1vpSetDepthCueing(3) Library Functions Manual vpSetDepthCueing(3)
2
6 vpSetDepthCueing - set depth cueing parameters
7
9 #include <volpack.h>
10 vpResult
12 vpSetDepthCueing(vpc, front_factor, density)
13 vpContext *vpc;
14 double front_factor;
15 double density;
16
18 vpc VolPack context from vpCreateContext.
19 front_factor
21 Depth cueing factor at the front clipping plane.
22 density
24 Fog density.
25
27 vpSetDepthCueing is used to set the parameters for depth cueing. Depth
28 cueing is a simulated fog that makes objects in the distance appear
29 darker than foreground objects. Depth cueing is independent of the
30 shading method. It can be used with lookup-table shading or callback-
31 function shading. Depth cueing is not stored in the shading lookup ta‐
32 ble; it is always applied to voxels at rendering time.
33 The front_factor argument is the transparency of the fog at the front
35 clipping plane (see vpWindow(3) or vpWindowPHIGS(3)). It must be a
36 positive number and is usually less than 1.0 (although larger numbers
37 can be used to brighten the foreground). The density argument controls
38 the "density" of the fog. Increasing the density causes objects to
39 recede into darkness over a shorter distance. The equation for the
40 transparency of the fog at a particular voxel is: T = front_factor *
41 exp(-density * depth) where depth is 0 at the front clipping plane and
42 1 at the back clipping plane. Each voxel color component is multiplied
43 by the fog transparency during rendering.
44 By default, depth cueing is not enabled. To enable it, call vpEnable
46 with the VP_DEPTH_CUE option.
47 Depth cueing is implemented using an internal lookup table (unrelated
49 to the shading lookup table). There are several VolPack state vari‐
50 ables that affect the depth cueing table, although they should not usu‐
51 ally be changed. The table contains depth cueing factors as a function
52 of depth. The table must contain entries for depths in the range
53 0.0-1.0, and possibly for negative depths depending upon the current
54 viewing parameters (the reason for this is beyond the scope of a man
55 page). The VP_DEPTH_CUE_QUANTIZATION variable (which can be set by
56 calling vpSetd) determines the depth difference between two adjacent
57 entries in the depth cueing table. For instance, the default quantiza‐
58 tion of 1/255 specifies that each entry in the table represents 1/255
59 times the distance between the front and back clipping planes. Reduc‐
60 ing the quantization increases the size and accuracy of the depth cue‐
61 ing table.
62 Normally the depth cueing table is resized and recomputed only when
64 necessary. Recomputation may be required if the viewpoint changes,
65 increasing the range of voxel depths that are encountered (recall that
66 there is no lower bound on the depth). You can use vpGeti with the
67 VP_DEPTH_CUE_TABLE_SIZE state variable code to get the current number
68 of table entries. You can also set the value of VP_DEPTH_CUE_TA‐
69 BLE_SIZE_HINT (using vpSeti) to suggest a size for the table. This
70 might be desirable if you plan to compute an animation loop and want to
71 ensure that the depth cueing table does not need to be recomputed in
72 the middle of the sequence. However, since recomputing the table is
73 relatively inexpensive, it should not be necessary to use this option.
74
76 Information about the current depth cueing properties can be retrieved
77 with the following state variable codes (see vpGeti(3)): VP_DEPTH_CUE,
78 VP_DEPTH_CUE_FRONT, VP_DEPTH_CUE_DENSITY, VP_DEPTH_CUE_TABLE_SIZE,
79 VP_DEPTH_CUE_SIZE_HINT, VP_DEPTH_CUE_QUANTIZATION.
80
82 The normal return value is VP_OK. The following error return value is
83 possible:
84 VPERROR_BAD_VALUE
86 The value of front_factor is not positive.
87
89 VolPack(3), vpCreateContext(3), vpSetLookupShader(3)
90VolPack vpSetDepthCueing(3)