Photographing Marine Gunwales and Narrow Areas
Insufficient target placement and weak photography usually manifest in not all photos solving (red X photos after processing). In particular if you have one part of the boat where none of the photos solved, especially if this area is a gunwale or an area connected by a gunwale, you might improve it by using the techniques described below.
Summary
- stagger targets in narrow areas
- place targets on other areas to help orientation – even if you don’t need to model the areas
- cover photos with targets – the photos don’t need to cover the water and the dock
- vary the distance and angle of photos
Stagger Targets in Narrow Areas
Do not place your targets on the gunwale in a straight line. A set of linear points is not sufficient to define a plane to draw on. Spread the targets out so they give a better definition of the surface shape.
Improve Target Spread
Include targets on both the gunwale and the near by boat deck or other regions to increase the spread of targets across the photos. Even if these other areas will not be traced it is important to have targets cover as much of your photo as possible. These extra targets help with photo orientation and help tie photos together.
Associated with this issue of photo coverage, ensure most of the photo is covered with targets. Do not center your photos over the gunwale. There is no benefit to capture the area outside of the boat (water, dock, etc.) since there are no targets there.
Another example of spread is shown below. Adding targets to areas where you might not need to model (the seat) and avoid large photo areas with no targets. The green area below shows adding targets to other areas to help the narrow area solve.
Photos Angle and Distance Variation
Instead of taking a straight row of photos that are all pointing down and are at the same height, add some variation. Take photos in a staggered pattern, take some angled photos looking along the gunwale, and take photos at different heights. These all help to tie the whole project together and improve overlap (this helps PhotoModeler solve all photos together).
Different angled photos. Courtesy of Marine Flooring Aruba.
Since the gunwale will be raised, if you are just standing on the main deck, your photos will be much closer to the targets. This will reduce the field of view, and you will capture a smaller area with each photo. Keep this in mind when you are moving between positions to ensure you get sufficient overlap with the previous photos. That is, if you are closer, you need more photos with small steps. You may need photos at and smaller steps to ensure overlap. If possible, take the photos from further back (perhaps by standing on the gunwale or a ladder).
Deal with Obstructions and Occlusions
If the gunwale has railings or other items that will cause occlusions (ie. block the view of the targets), extra care must be taken. Try to position the camera so the targets are not blocked, and keep this in mind when placing targets. In some cases, it can be helpful to clip targets to the railing.
For example, in this image the targets highlighted in red would not be detected. In these cases it is important to have redundancy by including targets in other locations, and taking extra photos to capture the area from different angles that limit the occlusions.
On some boats you might have a window that is highly reflective. This may lead to some targets being mirrored in the window so the photograph contains not only the original targets, but their mirrors. These extra mirrored targets can cause some confusion in connecting them to the wrong part of the boat (the mirrored targets have a different target ID). If you cannot position the camera to reduce these reflections, a solution is to cover the windows with anything non-reflective. Maybe use the opportunity to place some coded targets on top of the cover!
Additional Resources
In addition to the recommendations for target layout and photography described above, you may want to study:
- For general target and photography guidelines: PhotoModeler On-Boat Work Tutorial Video
- This more general page on red X / unsolved photos: Problems with a Deck Project – Red X Photos
Conclusion
While modeling narrow areas (such as marine gunwales) takes more care, it is possible. The keys are a) target placement (stagger smaller targets on gunwale and place targets in other areas – even if you do no need to model them, and b) more photos with better angles (esp. some angled photos with variety of distances to capture narrow and some wider areas). Do not hesitate to contact PhotoModeler Support if you are having trouble with such an area.