I have been browsing through my daily photos noticing the frequent use of perspective. See Photo Rotator
# Curiosity
Now I want to get quantitative by measuring the perspective for each photo and tally metrics for my collection as I proceed. But how?
If I recorded the vanishing point for each then I could compute a heat map of their locations. This could be another kind of tag for which I could retrieve subsets.
I could make a tool where I identify two strong edges and their intersection. Maybe some graphic edge detection algorithm would help? Ideas? post 
picnic table river gorge sky
Having slept on this for a bit now I have an idea of how I want to proceed. I'd like to concern myself only when a VP is associated with the subject of the photo. In the picnic tables I would choose one or the other as the subject or split the difference between them.
I could usefully identify by tag those images where a VP appears within the field of view as it does with the tables. When this comes from horizontal lines I can tag the VP as "outward", for verticals I would use tags "upward" or "downward".
Later, I decided against tags since I didn't want high frequency tags swamping my tag cloud associations. Instead I just eyeballed the VP for ever image and added those that qualified to one of three pages.