Each inked illustration consisted of about 30 minutes each, 2 hours over all work time.
Stable:
Ambigeous: 
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The OpenLab is an open-source, digital platform designed to support teaching and learning at City Tech (New York City College of Technology), and to promote student and faculty engagement in the intellectual and social life of the college community.
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I like your drawing because they are very simple and solid. Its very obvious which one is ambiguous and stable.
The second stable drawing is like a stamp. Interesting~