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       vpSetDepthCueing - set depth cueing parameters


       #include <volpack.h>

       vpSetDepthCueing(vpc, front_factor, density)
           vpContext *vpc;
           double front_factor;
           double density;


       vpc    VolPack context from vpCreateContext.

              Depth cueing factor at the front clipping plane.

              Fog density.


       vpSetDepthCueing is used to set the parameters for depth cueing.  Depth
       cueing is a simulated fog that makes objects  in  the  distance  appear
       darker  than  foreground  objects.   Depth cueing is independent of the
       shading method.  It can be used with lookup-table shading or  callback-
       function  shading.   Depth  cueing  is not stored in the shading lookup
       table; it is always applied to voxels at rendering time.

       The front_factor argument is the transparency of the fog at  the  front
       clipping  plane  (see  vpWindow(3)  or vpWindowPHIGS(3)).  It must be a
       positive number and is usually less than 1.0 (although  larger  numbers
       can be used to brighten the foreground).  The density argument controls
       the "density" of the fog.  Increasing the  density  causes  objects  to
       recede  into  darkness  over  a shorter distance.  The equation for the
       transparency of the fog at a particular voxel is:

           T = front_factor * exp(-density * depth)

       where depth is 0 at the front clipping plane and 1 at the back clipping
       plane.    Each   voxel   color  component  is  multiplied  by  the  fog
       transparency during rendering.

       By default, depth cueing is not enabled.  To enable it,  call  vpEnable
       with the VP_DEPTH_CUE option.

       Depth  cueing  is implemented using an internal lookup table (unrelated
       to  the  shading  lookup  table).   There  are  several  VolPack  state
       variables  that affect the depth cueing table, although they should not
       usually be changed.  The table  contains  depth  cueing  factors  as  a
       function  of  depth.   The table must contain entries for depths in the
       range 0.0-1.0, and possibly for  negative  depths  depending  upon  the
       current  viewing parameters (the reason for this is beyond the scope of
       a man page).  The VP_DEPTH_CUE_QUANTIZATION variable (which can be  set
       by calling vpSetd) determines the depth difference between two adjacent
       entries  in  the  depth  cueing  table.   For  instance,  the   default
       quantization of 1/255 specifies that each entry in the table represents
       1/255 times the distance between the front and  back  clipping  planes.
       Reducing  the quantization increases the size and accuracy of the depth
       cueing table.

       Normally the depth cueing table is resized  and  recomputed  only  when
       necessary.   Recomputation  may  be  required if the viewpoint changes,
       increasing the range of voxel depths that are encountered (recall  that
       there  is  no  lower  bound on the depth).  You can use vpGeti with the
       VP_DEPTH_CUE_TABLE_SIZE state variable code to get the  current  number
       of    table    entries.     You    can    also   set   the   value   of
       VP_DEPTH_CUE_TABLE_SIZE_HINT (using vpSeti) to suggest a size  for  the
       table.   This  might  be  desirable if you plan to compute an animation
       loop and want to ensure that the depth cueing table does not need to be
       recomputed  in  the middle of the sequence.  However, since recomputing
       the table is relatively inexpensive, it should not be necessary to  use
       this option.


       Information  about the current depth cueing properties can be retrieved
       with the following state variable codes (see vpGeti(3)):  VP_DEPTH_CUE,


       The normal return value is VP_OK.  The following error return value  is

              The value of front_factor is not positive.


       VolPack(3), vpCreateContext(3), vpSetLookupShader(3)