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The Volpack Volume Rendering Library


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http://www-graphics....ftware/volpack/

The Volume Rendering Pipeline

The input to the volume renderer is a three-dimensional array of data. Each element of the array is a C structure containing any number of fields of data, such as tissue density or temperature. Each element is called a "voxel." The first stage in the volume rendering pipeline is to classify the volume data, which means to assign an opacity to each voxel. Opacity is the inverse of transparency: an opacity of 0.0 indicates a fully-transparent voxel, while an opacity of 1.0 indicates a voxel which completely occludes anything behind it. Intermediate values between 0.0 and 1.0 indicate semi-transparent voxels. The purpose of classification is to assign low opacities to regions of the data set which are uninteresting or distracting and high opacities to regions of the data set which should be visible in the rendering. Intermediate opacity values are used for smooth transitions from transparent to opaque regions, and for effects such as semi-transparent voxels which should not completely occlude objects behind them.

VolPack provides a classification method based on lookup tables. To use this method you specify a transfer function which maps the scalar data in a particular array element into the opacity for that element. Alternatively you can implement other classification techniques such as context-sensitive segmentation and then provide VolPack with a pre-classified volume.

The second rendering stage is to assign a color to each voxel, an operation which is called shading (or more precisely, lighting). VolPack includes support for the standard Phong shading equation. To use this shading technique, the volume data is preprocessed before rendering in order to compute a gradient vector for each voxel. The gradient vector can then be used as a pseudo surface normal to compute how light reflects off of each voxel. The user specifies the position and color of one or more light sources, and the reflective properties of the volume data. See Computer Graphics: Principles and Practice (Chapter 16, 2nd ed.), by Foley, van Dam, Feiner and Hughes, for a detailed discussion of the Phong shading equation. Alternative shading models can be implemented through a callback function.

The third rendering stage is to specify a view transformation and to transform the volume accordingly. This step can be as simple as choosing the position from which to look at the volume, or it can include an arbitrary affine transformation of the volume including non-uniform scaling and shearing. The view transformation also specifies how the volume is projected onto a 2D image plane.

The fourth and final rendering stage is to composite the voxels into an image. Digital compositing is analogous to the compositing process used in the film industry: several layers of semi-transparent film are merged together into a final image. VolPack provides several rendering algorithms that use different techniques to accelerate the compositing stage. The next subsection briefly describes the available algorithms.

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Very hard to comprehend for me too technical. Hehe.

But basically this is a volumetric renderer similar to volumedic for lightwave.

Is there any reason why you started the thread like is the software updated? Developments?

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  • Advanced Member

Very hard to comprehend for me too technical. Hehe.

But basically this is a volumetric renderer similar to volumedic for lightwave.

Is there any reason why you started the thread like is the software updated? Developments?

It's a bit too technical for me too but I was curious about it and hoped to trigger a discussion on it...

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