undefined use of alpha channel

[christian-rauch]

Describe the bug The image on colour topic rgb/image_raw contains an alpha channel (bgra8). It's not clear why this channel is required or which information is stored there. Is this used to mark invalid pixels? If it is not used, I propose to remove the alpha channel and just encode the image in the native format of the sensor.

To Reproduce Steps to reproduce the behavior:

1. show the compressed format of the colour image rostopic echo /rgb/image_raw/compressed/format
2. recognise that the format contains an alpha channel ("bgra8; jpeg compressed bgr8")

Expected behavior The image should not contain an alpha channel unless it is used and documented.

Desktop (please complete the following information):

• OS: Ubuntu
• Version 20.04