原有内容中与新图形重叠的部分会被保留，其它区域都变成透明的。
The existing canvas content is kept where both the new shape and existing canvas content overlap. Everything else is made transparent.
原有内容中与新图形重叠的部分会被保留，其它区域都变成透明的。
The existing canvas content is kept where both the new shape and existing canvas content overlap. Everything else is made transparent.
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