Canvas Pocket Reference

Name

CRC.globalCompositeOperation — how colors are combined on the canvas

Synopsis

String globalCompositeOperation

Description

This property specifies how source pixels being rendered onto the canvas are combined (or “composited”) with the destination pixels that already exist in the canvas. This property is typically useful only when you are working with partially transparent colors or have set the globalAlpha property. The default is value is “source-over”. Other commonly used values are “destination-over” and “copy”.

The following table lists the allowed property values and their meanings. In the third column, the word source refers to the pixels being drawn onto the canvas, the word destination refers to the existing pixels on the canvas, and the word result refers to the pixels that result from the combination of the source and destination. The second column shows the formula for computing a result pixel from a source pixel S and a destination pixel D along with the alpha channel (the opacity) of the source pixel α_s and the alpha channel of the destination α_d:

Value	Formula	Meaning
copy	S	Draw the source pixel, ignoring the destination pixel.
destination-atop	(1-αd)S + αsD	Draw the source pixel underneath the destination. If the source is transparent, the result is also transparent.
destination-in	αsD	Multiply the destination pixel by the opacity of the source pixel, but ignore the color of the source.
destination-out	(1-αs)D	The destination pixel is made transparent when the source is ...