What are black pixels in ARKit Depth Map pixel buffer?

Question

When taking a depth pixel buffer from ARKit session like:

CVPixelBufferRef depthDataMap = frame.capturedDepthData.depthDataMap;

many pixels are in black. What do they represent? Why are they black? This doesn't reflect the real depth of the real world. How to interpret this?

Answer 1

It's some sort of "trail" artifact.

Those black pixels in a Depth channel are digital artifacts. The appearance of this black outline became possible due to the fact that the depth data is captured at a frequency of 15 fps (at every fourth frame), in contrast to the RGB image which is captured at a frequency of 60 fps. Also, a Depth channel image is of a low quality (low-res). Thus, if the subject quickly or abruptly moves its head at the time of AR session running, when every three out of four depth frames contain a completely black background – this leads to a situation when RGBA and Depth data don't match each other, so we get these nasty digital artifacts.

Look at a picture in a post How to improve People Occlusion in ARKit 3.0 (click on a link below).

If you need an additional info about a depth map and its trailing effect, please read this post.

Answer 2

This means no depth data for this pixel (depth = 0)