Shear warp Įxample of a mouse skull (CT) rendering using the shear warp algorithm Flat disks and those with other kinds of property distribution are also used depending on the application. These splats are rendered as disks whose properties (color and transparency) vary diametrically in normal ( Gaussian) manner. Here, every volume element is splatted, as Lee Westover said, like a snow ball, on to the viewing surface in back to front order. This is a technique which trades quality for speed. The process is repeated for every pixel on the screen to form the completed image. The RGBA color is converted to an RGB color and deposited in the corresponding image pixel. 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. Then the ray is sampled at regular or adaptive intervals throughout the volume. The ray is clipped by the boundaries of the volume in order to save time.
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. In this technique, a ray is generated for each desired image pixel. 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. It provides results of very high quality, usually considered to provide the best image quality. The technique of volume ray casting can be derived directly from the rendering equation. The image was rendered by Fovia's High Definition Volume Rendering® engine Rebecca Fahrig, Department of Radiology, Stanford University, using a Siemens SOMATOM Definition, Siemens Healthcare. Hearst Museum of Anthropology, UC Berkeley. Crocodile mummy provided by the Phoebe A. For instance, a shear warp implementation could use texturing hardware to draw the aligned slices in the off-screen buffer. The way this is done depends on the rendering technique.Ī combination of these techniques is possible. 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. This is done with a " transfer function" which can be a simple ramp, a piecewise linear function or an arbitrary table. Nevertheless, the epitomes of volume rendering models feature a mix of for example coloring and shading in order to create realistic and/or observable representations.Ī direct volume renderer requires every sample value to be mapped to opacity and a color. Still, technically, all volume renderings become projections when viewed on a 2-dimensional display, making the distinction between projections and volume renderings a bit vague. Volume rendering is distinguished from thin slice tomography presentations, and is also generally distinguished from projections of 3D models, including maximum intensity projection. Types of presentations of CT scans, with two examples of volume rendering. Direct volume rendering is a computationally intensive task that may be performed in several ways. The marching cubes algorithm is a common technique for extracting an isosurface from volume data. This is usually defined using an RGBA (for red, green, blue, alpha) transfer function that defines the RGBA value for every possible voxel value.įor example, a volume may be viewed by extracting isosurfaces (surfaces of equal values) from the volume and rendering them as polygonal meshes or by rendering the volume directly as a block of data. Also, one needs to define the opacity and color of every voxel. To render a 2D projection of the 3D data set, one first needs to define a camera in space relative to the volume. This is an example of a regular volumetric grid, with each volume element, or voxel represented by a single value that is obtained by sampling the immediate area surrounding the voxel. Usually these are acquired in a regular pattern (e.g., one slice for each millimeter of depth) and usually have a regular number of image pixels in a regular pattern. In scientific visualization and computer graphics, volume rendering is a set of techniques used to display a 2D projection of a 3D discretely sampled data set, typically a 3D scalar field.Ī typical 3D data set is a group of 2D slice images acquired by a CT, MRI, or MicroCT scanner. Volume rendered CT scan of a forearm with different color schemes for muscle, fat, bone, and blood