Ground control and markers

[JanS]

Hi,

In our last projects, using v0.8.4 we did some experimenting with the ground control settings. While certainly already functional, I think there are some improvements that can make this feature much more useful. Here are some comments and recommendations.

- placing GCP's (markers) in itself works well, zooming in and out is fast and it helps a lot that markers can be seen on the model as well as on the photo's
- as I understand now, when adding a marker before the model geometry is constructed, this markeris only on one photo. When adding it after model construction, it is also added on every photo where it is visible. This is a nice feature, but it can cause a lot of work to place every point right. e.g. in our last UAV project, overlap was between 65 and 85% so every marker was on between 10 and 15 photo's. We typically have between 10 and 20 markers per flight... My suggestion is to slightly modify the behavior as follows: when one adds a marker on a photo, it is automatically added on every other photo, but as a 'preliminary' or 'temporary' point, e.g. with a grey flag instead of a blue flag. When one opens a photo with a temporary point and moves the point, it becomes a 'definitive' point. Photoscan would then only use the 'definitive' markers for calculations, while the temporary points are used to assist the user.
- the use of markers as tie points to optimize the point cloud/camera calibration seems a very strong feature to me. Calibration parameters supplied by UAV manufacturers may not be that reliable and/or not entirely compatible with Photoscan. As I understand this feature now, there may however be an issue with the processing chain: 1. align photos, 2. build geometry as it is much easier to place markers, 3. optimize point cloud which detroys the model (am I right?).
To avoid building the model twice, it could help if PhotoScan could place preliminary markers on photo's directly after camera alignment. If a user enters or imports reference X,Y and Z coordinates, an initial estimate of the position on the photo could be made assuming a flat terrain. Now users can position the markers right (not necessarily all of them), optimize the point cloud and then build the geometry.

Best regards,

Jan

[Alexey Pasumansky]

Hello Jan,

Thank you for suggestions.

If I do understand correctly in pre-release version of 0.8.5 the function of "grey-flagged" marker is supported. If the marker is defined at least on two images (by Place marker command or using Create marker feature if the geometry is not built) its projections on other photos (if present) would be marked by grey flags.
Such flags are not taken into account during georeferencing or optimization and only shows the possible location of the marker. If clicked on grey flag will turn blue.

Markers positions and desirable accuracy of positioning/reprojection are involved during optimization process. Also in 0.8.5 additional camera calibration parameters (such as skew, aspect ratio, tangential and radial distortion coefficients) could refined during optimization.


Also in the latest pre-release of 0.8.5 (build 1398) inputting coordinates for marker instance (which is not defined on any image) shows the marker in the Model view, if the corresponding coordinate system is selected.
But we suggest that building geometry in point cloud quality for guided approach of marker placement is handy since point cloud geometry is built in seconds.

[JanS]

Thank you Alexey,

It seems we had the same ideas about the markers. I indeed only tested them in 0.8.4.
Does your optimization algorithm have a protection against overfitting (i.e. if too few markers are used) and - related to this - any suggestions on how many markers/on how many images are required for reliably fitting the camera model?

Keep up the good work,

Jan

[jan]

I'm using PS 0.8.5 now, but I don't see the "grey flag feature"? Do I have to enable it somewhere?

kind regards,
Jan

[Alexey Pasumansky]

Hello Jan,

Please try to remove one projection of any marker from image and you'll see "grey flag" in its estimated position based on other projections.

[jan]

Hello Alexey,

I've tried several things, but I only managed to manually grey a marker on individual pictures.

Is it possible to simultaneously grey them all out on each picture on which they occur, and enable (making blue) only those that I want to enable. In one project I have for instance 4 markers, and each marker occurs on more or less 25 pictures (yes a lot of overlap in that region). It still takes a lot of time if I have to manually grey out 20 (from 25) of them for each marker.

thank you,
Jan

[Alexey Pasumansky]

Hello Jan,

We will think about such functionality that will allow to remove selected markers from selected photos.
Currently it could be either done manually or using Python scripts.