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Volume rendering 240
Maximum intensity projection
As opposed to direct volume rendering, which requires
every sample value to be mapped to opacity and a color,
maximum intensity projection picks out and projects only
the voxels with maximum intensity that fall in the way of
parallel rays traced from the viewpoint to the plane of
projection.
This technique is computationally fast, but the 2D results
do not provide a good sense of depth of the original data.
To improve the sense of 3D, animations are usually
rendered of several MIP frames in which the viewpoint is
slightly changed from one to the other, thus creating the
illusion of rotation. This helps the viewer's perception to
find the relative 3D positions of the object components.
This implies that two MIP renderings from opposite
viewpoints are symmetrical images, which makes it
impossible for the viewer to distinguish between left or
right, front or back and even if the object is rotating
clockwise or counterclockwise even though it makes a
significant difference for the volume being rendered.
CT visualized by a maximum intensity projection of a mouse
MIP imaging was invented for use in nuclear medicine
[7] [8] [9]
by Jerold Wallis, MD, in 1988, and subsequently published in IEEE Transactions in Medical Imaging.
Suprisingly, an easy improvement to MIP is Local maximum intensity projection. In this technique we don't take the
global maximum value, but the first maximum value that is above a certain threshold. Because - in general - we can
terminate the ray earlier this technique is faster and also gives somehow better results as it approximates
occlusion [10] .
Hardware-accelerated volume rendering
Due to the extremely parallel nature of direct volume rendering, special purpose volume rendering hardware was a
rich research topic before GPU volume rendering became fast enough. The most widely cited technology was
VolumePro [11] , which used high memory bandwidth and brute force to render using the ray casting algorithm.
A recently exploited technique to accelerate traditional volume rendering algorithms such as ray-casting is the use of
modern graphics cards. Starting with the programmable pixel shaders, people recognized the power of parallel
operations on multiple pixels and began to perform general-purpose computing on (the) graphics processing units
(GPGPU). The pixel shaders are able to read and write randomly from video memory and perform some basic
mathematical and logical calculations. These SIMD processors were used to perform general calculations such as
rendering polygons and signal processing. In recent GPU generations, the pixel shaders now are able to function as
MIMD processors (now able to independently branch) utilizing up to 1 GB of texture memory with floating point
formats. With such power, virtually any algorithm with steps that can be performed in parallel, such as volume ray
casting or tomographic reconstruction, can be performed with tremendous acceleration. The programmable pixel
shaders can be used to simulate variations in the characteristics of lighting, shadow, reflection, emissive color and so
forth. Such simulations can be written using high level shading languages.