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Author Topic: Problem with DEM in shadows  (Read 8294 times)

d.portillo@habitatea.es

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Problem with DEM in shadows
« on: January 31, 2014, 12:50:43 PM »
Hi, I?ve a problem with shadows when I generate dense point cloud, shadows are below the surface (10cm aprox), the surface is a basketball court and the GCP were taken with very accurate GPS (double frequency), with a higth overlap 80%-80% and low noise sensor.
Also the surface (that is smooth) seems too noisy. If I set agressive map filtering, this improve but not enough.
I?m going to try mask shadows, but is tedius. Someone knows why this happen? I hadn?t this problem with others cases on irregular surfaces.
Thanks.

Marcel

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Re: Problem with DEM in shadows
« Reply #1 on: January 31, 2014, 12:56:13 PM »
Which camera are you using and at what ISO setting are you shooting?

If you look closely at the parts that are in shadow, do you see strong ISO noise? In general the darker areas of the photo have more noise, and noise can lead to unexpected results.

Alexey Pasumansky

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Re: Problem with DEM in shadows
« Reply #2 on: January 31, 2014, 12:59:09 PM »
Hello d.portillo,

Could you please post Chunk Info screenshot?

There's no dense cloud in the Workspace pane, are you sure you are generating mesh based on the Dense cloud? The number of faces also seems to be very low.
Best regards,
Alexey Pasumansky,
Agisoft LLC

hengefjes

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Re: Problem with DEM in shadows
« Reply #3 on: January 31, 2014, 01:56:35 PM »
It looks like the shadows moves with the sun. The longer the shadow is thrown, the more deviation.

d.portillo@habitatea.es

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Re: Problem with DEM in shadows
« Reply #4 on: January 31, 2014, 06:30:00 PM »
Thanks at all for responses:
Marcel: camera is sony qx100, ISO 160 and speed 400-800, photos was taken with multicopter at low speed. The images are clear no noisy, even dark areas.
Alexey: on the other screenshot you can?t see dense cloud because I erased it accidentally. And the process were at low quality, but the problem persist at high quality. On last process at high quality I masked the shadows and on sparse cloud filtering:
Reprojection error to 1, Reconstruction uncertainty to 10, Image count no filter
On previous process I filtering image count  to 2. Obtaining now better results of noise than the first process. But persist the altitude problem on shadows.
Hengefjes: I thought this too but the fly time was 5 minutes only.

James

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Re: Problem with DEM in shadows
« Reply #5 on: January 31, 2014, 07:23:51 PM »
you could mask the shadows quickly by deleting the 'sunken' areas from your dodgy mesh, and then use tools->import->import mask, then select 'from model' method and 'active chunk'.

i bet 5GBP on it being the shadows moving enough in 5 mins to cause this.

In many cases the shadows aren't prominent enough features to have such an effect, but on a plain surface like that perhaps they would be.

Imagine if the ground really was depressed there by an inch or so, the disparity in the aerial images would be barely noticeable, yet photoscan would have identified it, so it is sensitive to the slightest movements.

d.portillo@habitatea.es

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Re: Problem with DEM in shadows
« Reply #6 on: February 03, 2014, 11:49:10 AM »
James I think like you, but could be good that Alexey confirm it.
I don't understand your process to mask the model. I masked each photo by selection magic wizard. Can you explain better?I tried your steps but only corners and margins was masked. Thanks
« Last Edit: February 03, 2014, 01:18:02 PM by d.portillo@habitatea.es »

James

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Re: Problem with DEM in shadows
« Reply #7 on: February 03, 2014, 02:29:19 PM »
please see pictures attached below, hope it makes sense!

01. Use the freeform selection tool to select the faces of the textured mesh that correspond to the problematic shadow areas. you may need a slightly more dense mesh to get a better result.

02. delete those faces

03. import mask from model to active chunk

04. masks are created in all photos for areas where the mesh does not exist
« Last Edit: February 03, 2014, 02:31:29 PM by James »

d.portillo@habitatea.es

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Re: Problem with DEM in shadows
« Reply #8 on: February 03, 2014, 03:26:01 PM »
Thank you very much for your help, I masked over dense cloud . I'm going to do that with the mesh.


d.portillo@habitatea.es

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Re: Problem with DEM in shadows
« Reply #9 on: February 05, 2014, 08:00:14 PM »
Thanks James, I removed the undesirable shadows effect with your method.
Although the noisy surface not is all good I desire, but I think that is imposible to improve.
Is it possible close the holes on the ortophoto?
« Last Edit: February 05, 2014, 08:05:41 PM by d.portillo@habitatea.es »

James

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Re: Problem with DEM in shadows
« Reply #10 on: February 05, 2014, 11:57:36 PM »
You need to remove the masks again, once you've generated the dense cloud/mesh so that it's all used for the texturing/orthophoto generation. I think you can right click the chunk and select remove masks.

d.portillo@habitatea.es

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Re: Problem with DEM in shadows
« Reply #11 on: February 06, 2014, 06:44:47 PM »
Holes closed, thanks for your tips  :)