- when used with *any* alphaField and normalised (the usual case)
would largely give a 0-1 corresponding to the min/max of the first
component, but could also yield negative values.
- if the alpha field corresponds identically to colour field, it is
readily possible to combine as into RGBA sequences. However, if the
fields are different it potentially means referencing an opacity
field that has not yet been sampled. This impedes using the format
for a streaming sampler without additional overhead and/or rewriting
the alpha channel later.
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