#!F-adobe-helvetica-medium-r-normal--18*
#!N
#!CSeaGreen #!N #!Rtvolrnd Volume Rendering #!N #!EC
#!N #!N Volume rendering is a technique for using color and
opacity to visualize the data in a 3-dimensional data set. (In
contrast, surface techniques use tools like #!F-adobe-times-bold-r-normal--18* Isosurface #!EF and #!F-adobe-times-bold-r-normal--18*
MapToPlane #!EF to display a 2-dimensional surface, although in 3-dimensional space.)
The following are some simple examples. #!N #!I0 #!N #!F-adobe-times-medium-r-normal--18* #!N
#!N #!I30 #!N 1. Open and execute visual program .../VolumeRendering.net. As
the network in the canvas shows, the color of the volume
is determined by #!F-adobe-times-bold-r-normal--18* AutoColor #!EF . The data set contains
relatively few high values (red) and relatively many low values (blue).
No structure is apparent in the image. #!N #!I30 #!N 2.
Select #!F-adobe-times-bold-r-normal--18* Transformation #!EF and then #!F-adobe-times-bold-r-normal--18* Equalize #!EF in the
palettes. #!N #!I30 #!N 3. Position the #!F-adobe-times-bold-r-normal--18* Equalize #!EF icon
between #!F-adobe-times-bold-r-normal--18* Import #!EF and #!F-adobe-times-bold-r-normal--18* AutoColor #!EF in the VPE
canvas. #!N #!I30 #!N 4. Disconnect #!F-adobe-times-bold-r-normal--18* Import #!EF output from
#!F-adobe-times-bold-r-normal--18* AutoColor #!EF input and reconnect it to the first input
tab ("data") of #!F-adobe-times-bold-r-normal--18* Equalize #!EF . #!N #!I30 #!N 5.
Connect #!F-adobe-times-bold-r-normal--18* Equalize #!EF output to the first input tab ("data")
of #!F-adobe-times-bold-r-normal--18* AutoColor #!EF . #!N #!I30 #!N 6. Reexecute the
visual program. #!F-adobe-times-bold-r-normal--18* Equalize #!EF redistributes the data values more or
less uniformly between the minimum and maximum of the data. Although
the resulting image is somewhat diffuse, the structure of the data
(the electron density of an imide molecule) is now visible. #!N
#!I0 #!N #!EF #!N #!N #!N #!F-adobe-times-bold-r-normal--18* AutoColor #!EF parameters can
be used to add definition to the structure. #!N #!I0 #!N
#!F-adobe-times-medium-r-normal--18* #!N #!N #!I30 #!N 1. Delete the #!F-adobe-times-bold-r-normal--18* Equalize #!EF
module: Click on the icon and select #!F-adobe-times-bold-r-normal--18* Delete #!EF in
the #!F-adobe-times-bold-r-normal--18* Edit #!EF pull-down menu. The connections to #!F-adobe-times-bold-r-normal--18* Import
#!EF and #!F-adobe-times-bold-r-normal--18* AutoColor #!EF are automatically deleted along with the
icon. #!N #!I30 #!N 2. Reconnect the #!F-adobe-times-bold-r-normal--18* Import #!EF output
to the first input tab ("data") of #!F-adobe-times-bold-r-normal--18* AutoColor #!EF .
