Surface Rendering

With Surface Rendering, you first need to create a 3D surface model, which is made up of vertices and faces (not voxels). Other pre-processing steps may be required, often making surface rendering more time consuming than volume rendering, but the final product can appear more realistic and detailed. In this method, only the surface is rendered while the rest of the volume is ignored.

Surface Rendering is useful when you want to:

• Visualize a segmented internal structure (e.g. like a femur or the heart)
• Need to rotate or translate the surface in 3D space
• Measure the extent of your 3D surface (e.g. the length of a Femur)
• Compare the shapes of two different surfaces
• 3D print a surface