Ubuntu Manpage: vpSetDepthCueing - set depth cueing parameters

Provided by: libvolpack1-dev_1.0b3-9.1_amd64

NAME

       vpSetDepthCueing - set depth cueing parameters

SYNOPSIS

       #include <volpack.h>

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

ARGUMENTS

       vpc    VolPack context from vpCreateContext.

       front_factor
              Depth cueing factor at the front clipping plane.

       density
              Fog density.

DESCRIPTION

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.

STATE VARIABLES

       Information about the current depth cueing properties can be retrieved with the following state  variable
       codes  (see  vpGeti(3)): VP_DEPTH_CUE, VP_DEPTH_CUE_FRONT, VP_DEPTH_CUE_DENSITY, VP_DEPTH_CUE_TABLE_SIZE,
       VP_DEPTH_CUE_SIZE_HINT, VP_DEPTH_CUE_QUANTIZATION.

ERRORS