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The interface may be slightly challenging to learn because it may not be immediately obvious what to do at the beginning. However, I believe that the learning curve is shallow, because of the familiarity of drag and drop. I think the learnability of the gallery interface is also good because most people are immediately familiar with what can be done or what cannot be done with a gallery interface, and also drag and drop has become quite ubiquitous. Finally, there are not many buttons (mostly just the Analyze button), so the usage should be quite straightforward. As long as the interface is not cluttered and the instructions are clear, then the interface should be easy to learn how to use.
Efficiency
I believe that this interface should be quite efficient because there really isn't much for the user to do except to select the images he likes and to drag them to his sketchpad and analyze them. The simplicity of the layout should help also. There are also only two main modes of the interface - browsing, and analysis. This is easily toggled using the close button and the Analyse button, therefore I believe this interface is fairly efficient.
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