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How to Measure From a Picture

I am trying to make a table from a picture in a magazine, which has some intricate lathe work that I want to duplicate. The problem is the picture was taken at an unknown angle, and when I try to scale up from the picture my measurements come out screwed up. Is there something I can do that we allow me to find the right measurements?

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adityam815 months ago
Goodhart9 years ago
Is it possible to lay a "grid" on the page and then scale it up from there (copy each block onto a larger grid) ? This was a popular way of "copying" drawings and making them bigger when I was younger.
Sedgewick17 (author)  Goodhart9 years ago
I have seen grids used before, but I don't think that a grid can be accurately used for measuring on an angle. I might try to eye the part that I can't properly measure, like catlinsdad said and see how close it looks.
A picture is worth a quick drawing lesson... Get a copy of your picture and tape some more paper on it so you can establish the vanishing points from your table by extending parallel lines. If the table is round use your eye to determine the outermost edges of the "box" it is in. Extend all horizontal lines that go back to the vanishing points. Where the lines intersect you will recreate the figure. Extend the lines back to where you can fit a ruler vertically. When you extend the lines back to the vanishing point, what you get is the dimension that you can read off the ruler. Scale up the one side and go from there.
perspective.JPG
yeah, I was just wondering if you increased the size of the picture whether that would make it easier to make out what the measurements might be.
caitlinsdad9 years ago
If you had an artistic eye and understood how to draw in perspective, perhaps you could scale it from another known dimension in the picture or from another object in the picture. Extend parallel lines to find the vanishing point in the distance. You may be able to figure out or picture in your mind at what odd angle the picture was taken at. You can then "wireframe" your object to at least get relative dimensions, even with the foreshortened perspective view. Hope this helps.
caitlinsdad is essentially describing a mathematical projection, where 3d data is mapped to a 2d space. But it's difficult to map 2d data (photograph) back to a different 2d projection--you don't have any absolute reference points. Film makers use reference points on a green screen to convert the 2d space (they convert from the image) to a fairly accurate 3d reference--so a CGI background can be dropped in, and the background will sync with any camera movements. (maybe they can track camera movements inertially / electronically by now, but objects / people are still referenced...) The math isn't super-difficult, but unless you have a few fixed reference points it's very tough to get an accurate projection....Plus you probably have a very small sample (photos of the thingie.) There is software available that can convert 2d images into an approximate 3d model--but you'd need several frames of the object at different angles.
guyfrom7up9 years ago
lathework... as in on the legs?
iman9 years ago
could you ask the magazine for the plans ?
Sedgewick17 (author)  iman9 years ago
Yes, I contacted the magazine but unfortunately the man who built the table never made any plans, and he has since past away. I can find the general dimensions by using a caliper transferred to a ruler, and use scale factor. The big problem is that lathe work such as the beads and grooves on the closest leg in the picture are partially covered by another object in the picture.
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