Does RTI give repeatable and reliable normals of objects taken at different times and positions to facilitate detection of changes?

On the Linked-In discussion group Cultural Heritage Conservation Science. Research and practice’s discussion on 3-D digital imaging and photogrammetry for scientific documentation of heritage sites and collections http://linkd.in/RZMpFj , Greg Bearman wrote the following question:

“Does RTI give repeatable and quantitative set of normals good enough for looking for change? If I take an RTI set, rotate the object, let it warp a bit (flexible substrate), what do I get the second time? How do I align the datasets for comparison?

what is the system uncertainty? ie if I just take repeated images of the same object without moving anything, how well does the RTI data line up. Second, suppose I take something with some topography but is totally inflexible and cannot distort(make up test object here!) and I do repeated RTI on it in different orientations? Can I make the data all the same? If you are going to use an imaging method to determine changes in an object, the first thing to do is understand what is in inherent noise and uncertainty in the measuring system. It could be some combination of software, camera or inherent issues with the method itself”

I wrote back: “Hey Greg – tried sending response earlier last week but I do not see it!? Sorry. I’m on vacation until the 22nd – trying to recover and recharge. It is going well but I wanted to jot down my initial thoughts. One of my interns – Greg Williamson – is working on aberration recognition software that can recognize and highlight changes in condition captured by different H-RTI computational image assemblies – obviously taken at different times, but also with different equipment and with randomly different highlight flash positions. It seems, initally, that normal reflection is normal reflection, regardless of object or flash position and that the software correctly interpolates 3D positions of surface characteristics regardless of the precise position of the flash, because it is accustomed to calculating the highlights both the capture points and everywhere in between! Likewise, we have had promise with photogrammetry when the resolution of the images used to create the mesh and solids are similar. What may turn out to be key is a calibration set that will allow correction of the various lens distortions that would naturally come from different lenses. I know Mark Mudge at Cultural Heritage Imaging has suggested that we begin taking a calibration set before RTI capture, as we had before Photogrammetry. He may be working on incorporating a calibration correction into the highlight RTI Builder that CHI has made available. I’m sending this discussion along to the CHI forum at http://forums.cultur…ageimaging.org/ to see what others might have to add. When I return to work, I’ll ask Greg to give this some additional thought”

Whadaya think, Greg?

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The Evolution of Our Photogrammetry Workflow

Since we began working with photogrammetry we have developed a workflow that we follow every time we’re on site. That workflow has definitely changed and developed over the past several weeks. The process has become much smoother and what used to take 3-4 hours to shoot, now only takes 1.5-2 hours to shoot. Here are some of the changes we have made.

Topics covered within this post:

1. Tripod – Do you need it?
2. Creating Accurate Measurements
3. Image Composition
4. 60% Image Overlap
5. Calibration Sequence
6. Photogrammetry Around Corners or Objects

1. Tripod – Do you need it?

From what we have discovered, not really. Photogrammetry captures require a FIXED FOCUS and a FIXED APERTURE for all three capture orientations (90°, 180° and 270°). This means that your focal depth of field will be non-negotiable after you begin shooting. The only thing you can choose is your exposure time. You want an F-Stop of 11 or below to minimize distortion but high enough so that your depth of field will capture everything in the Z dimension (forward and back) in focus.  If you have enough ambient or artificial light to keep your shutter speed above 1/40th of a second, a tripod will not be necessary and hand-holding the camera, even with the inevitable shifts in camera orientation and slight distance irregularities, can work very well.  But remember, closer to the subject or further from the subject than the limits of the focal depth of field, and falling ambient light levels can result in unfocused regions. Blurry pixels contribute nothing in 3D or in 2D.  Marking the optimal distance from the subject using a string of chalk line, taping your lens focus ring to ensure it does not move, checking your F-Stop to be certain you have not moved it and checking your focus at EVERY capture will save you a lot of reshooting, whether you use a tripod or not. That said, moving the tripod along a chalkline laterally took up much of our time. Especially since we needed to overlap our images by at least 60%. Not only were we making sure the camera was almost always the same distance from the subject, but we always made sure it was level.

Dale adjusting the camera head

After many working days in the field and discovering just how powerful Agisoft PhotoScan is, we have decided the tripod is unnecessary except for low-ambient-lighting conditions,  haven’t experienced any true need for it yet in photogrammetry. As long as you walk along a measured line, keeping the distance between the subject and camera consistent, PhotoScan will have no problem creating the 3D mesh. We started handholding the camera a few weeks ago and the results have still been superb. PhotoScan is able to detect where each camera was positioned in relation to the subject AND at what angle. If the camera is angled slightly upward or downward, this doesn’t affect the processing at all. In fact, PhotoScan makes up for it and still creates an accurate digital surrogate. Now depending on your subject matter, it still may be necessary to use a tripod or monopod. We haven’t experienced any true need for it yet in photogrammetry.

