Filling Regions

An application fills the interior of a region by calling the FillRgn function and supplying a handle that identifies a specific brush. When an application calls FillRgn, the system fills the region with the brush by using the current fill mode for the specified device context. There are two fill modes: alternate and winding. The application can set the fill mode for a device context by calling the SetPolyFillMode function. The application can retrieve the current fill mode for a device context by calling the GetPolyFillMode function.

The following illustration shows two identical regions: one filled using alternate mode and the other filled using winding mode.

Alternate Mode

To determine which pixels the system highlights when alternate mode is specified, perform the following test:

1. Select a pixel within the region's interior.
2. Draw an imaginary ray, in the positive x-direction, from that pixel towards infinity.
3. Each time the ray intersects a boundary line, increment a count value.

The system highlights the pixel if the count value is an odd number.

Winding Mode

To determine which pixels the system highlights when winding mode is specified, perform the following test:

1. Determine the direction in which each boundary line is drawn.
2. Select a pixel within the region's interior.
3. Draw an imaginary ray, in the positive x-direction, from the pixel toward infinity.
4. Each time the ray intersects a boundary line with a positive y-component, increment a count value. Each time the ray intersects a boundary line with a negative y-component, decrement the count value.

The system highlights the pixel if the count value is nonzero.