Users should have the ability to highlight or mark up images and pdfs, with comments for each markup area.
Can you give more details about:
1. Where you would like to do this markup - mobile, web, desktop, etc.?
2. What types of annotations you'd like to create?
In order to make the commenting and collaboration somewhat useful, having the functionality of picking a location or area in the preview to add the comment would be needed. This funcionality is available in google, dropbox, and adobe. If in addition to a comment location a red pen and a highlight could be added that would be great. This would be used for knowing where to make a particular change and verifying that the change was made. if you click on the comment it would take you to the preview location of that comment or if you click on the the post it in the doc it would take you to the particular comment it references.
When you make the changes in the native file and save it it is uploaded and the comments should be maintained in roughly the same location. This might be annoying for large changes, but small changes it would be fine. the text of the comment would still be there.
The ability to reply to a particular comment or mark the comment complete would allow a reviewer to know what changes were done without deleting the original comments.
I just posted about something similar to this here but to give specifics about my requirements:
the comments would be on the web based preview in the form of sticky notes or drawn shapes (circling a particular region) that direct to the comments on the right side.
this functionality would be very useful not just for images, but for pdf engineering drawings, word docs, and excel spreadsheets (as long as there is only 1 sheet becasue only 1 is available in preview).
I agree, make sure there is connection between markup and comment (as described in my post)
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