Processed mesh of the roofless room. Images were captured handheld.

2. Creating Accurate Measurements

If one needs accurate measurements within the mesh of a certain subject, Dennison® dots have proved to be extremely valuable in this regard. Initially we were placing two parallel rows of dennison dots along our subjects (mainly walls and rooms). Each dot was 4 feet away from every other dot both vertically and horizontally. When it’s seen within the mesh, the dots help to know the exact scale of the object. Based on the distance between the dots, one is able to measure everything else within the mesh and know their exact dimensions.  If the distance between the dots is exactly 4 feet, or 122 cm, then any distance in the space is some fraction of 122 cm.  20% of the distance is 9.6 inches or 24.4cm. 1% is 0.48 inches or 1.22 cm.

The question becomes, what spacial resolution do you want to be able to resolve? If  you need to resolve characteristics as small as 5mm, then you need to be able to clearly resolve 5mm across several pixels in your initial Raw capture and your scale dots should be less than 2 feet apart.  For our first set of 3D condition captures at O’Keeffe’s historic home in Abiquiu, we went from using two rows, two feet apart to using one row. The only difference it made was making the process quicker.

3. Image Composition

When capturing our images we try to keep the whole subject tight within the frame. This speeds up the masking step in processing (see tutorial) by removing unnecessary objects and background from the photo.

The subject tight within the frame.

However, this isn’t always practical when working in tighter spaces. We had this experience last week when large cacti blocked our path.  We just had to do our best at the start of the shoot to plan ways of moving around them.  We could move the whole capture distance way-back and then shoot at 15 degrees to overlap areas behind the cacti, but this would include tons of sky and foreground.  Getting closer to the subject, capturing a smaller area in each shot and taking more photographs to cover the desired area was the best answer.   A 24mm lens helps when forced into tight places because it is a wide-angle lens. Wide angle lenses capture a broader area of the subject at a much closer distance than a 50mm lens. Sometimes you’ll need to take several photos vertically to capture the whole subject. And the distortion of the subject is much greater around the outer edges of the lens and capture area. But, thanks to the calibration sequence – taking the same area at 90°, 180° and 270°, the software will still process them without a problem as long as there’s still 60% overlap in the sequence of each shooting orientation. Like we said before, when PhotoScan detects these points, it also detects the angle of the camera and makes up for it.

4. 60% Image Overlap

When you’re making sure every sequential image overlaps at least 60% there are a couple of ways to do it. When looking through the viewfinder and you snap your shot. Locate a point on the subject that would be 1/3 of the frame and move to where that same point is at 2/3 of the frame. Having dennison dots helps in this regard if there isn’t anything distinct on the wall. We use the focusing squares within the viewfinder of our Canon Mark II to assist in overlapping ~60%.  They divide the view into 3rds in both the horizontal and vertical orientations.  We just find a feature at the center of our view, take the shot and then make the same feature be a third of the way from center, which ever way we are moving. Locate, shoot, move 33% and repeat!

Image depicting the 60% overlap between two sequential images.

5. Calibration Sequence

To calibrate the captured images and allow the software to correct for any lens distortion, we have to shoot photos at three different camera orientations per camera position before we move laterally. Our method of capturing the images at horizontal (the axis of the lens to the bottom of the camera at 180°), and two verticals (axis of the lens to the bottom of the camera at 90°and  270°) changed over a period of sessions.  First we would shoot our horizontal images moving laterally parallel to the wall, say from the left edge of the wall to the right edge of the wall.  Then we would come back and shoot the other two angles, each left to right again, for a total of 3 passes along the subject. When changing light and shadow movement rapidly changed the light on the subject area, we decided to begin shooting all three angles from the same spot, overlapping 60% on the verticals.

Since we were overlapping our images based on the narrower, vertical view, we knew the horizontal images would overlap more than 60% due to the increase in width of the capture area, giving us more than the necessary 60% overlap. However, this also resulted in us taking more images than were absolutely necessary.

So whether you want to shoot all angles from the same spot, or each angle one sequence at a time, it is up to your circumstances. Also whether you mind having extra images on the horizontal shots to have to process in your chunk.

6. Photogrammetry Around Corners or Objects

When moving around objects, or in our case, around an arced corner of a wall, the images have to be shot every 15 degrees. When doing photogrammetry on a smaller object, such as a sculpture or statue, the images must be captured every 15 degrees until a full 360 degree rotation is complete.

Photogrammetry around an object

In our case, going around a curved wall, we moved 15 degrees while maintaining our distance until we were once again parallel to the wall.

Hope you found this post useful!