How to make alpha image for Point Cloud example

In the github dev branch there’s a point cloud example that uses an images alpha as a depth map. Is there a tool or easy way to do this in Photoshop (turn an image into an alpha depth map)?

As a test, I tried to make half a sphere by making a gradient circle in photoshop and using the black -> white radial gradient as the alpha channel. However, it didn’t really work right and when loaded into the point cloud example it looked more like a bullet than a sphere.