A direct volume renderer requires every sample value to be mapped to opacity and a color. This is done with a "
transfer function" which can be a simple ramp, a
piecewise linear function or an arbitrary table. Once converted to an
RGBA color model (for red, green, blue, alpha) value, the composed RGBA result is projected on the corresponding pixel of the frame buffer. The way this is done depends on the rendering technique. A combination of these techniques is possible. For instance, a shear warp implementation could use texturing hardware to draw the aligned slices in the
off-screen buffer.
Volume ray casting The technique of volume ray casting can be derived directly from the
rendering equation. It provides results of very high quality, usually considered to provide the best image quality. Volume ray casting is classified as image based volume rendering technique, as the computation emanates from the output image, not the input volume data as is the case with object based techniques. In this technique, a ray is generated for each desired image pixel. Using a simple camera model, the ray starts at the center of projection of the camera (usually the eye point) and passes through the image pixel on the imaginary image plane floating in between the camera and the volume to be rendered. The ray is clipped by the boundaries of the volume in order to save time. Then the ray is sampled at regular or adaptive intervals throughout the volume. The data is interpolated at each sample point, the transfer function applied to form an RGBA sample, the sample is composited onto the accumulated RGBA of the ray, and the process repeated until the ray exits the volume. The RGBA color is converted to an RGB color and deposited in the corresponding image pixel. The process is repeated for every pixel on the screen to form the completed image.
Splatting This is a technique which trades quality for speed. Here, every volume element is
splatted, as Lee Westover said, like a snow ball, on to the viewing surface in back to front order. These splats are rendered as disks whose properties (color and transparency) vary diametrically in normal (
Gaussian) manner. Flat disks and those with other kinds of property distribution are also used depending on the application.
Shear warp The shear warp approach to volume rendering was developed by Cameron and Undrill, popularized by Philippe Lacroute and
Marc Levoy. In this technique, the
viewing transformation is transformed such that the nearest face of the volume becomes axis aligned with an off-screen image
data buffer with a fixed scale of voxels to pixels. The volume is then rendered into this buffer using the far more favorable memory alignment and fixed scaling and blending factors. Once all slices of the volume have been rendered, the buffer is then warped into the desired orientation and scaled in the displayed image. This technique is relatively fast in software at the cost of less accurate sampling and potentially worse image quality compared to ray casting. There is memory overhead for storing multiple copies of the volume, for the ability to have near axis aligned volumes. This overhead can be mitigated using
run length encoding.
Texture-based volume rendering and
diffuse reflection Many 3D graphics systems use
texture mapping to apply images, or textures, to geometric objects. Commodity PC
graphics cards are fast at texturing and can efficiently render slices of a 3D volume, with real time interaction capabilities.
Workstation GPUs are even faster, and are the basis for much of the production volume visualization used in
medical imaging, oil and gas, and other markets (2007). In earlier years, dedicated 3D texture mapping systems were used on graphics systems such as
Silicon Graphics InfiniteReality,
HP Visualize FX
graphics accelerator, and others. This technique was first described by
Bill Hibbard and Dave Santek. These slices can either be aligned with the volume and rendered at an angle to the viewer, or aligned with the viewing plane and sampled from unaligned slices through the volume. Graphics hardware support for 3D textures is needed for the second technique. Volume aligned texturing produces images of reasonable quality. The image quality degrades with coarse slice counts, though there is often a noticeable transition when the volume is rotated. ==Hardware-accelerated volume rendering==