Compositing to "erase" pixels about konva HOT 4 CLOSED

On a blank canvas I dram an image as shown in the figure below. Now I want to create a circle Shape which can be used to erase parts of the image. The red circle in the image is the eraser.

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For now I don't see any good ways to implement this inside framework.

Just for completeness I will add links to your questions from stackoverflow. As I think answear are very good for solving the issue.

http://stackoverflow.com/questions/28438034/erasing-parts-of-an-image-on-html5-canvas
http://stackoverflow.com/questions/28441093/how-can-i-combine-alpha-with-compositing-in-images

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I am thinking about implementing demo for Konva sandbox with tips from answears.

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都没解决咋就关闭了呢？橡皮擦和马赛克功能不是最常用的么？

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