property DrawMode: TDrawMode;
type TDrawMode = (dmOpaque, dmBlend, dmCustom);
Specifies how the bitmap should be combined with a background during pixel transfer and similar operations.
In dmOpaque mode, new pixels simply
replace the background pixels.
In dmBlend mode,
they are combined using the alpha blending operation defined by CombineMode. This property
is used while copying one bitmap into another, scaling, performing linear transformations
etc.
The dmCustom mode allows implementation of
custom blending functions using the OnPixelCombine call-back
event.