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I am looking to render the vector pass in a OpenExr format with all channels. I have rendered on both 2.7 and 2.8 and the generated images have an empty alpha channel. The compositor is set as follows:

enter image description here

Two issues arise, the first is that the alpha channel is missing on every image as can seen by the recorded rgba values at the bottom of the window in the image below:

enter image description here

Also where a zero value would be expected, it seems to be set to RGBA(0.0039, 0.0039, 0.0039, 0). As seen below:

enter image description here

The same issues spans different versions and scenes, is there something wrong in my configuration or is this a bug? Thanks for any help in advance!

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Did you make sure to disable the Motion Blur check box under the Cycles render properties panel? If not, the Vector Pass won't work properly

Also, for some reason, in order to save the Vector pass to OpenEXR with proper values, you first need to convert it from to RGBA then to Image format.

Something like this should work:

enter image description here

Also, to verify the values stored in the OpenEXR, just drag and drop the file into the compositor where you can also doublecheck them (that's what I did).

  1. Alpha channel is not used for the Vector Pass which uses XYZ components of each pixel vector stored as RGB only for the OpenEXR file. (see comment below)
  2. The non zero value could be attributed to movement of your camera, the background of the scene, rotation... or whatever image/color processing of the viewer used.
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    $\begingroup$ The Vector pass consists of two vectors according to docs.blender.org/manual/en/latest/render/layers/passes.html, one for (R) x and (G) y difference between current and previous frame and the second for the (B) x and (A) y between the current and the next frame. And unfortunately the camera is static so it is not the source of the RGBA(0.0039, 0.0039, 0.0039, 0) value. $\endgroup$
    – Sam
    Apr 23, 2020 at 19:27
  • $\begingroup$ I stand corrected on point 1. $\endgroup$
    – Bruno
    Apr 24, 2020 at 7:40
  • $\begingroup$ (also verified in code). $\endgroup$
    – Bruno
    Apr 24, 2020 at 7:46

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