### Global transparency Settings
globalAlpha = value
This property affects the transparency of all graphics in the canvas,
Valid values range from 0.0 (completely transparent) to 1.0 (completely opaque)
The default is 1.0
### overlay composition
Source: New image (source)
Destination: Graph already drawn (destination)
globalCompositeOperation
Source-over (default): The source is above and the new image has a higher level
Source-in: leaving only the overlap between the source and the target (that part of the source)
Source-out: Leaves only the portion of the source that exceeds the target
Source-atop: Remove the overflow from the source
Destination-over: The destination is on top. The old image has a higher level
Destination-in: leaving only the overlap between the source and the destination (that part of the destination)
Destination-out: leaves only the portion of the destination that exceeds the source
Destination-atop: Remove the target from the top