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Charles Walbridge

Photographing scales on the turntable

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At the Minneapolis Institute of Art we're now doing photogrammetry of medium-sized objects with a robot turntable/swing arm, and with each object we've been photographing a data set where the CHI photogrammetry scales occlude the object in many images. For now, I'm also photographing the objects and scales as 'flat,' just like I learned at CHI, but I'm theorizing that the measurement data we'll get from the scales on the turntable will be much more robust.
Here's the shooting and PhotoScan breakdown:
- Photograph the object on the turntable with no scale bars from multiple rotations and elevations (we've been making 36 columns and nine elevations, from 0-88 degrees, but fewer columns for the top elevations). Also photograph empty backgrounds for auto masking.
- Photograph the object with two scales occluding the object and as close as possible to the object, and two scales on the turntable's surface (with fewer photos in this set; four elevations, from 0-66 degrees).
- Use the first set of images as one Camera Group in PhotoScan, and the scales as another Camera Group. Align photos to make a sparse point cloud.
- Refine the sparse cloud using CHI/BLM's magical method. Add scale.
- Remove (or turn off) all images from the scales dataset.
- Build the dense cloud, et cetera.

P​hotoScan is identifying the scales on the turntable with no problem; it feels to me like having a much larger data set full of scales will produce better scale information than a set where the three-dimensional art is treated as a flat object.

And I'm happy to report that scale is traveling with exported objects - this figure arrived in an OBJ-reading program with a size of .67 units (apparently there's no set unit in a lot of these programs), and it's 67 cm tall: https://sketchfab.com/models/8217886808944db3b3a01734d604cdd6

What do you think?

 

post-185-0-23408800-1440608527_thumb.jpg

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Hello,

 

Putting scales on the turntable works just fine as you have proven yourself. In our testing it won't necessarily provide a better solution than scale from a flat project. We have found that a proper flat project - proper lens and base to height, photos at 90's and 270's etc. will provide a highly reliable camera calibration - perhaps very slightly better than from a single circuit in the round. The number of tie points that connect the in the round captures and the flat capture is usually in the thousands and will more than adequately tie the multiple captures together for highly accurate transfer of scale.

 

Than being said, the biggest reason we tend to use the flat project method is that it always seems more difficult to make sure the target sticks don't move and are in good focus when capturing them on a turntable or occluding the subject. If you like that approach better that is great. There should not be any difference in the final results.

 

Tom

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Matt, the auto-masking is a feature in Photoscan. If you have a photo of the empty set and feed that into Photoscan's Import Mask, it will make masks for you for all the photos in that set. In our case with the turntable/swing arm combination, we're making one empty background for each camera elevation, then having Photoscan auto-mask the 36 images from that elevation. (Again, the highest elevations get fewer than 36 images per elevation.)

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