ffog[w] is computed by interpolating between entries in a fog table indexed with w
GR_FOG_ADD2
|
cout = (1-f )cin
|
f can be either the high order byte of iterated alpha or computed from the fog table
|
GR_FOG_MULT2
|
cout = f cfog
|
f can be either the high order byte of iterated alpha or computed from the fog table
|
The fogging factor f is determined by mode. If mode is GR_FOG_WITH_ITERATED_ALPHA, then f is equal to the integer bits of iterated alpha. If mode is GR_FOG_WITH_TABLE, then f is computed by interpolating between fog table entries, where the fog table is indexed with a floating point representation of the pixel’s w component. Fog is applied after color combining and before alpha blending.
The global fog color (cfog) is set by calling grFogColorValue(). The argument, value, is an RGBA color and is specified in the format defined in the cFormat parameter to grSstWinOpen() (see Chapter Chapter 3. ).
void grFogColorValue( GrColor_t value )
To fog with iterated alpha, the fog mode must be set to GR_FOG_WITH_ITERATED_ALPHA. In this mode the high order eight bits of the value produced by the alpha iterator are used as the fog blending factor f. The fog equation becomes
cout = ai cfog + (1-ai)cin
Example Special Effects.1 presents a code segment that adds iterated alpha fog to a scene.
Example Special Effects.1 Fogging with iterated alpha.
The following code segment demonstrates fogging with iterated alpha. No setup is required beyond specifying a fog color and fog mode.
/* fog is white… color is ARGB format */
grFogColorValue( 0xFFFFFF);
grFogMode(GR_FOG_WITH_ITERATED_ALPHA);
/* vertices have alpha values that grow as the object gets more indistinct */
draw_objects();
Share with your friends: |
