OpenGL Fill-Pattern Function

► We generate displays of filled convex polygons in four steps:

1. Define a fill pattern.
2. Invoke the polygon-fill routine.
3. Activate the polygon-fill feature of OpenGL.
4. Describe the polygons to be filled.

► A polygon fill pattern is displayed up to and including the polygon edges. Thus, there are no boundary lines around the fill region unless we specifically add them to the display.

OpenGL Fill-Pattern Function

➢ To fill the polygon with a pattern in OpenGL, we use a 32 × 32 bit mask.
➢ A value of 1 in the mask indicates that the corresponding pixel is to be set to the current color, and a 0 leaves the value of that frame-buffer position unchanged.
➢ The fill pattern is specified in unsigned bytes using the OpenGL data type Glubyte
                → GLubyte fillPattern [ ] = { 0xff, 0x00, 0xff, 0x00, ... };
➢ The bits must be specified starting with the bottom row of the pattern, and continuing up to the topmost row (32) of the pattern.
➢ This pattern is replicated across the entire area of the display window, starting at the lower-left window corner, and specified polygons are filled where the pattern overlaps those polygons
➢ Once we have set a mask, we can establish it as the current fill pattern with the function
glPolygonStipple (fillPattern);
➢ We need to enable the fill routines before we specify the vertices for the polygons that are to be filled with the current pattern
glEnable (GL_POLYGON_STIPPLE);
➢ Similarly, we turn off pattern filling with
glDisable (GL_POLYGON_STIPPLE);