Forum

Author Topic: Shadows Causing Deformations in DSM  (Read 2245 times)

imageprocessing1

  • Newbie
  • *
  • Posts: 3
    • View Profile
Shadows Causing Deformations in DSM
« on: October 17, 2018, 08:09:20 AM »
Hi,

We are having issues where we get large deformations in our DSM due to shadows of various objects like light posts. 

See the attached two images.  In the top left you can see the shadow from the light post, and then in the DSM you can see the deformation from this.  The ground is known to be smooth yet, the DSM shows it as deformed right where the shadow is. 

Does anyone know a way to process the data without this occurring?  What settings might we try?

We believe that this effect is particularly pronounced when the aircraft has taken a long time to return and the shadows have moved and/or the surface is fairly featureless.

JMR

  • Hero Member
  • *****
  • Posts: 502
    • View Profile
Re: Shadows Causing Deformations in DSM
« Reply #1 on: October 17, 2018, 03:08:40 PM »
The case is quite extreme. yes, the moving shadows will be understood by SFM as features in motion relatively to the context thus having different elevation than surounding ground (lower or higher) so, sadly there is nothing you can do to prevent that but by masking shadows that are likely to have changed significanlty during flight turn time.
you could edit dense cloud and delete those dark points prior to build the DEM or MESH
I wonder also why the cabinet is so poorly modeled... flying lower and might help to grab more texture.
Regard

imageprocessing1

  • Newbie
  • *
  • Posts: 3
    • View Profile
Re: Shadows Causing Deformations in DSM
« Reply #2 on: October 17, 2018, 06:16:51 PM »
Thanks for your reply!

What about cases where the shadows haven't moved and we still see the issue?  Is there anything we could do about that?

This is an area we re-fly consistently - is there any way we could use data from other captures to assist in the processing of the current capture?

Is there any way to remove shadows automatically?  Such if we know where the lightposts are could we predict the shadow location (based on time of day and year) and remove them?

bisenberger

  • Sr. Member
  • ****
  • Posts: 329
    • View Profile
    • Digital Mapping & Graphics
Re: Shadows Causing Deformations in DSM
« Reply #3 on: October 18, 2018, 04:44:47 AM »
Flying it when overcast with diffuse lighting should take care of the shadows. This is also done for generating clean textures (without shadows) for 3d modeling.
Digital Mapping & Graphics LLC
https://digital-mapping.net/

James

  • Hero Member
  • *****
  • Posts: 748
    • View Profile
Re: Shadows Causing Deformations in DSM
« Reply #4 on: October 18, 2018, 12:23:26 PM »
I came up with a solution to something like this in this post http://www.agisoft.com/forum/index.php?topic=2004.msg10673#msg10673 which involved deleting sunken areas in the mesh caused by moving shadows, and then using the model as a source to generate masks, but your mesh looks so undulating that i don't think it will help you out so much here.

Also the 5GBP bet was a one time offer and i won't be repeating it this time round!

James

  • Hero Member
  • *****
  • Posts: 748
    • View Profile
Re: Shadows Causing Deformations in DSM
« Reply #5 on: October 18, 2018, 12:27:10 PM »
something else you can try is using this masking technique, and just manually mask all shadows once only, and this may be enough for it to ignore them in all photos.

http://www.agisoft.com/forum/index.php?topic=8491.0

imageprocessing1

  • Newbie
  • *
  • Posts: 3
    • View Profile
Re: Shadows Causing Deformations in DSM
« Reply #6 on: October 18, 2018, 06:46:00 PM »
thanks much James! I  will give these a try.