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Thread Subject:
area of 2D point cloud

Subject: area of 2D point cloud

From: Jveer

Date: 17 Jan, 2009 13:15:04

Message: 1 of 13

does anyone know how to calculate the area of a 2D point cloud?

the point cloud irregular and has random holes. i need the area to be as accurate as possible

any suggestions?

Subject: area of 2D point cloud

From: ImageAnalyst

Date: 17 Jan, 2009 13:24:15

Message: 2 of 13

Jveer
If they are just a bunch of points that are not connected to each
other (randomly placed isolated dots), you'd have to use something
like the convex hull or alpha shapes. If the points are really like a
solid blob but with some holes here and there, you could just
threshold and sum, like sum(imageArray(:) > thresholdValue). This
would also work for the first case if you want to consider the area of
a point as 1. It really depends on how you want to define "area" and
"point cloud."

Maybe you could post a picture or plot for us to look at.
-ImageAnalyst

Subject: area of 2D point cloud

From: Jveer

Date: 17 Jan, 2009 13:40:18

Message: 3 of 13

thanks for the reply

i've uploaded a picture called Area.tiff on http://idisk.mac.com/jveer-Public?view=web

the points themselves dont have any area. they are just spatial coordinates. i need the area bound by the perimeter of those coordinates.

Subject: area of 2D point cloud

From: Image Analyst

Date: 17 Jan, 2009 14:06:02

Message: 4 of 13

Can you consider that it's a 3D volume image where each coordinate fills a voxel? If so, then just use the second method. Convex hull won't work because it's kind of a rectangular torus and convex hull is like if you wrapped the object in a balloon.

Subject: area of 2D point cloud

From: Rune Allnor

Date: 17 Jan, 2009 14:07:11

Message: 5 of 13

On 17 Jan, 14:40, "Jveer " <jv...@jveer.com> wrote:
> thanks for the reply
>
> i've uploaded a picture called Area.tiff onhttp://idisk.mac.com/jveer-Public?view=web
>
> the points themselves dont have any area. they are just spatial coordinates. i need the area bound by the perimeter of those coordinates.

doc convhull

Rune

Subject: area of 2D point cloud

From: Jveer

Date: 17 Jan, 2009 14:29:01

Message: 6 of 13

solved. thank you all for your suggestions.

Rune, as correctly pointed out by ImageAnalyst, convhull is not an option.

the solution is actually very simple- note that the points are equidistant. the known variables are the coordinates of each point and the number of points forming the area.

therefore considering a small area dx*dy, the area of 4 adjacent points forming a square can be found. the area of each point is then area of dx*dy/4.

final area of no matter how irregular the shape is will simply be (dx*dy/4)*number of points

solved.

Subject: area of 2D point cloud

From: John D'Errico

Date: 17 Jan, 2009 16:45:04

Message: 7 of 13

"Jveer " <jveer@jveer.com> wrote in message <gkspvd$go9$1@fred.mathworks.com>...
> solved. thank you all for your suggestions.
>
> Rune, as correctly pointed out by ImageAnalyst, convhull is not an option.
>
> the solution is actually very simple- note that the points are equidistant. the known variables are the coordinates of each point and the number of points forming the area.
>
> therefore considering a small area dx*dy, the area of 4 adjacent points forming a square can be found. the area of each point is then area of dx*dy/4.
>
> final area of no matter how irregular the shape is will simply be (dx*dy/4)*number of points
>
> solved.

Use an alpha shape for a general domain.

John

Subject: area of 2D point cloud

From: maya

Date: 18 Jan, 2009 02:50:03

Message: 8 of 13

Hai,
 can u suggect a method to find point cloud on CT slices for 3D reconstruction"Jveer " <jveer@jveer.com> wrote in message <gkslko$93j$1@fred.mathworks.com>...
> does anyone know how to calculate the area of a 2D point cloud?
>
> the point cloud irregular and has random holes. i need the area to be as accurate as possible
>
> any suggestions?

Subject: area of 2D point cloud

From: Jveer

Date: 18 Jan, 2009 07:58:06

Message: 9 of 13

hello Maya

here is a an example of a point cloud slice of a 3D solid (the file is named 3DPointCloudSlice)
http://idisk.mac.com/jveer-Public?view=web

i apologise but i'm not sure i understand your question. what do u mean by CT slice?

Subject: area of 2D point cloud

From: John D'Errico

Date: 18 Jan, 2009 11:36:02

Message: 10 of 13

"Jveer " <jveer@jveer.com> wrote in message <gkunee$b20$1@fred.mathworks.com>...
> hello Maya
>
> here is a an example of a point cloud slice of a 3D solid (the file is named 3DPointCloudSlice)
> http://idisk.mac.com/jveer-Public?view=web
>
> i apologise but i'm not sure i understand your question. what do u mean by CT slice?

Sorry, but I can't view an image from that location.
When I try to, all I get is a 16x16 thumbnail of a
general file icon, not the 146K image file itself.

John

Subject: area of 2D point cloud

From: Jveer

Date: 18 Jan, 2009 18:19:01

Message: 11 of 13


> Sorry, but I can't view an image from that location.
> When I try to, all I get is a 16x16 thumbnail of a
> general file icon, not the 146K image file itself.
>
> John
>

very strange. normally all you have to do is click the little download arrow on the right hand side of the file.

for simplicity, i emailed you the files and a question on rendering point clouds back to CAD style objects.

Subject: area of 2D point cloud

From: maya

Date: 18 Jan, 2009 23:55:04

Message: 12 of 13

Hello jveer,
sorry,i was not clear in my question. My work is to reconstruct a 3D structure from 2D CT(computed Tomography) scan iimages(slices).there are 236 2D images .these images are in dicom format.now i want t find the coordinates of the points on these images to form a point cloud.
Thanks
Maya

Subject: area of 2D point cloud

From: ImageAnalyst

Date: 19 Jan, 2009 23:36:28

Message: 13 of 13

I had the same trouble as John initially. The problem is that the
image is not displayed right off the bat like on most photo sharing
sites. It is very non-intuitive to click the little down arrow in
order to download and then see the file. A different web site would
be better.

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