You can implement transparency when doing any of the following:
- Region processing. Defining a region in an image makes it possible to treat the area outside the region as transparent. The PaintRegion method paints only the region that is defined for an image. The CombineFastCommand class combines only the area inside the region. You can define a region in a number of ways, including specification of a color. For example, if a transparent color is defined for an image, you can create a region that omits pixels of that color. For details, refer to Creating a Region.
Once a region is defined, you can create a mask from it, using the CreateMaskFromRegion method, which you can save in the alpha channel of a 16- or 32-bit file. Refer to the SetAlphaImage method for an example of how to save a region in an alpha channel.
When specifying a transparent color, you can use the Windows PALETTEINDEX macro to refer to a particular palette index (where the RasterColor value is >= 0x01000000). Using a palette index for transparency means that one of the remaining colors in the palette can have the same RGB value as the transparent color, without being interpreted as transparent.