orthophoto from standard editon

[RalfH]

In the PhotoScan standard edition, I have the possibility to create and export orthophotos. It is not possible to set control points in the standard edition, and the model is created in an arbitrary coordinate system. Does anybody know how PhotoScan creates and aligns the orthophoto in this case?

[Alexey Pankov]

In the PhotoScan standard edition, I have the possibility to create and export orthophotos.

But.. How it is possible?

[RalfH]

After you have created a 3D model, you can select "Build Texture" under "Workflow". There, you can select the mapping mode "Orthophoto". After creating the texture, select "Tools" --> "Export Texture".

As far as I can judge, the resulting image is an orthophoto (without georeferencing). I have tried this with legacy aerial photographs (from 1944, n=20) and also with some ground based images (n=33). Using only a few reference points, the orthophotos can be georeferenced, and they align well with Google Earth.

Now, the interesting thing is that in the standard edition I can not set ground control points. The 3D model is in an arbitrary coordinate system, with the X, Y and Z axis definitely not aligned with real world coordinates.

My question is how PhotoScan choses the plane on which it projects the input images to create the orthophoto. Does it simply use the plane defined by the maximum extents of the model? Does it use the bounding box? Any ideas out there? Anyone from Agisoft who wants to tell me?

[Alexey Pankov]

As far as I understand how PhotoScan works, it projects texture on base plane of bounding box, the one that is shown by red frame.
PS Standard doesn't have any referencing tools, so the only way to reference such "orthophoto" is to perform it in external tools. I think every GIS has a tool for manual referencing of surface images, even Google Earth.
However, I always thought that texture size is very limited by RAM available.

[RalfH]

Hmmm, I just played with changing the bounding box; this does not have any effect on the created orthophoto. Must me something else.

P.S.: I have now tested changing the model extensions by exporting it to .obj, arbitrarily changing the coordinates of the first point by multiplying them by 10 and re-importing the mesh to PhotoScan. The orthophoto created on the basis of this model changed accordingly (i.e., the actual image in now small and in one corner of the exported texture). Looks as if by tweaking the model the orientation of the orthophoto could be changed. The plane on which the orthophoto is projected did not seem to change though. So it looks like PhotoScan uses a plane that is not optimised to the extreme extents of the model point coordinates but rather to all points of the model.

[Alexey Pankov]

Hmmm, I just played with changing the bounding box; this does not have any effect on the created orthophoto. Must me something else.

I also tried to change (orientation of the) bounding box.
When you change it after model is build it doesn't have any effect. But if you change bounding box, rebuild model and export texture, the texture plane corresponds to PS base plane very well. So for me it looks like PS projects texture on the base plane that it 'remembered' from the model generation.
However your experiments with changing point coordinates break this theory.
I dont't have other ideas.
Will wait when you turn Standard into Pro

[RalfH]

OK, I will keep playing with it when needed. For the moment, what I get is good enough for me (although it would be nice to know how exectly this works). As for upgrading to pro, I can't really spend that much money for software which I only use sporadically.