1vpSetShadowLookupShader(3) Library Functions Manual vpSetShadowLookupShader(3)
2
6 vpSetShadowLookupShader - specify shading lookup tables for rendering
7 shadows
8
10 #include <volpack.h>
11 vpResult
13 vpSetShadowLookupShader(vpc, color_channels, num_materials,
14 color_field, color_table, color_table_size, weight_field,
15 weight_table, weight_table_size, shadow_table, shadow_ta‐
16 ble_size)
17 vpContext *vpc;
18 int color_channels, num_materials;
19 int color_field;
20 float *color_table;
21 int color_table_size;
22 int weight_field;
23 float *weight_table;
24 int weight_table_size;
25 float *shadow_table;
26 int shadow_table_size;
27
29 vpc VolPack context from vpCreateContext.
30 color_channels
32 The number of color channels per pixel (1 or 3).
33 num_materials
35 The number of material types.
36 color_field
38 Field number for voxel field containing color lookup table
39 index.
40 color_table
42 Color lookup table.
43 color_table_size
45 Size of color lookup table in bytes.
46 weight_field
48 Field number for voxel field containing material weight lookup
49 table index.
50 weight_table
52 Material weight lookup table.
53 weight_table_size
55 Size of material weight lookup table in bytes.
56 shadow_table
58 Shadow color lookup table.
59 shadow_table_size
61 Size of shadow color lookup table in bytes.
62
64 vpSetShadowLookupShader is used to specify lookup tables that define a
65 shading function for rendering a volume with shadows. It should be
66 used instead of vpSetLookupShader when shadows are enabled.
67 VolPack supports a fast, one-pass shadow algorithm. The algorithm com‐
69 putes the fraction of light from a given light source that reaches each
70 voxel. The fraction is then used to attenuate the diffuse and specular
71 shading terms associated with that light source, producing a dark
72 shadow in areas that are hidden from the light source. The implementa‐
73 tion uses lookup tables so that most of the shading calculation can be
74 precomputed.
75 In order to compute the shadows in a single pass the algorithm places a
77 restriction on the direction of the light source: the light casting the
78 shadows must not be more than 45 degrees from the viewing direction.
79 The quality of the shadows may degrade if the angle approaches 45
80 degrees. The current implementation allows shadows to be cast only
81 from one light source. Additional lights may be enabled, but they will
82 not cast shadows.
83 To make a rendering with shadows, the following steps must be per‐
85 formed:
86 Call vpSetShadowLookupShader to define the lookup tables (see
88 discussion below).
89 Call vpSeti with the VP_SHADOW_LIGHT option to specify which
91 light will cast shadows. The current implementation only allows
92 one light to be specified.
93 Call vpSeti with the VP_SHADOW_BIAS option to set the shadow
95 bias value (see discussion below).
96 Call vpEnable with the VP_SHADOW option to enable shadows.
98 Call vpShadeTable as usual to initialize the lookup tables.
100 Call one of the rendering routines.
102 vpSetShadowLookupShader defines the lookup tables required for the
104 shading and shadowing algorithm. The first nine arguments are identi‐
105 cal to the arguments for vpSetLookupShader (see the corresponding man
106 page). The remaining two arguments specify an additional color lookup
107 table, shadow_table, with the same dimensions as color_table. The con‐
108 tents of the table will be initialized by vpShadeTable.
109 The call to vpSeti with the VP_SHADOW_BIAS option specifies an offset
111 to eliminate self-shadowing. Self-shadowing is an intrinsic problem
112 when implementing shadow algorithms for volume rendering. Consider a
113 single voxelized object consisting of an opaque core surrounded by a
114 "halo" of lower-opacity voxels (necessary for the scene to be band-lim‐
115 ited). As a light ray pierces the halo its strength is attenuated. By
116 the time the light reaches the high-opacity region a significant frac‐
117 tion of the light may be obscured, resulting in a general darkening of
118 the image even if no shadows should be present. The problem can be
119 corrected by moving the shadow a small distance along the direction of
120 the light rays. VP_SHADOW_BIAS specifies the distance of the bias in
121 units of voxels. The optimal value depends on the data set. Increase
122 the bias until it has no more effect on overall image brightness, but
123 do not increase it too far or small features in the data will no longer
124 produce correct shadows.
125 vpShadeTable initializes the shading lookup tables. It operates dif‐
127 ferently when shadows are enabled. Instead of computing one color for
128 each surface normal vector and storing the results in color_table, the
129 routine computes two colors terms. The first term is the portion of
130 the voxel color due to the diffuse and specular components of the
131 shadow light. This value is stored in shadow_table. The second term
132 contains the contribution of all other light source and the ambient
133 light term, and is stored in color_table. During rendering the color
134 of a voxel is computed by extracting a surface normal from the voxel,
135 using the surface normal to index both color_table and shadow_table,
136 attenuating the value from shadow_table by the local strength of the
137 shadow light, and then adding the two terms together. The local
138 strength of the shadow light is found by extracting a value from the
139 shadow buffer, an internal data structure that is updated during ren‐
140 dering.
141 The values in the shading lookup tables may be initialized before or
143 after calling vpSetShadowLookupShader. Typically vpSetShadowLookup‐
144 Shader is called once at the beginning of a rendering session, and then
145 vpShadeTable is called whenever the user changes the lighting and shad‐
146 ing parameters or the viewing direction.
147 The shadow buffer is an internal 2D array used to maintain state during
149 rendering. There are several state variables that can be used to query
150 its current size in pixels (VP_SHADOW_WIDTH and VP_SHADOW_HEIGHT) and
151 to suggest a size (VP_SHADOW_WIDTH_HINT and VP_SHADOW_HEIGHT_HINT).
152 The required size depends on the volume size and the shadow light's
153 direction. Normally the buffer is automatically resized when neces‐
154 sary.
155
157 Information about the current shading table parameters can be retrieved
158 with the following state variable codes (see vpGeti(3)): VP_COLOR_CHAN‐
159 NELS, VP_SHADE_COLOR_TABLE, VP_SHADE_COLOR_SIZE, VP_SHADE_WEIGHT_TABLE,
160 VP_SHADE_WEIGHT_SIZE, VP_SHADE_COLOR_FIELD, VP_SHADE_WEIGHT_FIELD,
161 VP_MATERIAL_COUNT, VP_SHADOW, VP_SHADOW_LIGHT, VP_SHADOW_WIDTH_HINT,
162 VP_SHADOW_HEIGHT_HINT, VP_SHADOW_WIDTH, VP_SHADOW_HEIGHT,
163 VP_SHADOW_COLOR_TABLE, VP_SHADOW_COLOR_SIZE, VP_SHADOW_BIAS
164
166 The normal return value is VP_OK. The following error return values
167 are possible:
168 VPERROR_BAD_VALUE
170 One or more of the arguments has an invalid value or is out of
171 range.
172 VPERROR_LIMIT_EXCEEDED
174 The num_materials argument has exceeded an internal limit.
175 Change the value of VP_MAX_MATERIAL in volpack.h and recompile
176 the VolPack library.
177
179 VolPack(3), vpCreateContext(3), vpShadeTable(3), vpSetLookupShader(3)
180VolPack vpSetShadowLookupShader(3)