How to remove the Sky from the Point Cloud using the Annotation Tool - PIX4Dmapper


In aerial oblique or terrestrial projects, part of the images is covered by sky. The sky does not consist of meaningful keypoints; however, it can happen that some of the sky keypoints extracted are matched in the images and reconstructed in the 3D model introducing noise to the model. It is possible to annotate the sky not to take it into account for processing.

To remove the sky from the Point Cloud:

1. Click View > rayCloud.
2. In the section Layers of the left sidebar, select one image that is covered by sky.
3. In the right sidebar, click the Annotation tool button pencil.png.
4. In Annotate, select Carve.
5. Annotate the sky in the image, following How to Annotate Images in the rayCloud (steps 7-13).

  • If step 2. Point Cloud and Mesh is already processed, the effect of the annotation is immediately visible in the 3D View of the rayCloud, but the results saved in the results folder are not updated automatically. The annotated point cloud can be saved by reprocessing step 2. Point Cloud and Mesh or by exporting the point cloud. More here: How to export the Point Cloud
  • If step 2. Point Cloud and Mesh is not processed, the annotation will affect the point cloud displayed in the rayCloud after processing and the results saved in the results folder. 
Important: The Carve annotation is not "forgiving"; i.e. all the 3D points that are located on the rays connecting the camera center and the annotated pixels are deleted. For more information: What is Carve?

6. Repeat steps 2-5:

    • If step 2 is already processed, till the sky points are removed from the 3D View of the rayCloud.
    • If step 2 is not processed yet, till the sky is annotated in all the images that it appears. 

7. Make sure that the Use Annotation box is selected in the Processing OptionsHow to Annotate Images in the rayCloud (step 14).
8. Process step 2. Point Cloud and Mesh to remove the annotated pixels from all the results.

Tip: Since the Carve annotation is not "forgiving", if some pixels are wrongly annotated, the Point Cloud will be affected.
It is recommended to annotate the images once step 2. Point Cloud and Mesh is processed. The effect of the annotation is immediately visible in the 3D View of the rayCloud and it is easy to notice if some pixels were erroneously annotated so as to be corrected immediately. Once the annotation is completed, step 2 needs to be processed again, to update all the results saved in the results folder. 
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  • NV CTG

    How many pictures are recommended to be carved in order to remove the sky? 

    All of the pictures with sky in it or just a few so the software "gets the idea?"

  • Avatar
    Blaž (Pix4D)

    Hi Tylor,

    We recommend carving the sky on multiple images in order to cover all the side of the project. 

    You can see the effect of the annotation on the point cloud after clicking "Apply". In case that you see that some parts were not carved out, I recommend repeating the procedure on more image in order to get the desired results. 

    I also recommend checking the How to select / draw the Processing Area article as it an insight on how to use the processing area in order to exclude some parts from processing. It can be used to limit the generation of the points that represent the sky. 


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