SHADE_VOLUME Procedure

Given a 3D volume and a contour value, produces a list of vertices and polygons describing the contour surface.

SHADE_VOLUME, volume, value, vertex, poly

volume An array with three dimensions containing the dataset to be contoured. If the volume array is dimensioned (D₀, D₁, D₂), the resulting vertices range as follows:

• In X, they range between 0 and D₀ - 1.

• In Y, they range between 0 and D₁ - 1.

• In Z, they range between 0 and D₂ - 1.

value A scalar containing the contour value.

vertex The name of a variable to receive the vertex array. This variable will be set to a (3, n) floating-point array, suitable for input to the MESH or POLYSHADE procedure.

poly The name of a variable to receive the polygon list, an m-element longword array. This list describes the vertices of each polygon and is suitable for input to MESH or POLYSHADE.

Low If present and nonzero, indicates that the low side of the contour surface is to be displayed and that the contour surfaces enclose high data values. Otherwise, it is assumed that the high side of the contour surface is to be displayed and that the contour encloses low data values. If this parameter is incorrectly specified, errors in shading will result.

Shades An array with the same dimensions as volume. On input, it contains the user-specified shading color index for each volume element (voxel), and is converted to byte type before use. On output, this input array is replaced by another array containing the shading value for each vertex, contained in vertex.

SHADE_VOLUME computes the polygons that describe a three-dimensional contour surface. Each voxel is visited to find the polygons formed by the intersections of the contour surface and the voxel edges.

You can obtain shading from either a single light-source model or from the values you specify with Shades.

The surface produced by SHADE_VOLUME may then be displayed as a shaded surface with the POLYSHADE procedure.

This routine is limited to processing datasets that will fit in memory.

The following procedure shades a volume passed as a parameter. It uses SURFACE to establish the viewing transformation. It then calls SHADE_VOLUME to produce the vertex and polygon lists, and POLYSHADE to draw the contour surface.

PRO ShowVolume, vol, thresh, Low=low

; Display the contour surface of a volume.

s = SIZE(vol)

; Get the dimensions.

IF s(0) NE 3 THEN PRINT, 'Error'

; Flag an error if s is not a 3D array.

SURFACE, FLTARR(2,2), /Nodata, /Save, $
    XRange=[0,s(1)-1], YRange=[0,s(2)-1],$ ZRange=[0,s(3)-1]

; Use SURFACE to establish 3D transformation and 

; coordinate ranges.

IF N_ELEMENTS(low) EQ 0 THEN low = 0

; Make the default be to view the high side of the contour surface.

SHADE_VOLUME, vol, thresh, v, p, Low = low

; Produce the vertices and polygons.

TV, POLYSHADE(v, p, /T3D)

; Produce the image of the surface and display.

END

POLYSHADE, SURFACE

For information on volume visualization, see Chapter 7, Advanced Rendering Techniques, in the PV-WAVE User's Guide.

The method used by SHADE_VOLUME is that described by Klemp, McIrvin and Boyd in "PolyPaint A Three-Dimensional Rendering Package," American Meteorology Society Proceedings, Sixth International Conference on Interactive Information and Processing Systems, 1990.

This method is similar to the marching cubes algorithm described by Lorenson and Cline in "Marching Cubes: A High Resolution 3D Surface Construction Algorithm," Computer Graphics, Vol. 21, pp. 163-169, 1987.