finding systematic misalignment in points from PTX files, when angles/range calculated.

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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by VXGrid »

psiplace wrote: Thu Apr 02, 2020 5:12 pm > Faro scan not registered (inclinometer not used) no resampling

i have taken a copy.
Would be great if you share your findings with us once you had time to check the file :)
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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by psiplace »

> Would be great if you share your findings with us once you had time to check the file

for sure.

i looked at the scan (inside a barn?) and its hard to see the issue, but maybe the roughness of all the wood/plaster in the scene is hiding it. i am trying to filtering out parts of it to see if it helps.

is it OK to share? i'd like to split it into a couple of parts, introduce an offset to one part, then see what a friend of mine's software does registering them back to together. software (i forget its name.) i know produced files with the issue.
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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by psiplace »

OK, i found a smooth region, close to the scanner with a step change in distance.

this is the handle of a shovel (going left/top to right/bottom.)

(there's also some vibration, i think, down the middle, it goes all the way from top to bottom of the full scan.)

Screenshot_2020-04-04 WebGL Pointcloud Viewer(4).png
what it shows, quite nicely, is that the light beam and the detection sensor optics are significantly offset.

the light is in lines but the sensor sees it off to the side.(giving a 3d effect)

seems conclusive because there is also an empty 'halo' along the sides, the outgoing beam is being shadowed from the point-of-view of the optics, on one side, and the optics can't see the beam spot, as it goes behind the object, on the other side.

do some scanners work by measuring angle, like Kinect, i thought they used time-of-flight? (lidar) surely the angle wouldn't be precise enough, or is this to simplify the optics?

notice: the points are not in the wrong place or anything.

if you know the offset then its easy to allow for.

in a sense its like two scans, from two slightly different points, with some holes at object edges.
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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by Tookie »

I might be wrong, it happens regularly but.

Scanners know the azimuth and elevation angles very precisely they then send out the laser and measure either time of flight or phase ( of the return signal compared to the original) to determine the range.

Faro use the phase method.

In your example there is some shadowing but also not all of the lines are straight, I wonder if this is either because the Spot size is large enough that is on both the fork handle and the wall behind or that the software has run out of precision and is missplacing the point.

I would imagine it is all within the specified 3d point accuracy of the device used.
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Re: finding systematic misalignment in points from PTX files, when angles/range calculated.

Post by VXGrid »

psiplace wrote: Sat Apr 04, 2020 9:49 pm OK, i found a smooth region, close to the scanner with a step change in distance.

this is the handle of a shovel (going left/top to right/bottom.)

(there's also some vibration, i think, down the middle, it goes all the way from top to bottom of the full scan.)


Screenshot_2020-04-04 WebGL Pointcloud Viewer(4).png

what it shows, quite nicely, is that the light beam and the detection sensor optics are significantly offset.

the light is in lines but the sensor sees it off to the side.(giving a 3d effect)

seems conclusive because there is also an empty 'halo' along the sides, the outgoing beam is being shadowed from the point-of-view of the optics, on one side, and the optics can't see the beam spot, as it goes behind the object, on the other side.

do some scanners work by measuring angle, like Kinect, i thought they used time-of-flight? (lidar) surely the angle wouldn't be precise enough, or is this to simplify the optics?

notice: the points are not in the wrong place or anything.

if you know the offset then its easy to allow for.

in a sense its like two scans, from two slightly different points, with some holes at object edges.
Interesting to see that this is happening in other scans as well.

You can share this scan if you like to.
In the middle and at the boundaries of a scan you have the overlapping area (a scanner is only measureing 180°+DELTA but in two view directions), therefore the overlapping area.

Scanners are using either time of flight or phase comparison, no grids like kinect.

Just curious, but is it possible that this effect is coming from your calculation when using floats, instead of doubles together with sin, cos, tan functions which results in precision loss?
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