Allow pointcloud create_cloud function to set specific point_step

[broughtong]

If I try to convert a pointcloud message to an array, and then use create_cloud to convert it back to a message, it will not work if there is padding at the end.

The original lidar data is laid out like this (note the itemsize/padding):

{'names':['x','y','z','intensity','t','reflectivity','ring','ambient','range'], 'formats':['<f4','<f4','<f4','<f4','<u4','<u2','<u2','<u2','<u4'], 'offsets':[0,4,8,16,20,24,26,28,32], 'itemsize':48}

The current create_cloud will try to reconstruct the point layout from the field info with dtype_from_fields, which in this case will take into account field padding, but not the padding at the end. It generates the following (again note the itemsize):

{'names':['x','y','z','intensity','t','reflectivity','ring','ambient','range'], 'formats':['<f4','<f4','<f4','<f4','<u4','<u2','<u2','<u2','<u4'], 'offsets':[0,4,8,16,20,24,26,28,32], 'itemsize':36}

The mismatch in data layout will fail an assertion.

The dtype_from_fields function does allow for the point_step (itemsize) to be specficied explicitly when there is padding present at the end of the data, but there is currently no way to pass this in from the create_cloud function.

Fix adds an optional parameter to allow users to pass in a custom point_step, allowing the data to align properly.

[clalancette]

Can you please add a test showing that this PR fixes the problem you are describing?

[broughtong]

Done :)

[clalancette]

This looks reasonable to me, thanks for the additional tests. I'll run CI on it next.

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows