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Thank you for your interest in the Intel RealSense D435 camera.
If a device fails to determine the depth of a given image pixel, a value of zero will be stored in the depth image. This is a reasonable sentinel for "no depth" because all pixels with a depth of zero would correspond to the same physical location, the location of the imager itself. Undefined depth is 0.
You should try the "High Density" preset and Hole filling Post processing filter toggled on in RealSense Viewer. This should generate the most points. Then you can use the align example to align the depth image to color.
Let me know if this helps.