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Author Topic: Mesh from Dense Point Cloud vs. Depth Maps  (Read 3302 times)

bgreenstone

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Mesh from Dense Point Cloud vs. Depth Maps
« on: October 05, 2021, 02:30:43 AM »
For the last 9 months that I've been using Metashape I have been generating my Meshes from the Depth Maps, mainly because it's faster.  However, something broke with 1.7.4 and 1.8 beta which is causing all sorts of blobs to appear on the underside of my meshes when generated from Depth Maps.  The blobs are impossible to remove, thus, leaving me with unusable meshes.

However, if I generate my meshes from a Dense Point Cloud everything is perfectly fine.  So, for now I'm staying with v1.7.3 so I can use the Depth Maps, but inevitably I will need to upgrade and will therefore be forced to use Dense Point Clouds.

So, my question is this:  generally speaking which method gives the most accurate and detailed results?  I've read conflicting information on the topic.  Some people say Depth Maps are best in every way while others say Dense Clouds are better for details despite the extra noise.  What say you guys?

-Brian
« Last Edit: October 05, 2021, 03:55:19 AM by bgreenstone »

Steve003

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Re: Mesh from Dense Point Cloud vs. Depth Maps
« Reply #1 on: October 15, 2021, 12:18:47 AM »
No one has answered this so far, its important !
This forum is a bit slow at times.
I also am itching to know this, and that 1.7.3 is not broken whilst 1.7.4 is, is worrying as I just updated thinking I am doing the right thing.
An internet article on Dive3D shows better definition from Depth Map, its better at fine lines, thin pipes, cables etc.
However I deleted out grass around  a wheel and mesh firstly in sparse point cloud then again in densePointCloud when in the latter it appeared as if I hadnt even deleted it from those coloured spots after align photos was done.
Then without realising, and going with default settings, I found out I had chosen depth map for generate mesh, and this depth map put the grass back in, I was told use mesh from DensePoint cloud to have it honour the deletion I did in DensePointCloud. As I want the extra definition on pipes etc how do I get rid of grass then ?  There is no stage one can delete such and influence depth map mesh generation or is there, that aspect of going straight from sparse cloud to a final mesh and no means of seeing a surface to edit away is UNSETTLING, and is a minus vote it seems for depth maps.

How is editing out areas and unwanted structures done if using depth maps  ?

Steve

sfmdd

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Re: Mesh from Dense Point Cloud vs. Depth Maps
« Reply #2 on: October 18, 2021, 09:26:24 AM »
You could delete the polygons and fill the voids with the "close holes"-command. This command is sometimes sketchy and not working as supposed but you could also try to fix the holes with 3rd-party software like Blender.

bgreenstone

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Re: Mesh from Dense Point Cloud vs. Depth Maps
« Reply #3 on: October 18, 2021, 03:20:43 PM »
Unfortunately, the mess that is being created is too complex to be manually deleted and repaired. 

Bzuco

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Re: Mesh from Dense Point Cloud vs. Depth Maps
« Reply #4 on: October 18, 2021, 05:35:57 PM »
Maybe select mesh by element function in 3D apps.
Slelect biggest mesh elements and then invert selection and delete the mess.
Or select vertices by vertex color, convert selection vertex => faces/elements and delete.
Or simply create mesh from dense cloud, where you can select points by color or some other metrics. Use metashape/cloudcompare for creating mesh and filtering points, what will be better for you or what will create better result. I think if dense cloud is good and dense enough, there should be not so difference quality in mesh compared with pure depth maps workflow.

bgreenstone

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Re: Mesh from Dense Point Cloud vs. Depth Maps
« Reply #5 on: October 18, 2021, 07:28:55 PM »
Heh, well, thanks for the suggestions, but the geometry it's creating is so bad under there and so interconnected to the top of the mesh that there's simply no way to remove it.  For now I'm just using 1.7.3 since that's the last version that worked properly.


Eric Baird

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Re: Mesh from Dense Point Cloud vs. Depth Maps
« Reply #6 on: October 18, 2021, 09:47:08 PM »
For the material I'm currently working on, depth maps do seem to give a nicer result.

The difference is probably slightly exaggerated with 1.7.4, as the "blobbiness" problem will be using up some of the polys, leaving with fewer faces for the "proper" model for a given face-count ... but even without that, depth maps seem to yield generally better results for things like architecture, IMO. I think this was one of the selling points of the new approach when Agisoft introduced the new method -- improved speed and visibly better results.

I used to use dense maps "select points by colour" to select and delete white and blue points that represented stray pieces of sky and false "sky flare" detail that appeared on roofs when when a building was backlit by over-bright sunlight. But this problem seems to be less severe with the depth map approach (either that, or I'm now taking fewer bad photos!).

I suppose that one thing that might help would be if Agisoft copied the existing "select points by color" feature (for dense clouds), as a "select faces by color" feature for meshes. If that new feature appeared, you could then delete all the green and greenish patches of the mesh that belonged to grass.


Bzuco

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Re: Mesh from Dense Point Cloud vs. Depth Maps
« Reply #7 on: October 18, 2021, 11:29:05 PM »
@bgreenstone

Blobbiness issue: The blob mesh should have very large polygons.  If you export the mesh to meshlab, there is function "select faces with edges longer than..." with edge threshold slider and preview options. You should be able to select and delete faces of the blob only, because the rest of the mesh should have much smaller polygons. Then export/import back to metashape. Maybe this will help you until it will be corrected somehow in metashape.

As I know from cloudcompare/meshlab, when creating mesh from points using poison surface reconstruction function, one of it's parameter/options is creating this blob shape unwanted mesh, other options are creating just flat surface at the edges of the object. Maybe metashape is using this method or something similar. It also does not happend everytime and on every object, but I dont know exactly what is needed to prevent the blob effect.