#!N #!I30 #!N 3. Open the #!F-adobe-times-bold-r-normal--18* AutoColor #!EF configuration dialog
box. #!N #!I30 #!N 4. Set the value of the #!F-adobe-times-bold-r-normal--18*
min #!EF parameter to ".1" and click on #!F-adobe-times-bold-r-normal--18* OK #!EF
. #!N #!I30 #!N 5. Reexecute the visual program. All data
values smaller than 0.1 are rendered invisible (black). The image is
much darker, but still visible. #!N #!I30 #!N 6. To increase
the visibility of the data, increase the value of the #!F-adobe-times-bold-r-normal--18*
intensity #!EF parameter in the #!F-adobe-times-bold-r-normal--18* AutoColor #!EF configuration dialog box
to "5." #!N #!I30 #!N 7. Click on #!F-adobe-times-bold-r-normal--18* OK #!EF
and reexecute the visual program. The structure of image is now
fairly distinct. #!N #!I0 #!N #!EF #!N #!N #!N A color
map gives you much greater control over the appearance of the
image: #!N #!I0 #!N #!F-adobe-times-medium-r-normal--18* #!N #!N #!I30 #!N 1. Disconnect
#!F-adobe-times-bold-r-normal--18* AutoColor #!EF from #!F-adobe-times-bold-r-normal--18* Image #!EF and connect the #!F-adobe-times-bold-r-normal--18*
Color #!EF output to the #!F-adobe-times-bold-r-normal--18* Image #!EF input. #!N #!I30
#!N 2. Reexecute the visual program. #!N #!I30 #!N 3. Bring
up the Colormap Editor by double clicking on the #!F-adobe-times-bold-r-normal--18* Colormap
#!EF icon. The color-bar, Hue, and Opacity settings are clearly reflected
in the image: regions of low data values (green) and smaller
regions of higher data values (red). All other data values have
been rendered invisible. Note: To make a region or volume invisible,
it is necessary to set #!F-adobe-times-medium-i-normal--18* both #!EF its intrinsic opacity
and its color value to zero. The reason is that the
volume rendering model assumes that regions emit light as well as
absorb it. So even if its opacity is zero (no absorption),
a region will still emit light unless its color is black
([0 0 0]). #!N #!I0 #!N #!EF #!N #!N #!N It
is interesting to contrast the volume rendering of previous images with
a surface technique. For example: #!N #!I0 #!N #!F-adobe-times-medium-r-normal--18* #!N #!N
#!I30 #!N 1. Disconnect #!F-adobe-times-bold-r-normal--18* Color #!EF from #!F-adobe-times-bold-r-normal--18* Image #!EF
and connect the #!F-adobe-times-bold-r-normal--18* Isosurface #!EF output to the #!F-adobe-times-bold-r-normal--18* Image
#!EF input. #!N #!I30 #!N 2. Reexecute the visual program. The
resulting image is an isosurface representation of the structure of an
imide molecule. #!N #!I0 #!N #!EF #!N #!N #!N You can
also combine surface techniques with volume rendering. For example: #!N #!I0
#!N #!F-adobe-times-medium-r-normal--18* #!N #!N #!I30 #!N 1. Select #!F-adobe-times-bold-r-normal--18* Structuring #!EF
and then #!F-adobe-times-bold-r-normal--18* Collect #!EF in the palettes. #!N #!I30 #!N
2. Position the #!F-adobe-times-bold-r-normal--18* Collect #!EF icon above #!F-adobe-times-bold-r-normal--18* Image #!EF
in the VPE canvas. #!N #!I30 #!N 3. Disconnect #!F-adobe-times-bold-r-normal--18* Isosurface
#!EF from #!F-adobe-times-bold-r-normal--18* Image #!EF . #!N #!I30 #!N 4. Connect
the first output tab ("mapped") of #!F-adobe-times-bold-r-normal--18* AutoColor #!EF to either
of the #!F-adobe-times-bold-r-normal--18* Collect #!EF input tabs. #!N #!I30 #!N 5.
Connect the #!F-adobe-times-bold-r-normal--18* Isosurface #!EF output to the other #!F-adobe-times-bold-r-normal--18* Collect
#!EF input tab. #!N #!I30 #!N 6. Connect the #!F-adobe-times-bold-r-normal--18* Collect
#!EF output to the #!F-adobe-times-bold-r-normal--18* Image #!EF input. #!N #!I30 #!N
7. Reexecute the visual program. The result is a combination of
the volume-rendering and isosurface images of the imide molecule. #!N #!I30
#!N 8. To make the isosurfaces translucent, insert a new #!F-adobe-times-bold-r-normal--18*
Color #!EF module (from the #!F-adobe-times-bold-r-normal--18* Transformation #!EF category) into the
network between #!F-adobe-times-bold-r-normal--18* Isosurface #!EF and #!F-adobe-times-bold-r-normal--18* Collect #!EF . (Use
the first, or "input," tab of the #!F-adobe-times-bold-r-normal--18* Color #!EF icon.)
#!N #!I30 #!N 9. Open the #!F-adobe-times-bold-r-normal--18* Color #!EF configuration dialog
box and set the #!F-adobe-times-bold-r-normal--18* opacity #!EF parameter to ".3." (You
can try other values as well.) #!N #!I30 #!N 10. Click
on #!F-adobe-times-bold-r-normal--18* OK #!EF and reexecute the visual program. The isosurfaces
are now translucent. #!N #!I0 #!N #!EF #!N #!N #!N #!N
#!F-adobe-times-medium-i-normal--18* Next Topic #!EF #!N #!N #!Lttandt,dxall56 h Tasks and Tools #!EL #!N #!F-adobe-times-medium-i-normal--18* #!N
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