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Author Topic: What am I doing wrong? coins photogrammetry  (Read 1678 times)

spogna

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What am I doing wrong? coins photogrammetry
« on: January 15, 2021, 06:33:43 PM »
Hello everyone.
I'm doing some tests with the Macro lens and I'm trying to reconstruct some coins
I'm trying to figure out how to do this type of reconstruction because I will have to do some reconstructions of ancient coins for a museum.
I have been involved in photogrammetry for some years but never with such thin subjects and surfaces.
I approached the object as if it were a wall, it seemed the most logical thing to do, but the results are really strange
Metashape doesn't give me back the thickness but it does the exact opposite.

To take the images I used a Sony a7R3 with the Canon 100mm IS 2.8 and the "Super 35mm / APS-C" mode.

What am I doing wrong?

Andreas1206

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Re: What am I doing wrong? coins photogrammetry
« Reply #1 on: January 15, 2021, 07:03:44 PM »
hello spogna,

I am doing a lot of close range photogrammetry on cultural heritage objects as I am an art restorer specializing in bowed musical instruments. A couple of ideas concerning your project from my side:

  • try maybe using a lens with less focal length (a wider angle of view), so the images will have more z-axis information to process. I've noticed a couple of times when working with my 105mm Nikon macro lens, that precision of the "height" reconstroction was quite bad when working with images taken only parallel to the surface. Of course, working at such a short distance you immediately loose the necessary depth of field when rotating the camera - so using a wider angle of view with for example a 24mm lens usually gave me much better results
  • the artefacts you're getting in the mesh look to me, as if they could be related to strong reflexions of light on the metallic surface. What kind of lighting are you using? Could you maybe try taking the images in a light tent in order to get the most diffuse lighting possible? Is matting the surface (for example with a volatile substance) an option?

hope you find a solution - best whishes

Andreas


spogna

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Re: What am I doing wrong? coins photogrammetry
« Reply #2 on: January 15, 2021, 07:25:32 PM »
Thanks Andreas for your suggestions.
What you see is only a test made on 50 euro cents
To illuminate the coin I used a Ring-Flash, but being very close to the subject the light practically hit the object only from the sides and not from the front.
Unfortunately, on the objects that I will have to photograph for the museum, I cannot use anything to make them less shiny
I will definitely try to use a light tent to diffuse the light and I will try, at this point, to use other lenses.
Thanks again

cbnewham

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Re: What am I doing wrong? coins photogrammetry
« Reply #3 on: January 15, 2021, 10:55:53 PM »
I've done some close range work using macro lenses of objects smaller than your coin. You need to take some pictures from shallower angles, not just face-on. Treat it as a 3D object - that can be difficult because of reduce depth of field, but with enough pictures and some masking you should be able to reproduce it very accurately.

spogna

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Re: What am I doing wrong? coins photogrammetry
« Reply #4 on: January 15, 2021, 11:52:06 PM »
Thanks "Andreas1206" and "cbnewham"
your advice was very useful !!!
I did a new quick test
I would say excellent as a second attempt, considering the quality of the first (from my point of view, of course)
In the next few days I will do other tests to improve