Meaning of depth image from kinect by openni_launch [closed]

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Hi,

openni_launch package publishes a topic /camera/depth/image from kinect.

I observed the data and I found that it ranges from 0 to 255.

Can any one give some meaning to those values,

1.Does 255 or 0 mean that the object is close or far away?

2.What will be the values for areas where there is no depth information like shadows of object?

3.I want to extract keypoints on the depth images ?

4.What is the difference between depth image and range image?

Any suggestions