Fix color update in gpu

[cdh981009]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

There was a typo, UpdateGeometry called GetPointPositions instead of GetPointColors for the color data when updating the colors of a pointcloud in a gpu.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

Fixed the typo.

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[benjaminum]

Hi @cdh981009 , thanks for creating this PR. Do you have a minimal example to produce the problem? Does the problem only occur when using a point cloud stored on the GPU?

[cdh981009]

Hi @benjaminum.

I modified examples/python/visualization/multiple_windows.py and removed irrelevant part. Hope this works as a minimum example.

import open3d as o3d
import threading
import time

CLOUD_NAME = "points"

def main():
    App().run()

class App:
    def __init__(self):
        self.is_done = False
        self.cloud = None
        self.main_vis = None

    def run(self):
        app = o3d.visualization.gui.Application.instance
        app.initialize()

        self.main_vis = o3d.visualization.O3DVisualizer()
        self.main_vis.set_on_close(self.on_main_window_closing)

        app.add_window(self.main_vis)

        threading.Thread(target=self.update_thread).start()

        app.run()

    def on_main_window_closing(self):
        self.is_done = True
        return True

    def update_thread(self):
        pcd_data = o3d.data.DemoICPPointClouds()
        self.cloud = o3d.io.read_point_cloud(pcd_data.paths[0])
        
        self.cloud = o3d.t.geometry.PointCloud.from_legacy(self.cloud).cuda() # <-- remove cuda() here to test on CPU
        bounds = self.cloud.get_axis_aligned_bounding_box().to_legacy()

        def add_first_cloud():
            mat = o3d.visualization.rendering.MaterialRecord()
            mat.shader = "defaultUnlit"
            self.main_vis.add_geometry(CLOUD_NAME, self.cloud, mat)
            self.main_vis.reset_camera_to_default()
            self.main_vis.setup_camera(60, bounds.get_center(),
                                       bounds.get_center() + [0, 0, -3],
                                       [0, -1, 0])

        o3d.visualization.gui.Application.instance.post_to_main_thread(
            self.main_vis, add_first_cloud)

        while not self.is_done:
            time.sleep(0.01)

            def update_cloud():
                mat = o3d.visualization.rendering.MaterialRecord()
                mat.shader = "defaultUnlit"
                flags = o3d.visualization.rendering.Scene.UPDATE_COLORS_FLAG
                self.main_vis.update_geometry(CLOUD_NAME, self.cloud, flags)

                self.main_vis.post_redraw()

            if self.is_done:
                break
            o3d.visualization.gui.Application.instance.post_to_main_thread(
                self.main_vis, update_cloud)

if __name__ == "__main__":
    main()

color values are using position values.

Does the problem only occur when using a point cloud stored on the GPU?

As far as I know, yes.

For some context, I was trying using update_geometry instead of removing and re-adding (because of #4992 related problem. Do you know anything about this issue?). And for some reasons, updating a point cloud stored on the CPU sometimes makes flickering artifact (I'm guessing it is related to buffering speed, as slowing down the updating rate mitigates the artifact). So I tried sending the point cloud to GPU, which solved the flickering problem, but caused weird coloring artifact. I looked into the code and found that on GPU, position buffer was being used instead of color buffer.