Doubt about crop_point_cloud working

[VictorGimenez]

Hi,

I would like to know how the o3d.visualization.read_selection_polygon_volume.crop_point_cloud works, for that I followed this example in this link: https://www.open3d.org/docs/release/tutorial/geometry/pointcloud.html#Crop-point-cloud and reproduced this example by creating my own cropped.json file.

I executed the script /home/victorbg/miniconda3/envs/3Dbce_env/lib/python3.12/site-packages/open3d/examples/visualization/interactive_visualization.py and I was selecting the chair little by little until reaching this result:

After, I created a script (if anyone would like to see the code I can show) without downsampling to get the numpy.array from this .ply with approximately this content:

np.asarray(pcd.points):
[[1.99609375 1.49525154 1.90234375]
 [1.99609375 1.49609375 1.90226912]
 [1.98046875 1.49609375 1.90806305]
 ...
 [2.30332327 2.40234375 1.80078125]
 [2.30078125 2.402771   1.80078125]
 [2.30078125 2.40234375 1.80207741]]

With this script I generated a custom cropped.json file for test and understanding purposes, the file different from the original one (with only 31 lines being 20 np.arrays) from the example had 214460 lines with this content, also, I decided to use the same values to axis_max, axis_min and still maintained the orthogonal_axis in Y:

{
    "axis_max" : 4.022921085357666,
	"axis_min" : -0.76341366767883301,
    "bounding_polygon": [
        [
            1.99609375,
            0.0,
            1.90234375
        ],
        [
            1.99609375,
            0.0,
            1.9022691249847412
        ],
        [
            1.98046875,
            0.0,
            1.9080630540847778
        ],...
        ....
        ,
        [
            2.30078125,
            0.0,
            1.80078125
        ],
        [
            2.30078125,
            0.0,
            1.8020774126052856
        ]
    ],
    "class_name": "SelectionPolygonVolume",
    "orthogonal_axis": "Y",
    "version_major": 1,
    "version_minor": 0
}

This is the original cropped.json one:

{
	"axis_max" : 4.022921085357666,
	"axis_min" : -0.76341366767883301,
	"bounding_polygon" : 
	[
		[ 2.6509309513852526, 0.0, 1.6834473132326844 ],
		[ 2.5786428246917148, 0.0, 1.6892074266735244 ],
		[ 2.4625790337552154, 0.0, 1.6665777078297999 ],
		[ 2.2228544982251655, 0.0, 1.6168160446813649 ],
		[ 2.166993206001413, 0.0, 1.6115495157201662 ],
		[ 2.1167895865303286, 0.0, 1.6257706054969348 ],
		[ 2.0634657721747383, 0.0, 1.623021658624539 ],
		[ 2.0568612343437236, 0.0, 1.5853892911207643 ],
		[ 2.1605399001237027, 0.0, 0.96228993255083017 ],
		[ 2.1956669387205228, 0.0, 0.95572746049785073 ],
		[ 2.2191318790575583, 0.0, 0.88734449982108754 ],
		[ 2.2484881847925919, 0.0, 0.87042807267013633 ],
		[ 2.6891234157295827, 0.0, 0.94140677988967603 ],
		[ 2.7328692490470647, 0.0, 0.98775740674840251 ],
		[ 2.7129337547575547, 0.0, 1.0398850034649203 ],
		[ 2.7592174072415405, 0.0, 1.0692940558509485 ],
		[ 2.7689216419453428, 0.0, 1.0953914441371593 ],
		[ 2.6851455625455669, 0.0, 1.6307334122162018 ],
		[ 2.6714776099981239, 0.0, 1.675524657088997 ],
		[ 2.6579576128816544, 0.0, 1.6819127849749496 ]
	],
	"class_name" : "SelectionPolygonVolume",
	"orthogonal_axis" : "Y",
	"version_major" : 1,
	"version_minor" : 0
}

After I reproduced the same code, only modifying the o3d.visualization.read_selection_polygon_volume for the path where there is my cropped_chair.json and I executed the code:

demo_crop_data = o3d.data.DemoCropPointCloud()
pcd = o3d.io.read_point_cloud(demo_crop_data.point_cloud_path)
    vol = o3d.visualization.read_selection_polygon_volume("/home/victorbg/open3d_data/extract/DemoCropPointCloud/cropped_chair.json")
chair = vol.crop_point_cloud(pcd)
o3d.io.write_point_cloud("chair.ply", chair)

I got this result after a very long time:

and this amount of points:
chair:PointCloud with 27027 points.
len np.array(chair.points):27027

Less than in the original one from the example:
chair:PointCloud with 31337 points.
len np.array(chair.points):31337

With lots of holes, also I am without any idea how should I select the correct points to crop the original .ply. Also, I would like how could I do to crop the armrest, isolate the chair from the ground for example to more examples to understand better this concept