Best way to store "doodle" data?

On one of the applications that I am writing, I was asked to provide the feature for "pencil and eraser" to allow the user to doodle randomly on a document (for proofreading, note-taking, etc.)

What would be the best way to store such data?

I was thinking of using an image with transparency for each doodle (so that I can also support multiple colors of "doodles") but it seems like it will very quickly make any saved project with doodles grow large in file size.

I am looking if there is a better (existing) alternative (e.g. is there a Doo开发者_JAVA技巧dleXML spec out there?) or just any suggestions.

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I think the "DoodleXML" spec you're looking for might just be SVG. Simply save the doodles as a series of lines. You don't need a full SVG engine as long as you're only supporting the subset that you generate in the first place.