Master's Thesis - Studierstube Augmented Reality Project - Graz ...

5.3 Visualization nodes and subgraphs
Figure 5.12: This Figure shows path lines with a red colored one-dimensional seedregion
and red-colored sections of the lines which are placed in this part related to the
volume currently shown in the background by the morphological slice renderer in the
background.
that always the same side of the strip is placed below the actual line and the other
above. Due to the fact that a line strip is not a tube when looking directly in line
direction a terminator at the start and the end of each tube has to be added. This
might be a pre-shaded circle rendered on an impostor-quad perpendicular to the line’s
tangent direction or a sphere or anything else. The start and the end point of each
line can be easily determinated since they always refer to the first, respectively the last
texture coordinate of each column in the already calculated supporting points texture.
Consequently, the terminator structures only have to be assigned to these coordinates.
For realistic lighting and coloring of the tube, the interpolations abilities of built in
shader varyings can be utilized. A smooth color course can be established by setting
the variable gl_FrontColor for each vertex to a value looked up in a former introduced
color gradient for some arbitrary parameter. Moreover the correct reflection of light
is important. To give an impression of a dilated tube an additional varying for the
vertex normals has to be defined. By setting the normals always in the same direction
as the vertices have been displaced relatively to the line, correct spherical interpolated
normals will result for each fragment between the borders of the strip. The blue arrows
in figure 5.13 try to explain these coherences. Phong-shading with a fragment shader is
consequently possible directly. Due to a well defined normal vector for each fragment,
equation 2.6 can now be solved directly without a maximization criterion as used for
the illumination of lines. This approach still performs with about 60 to 120 frames per
second on the reference systems. The line length and number were each defined with
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