PNG encoding does not recognize band labeling

[Piero Campalani]

It seems that when encoding a PNG with a WCPS query with explicit definition of the single bands, the order of the bands is fixed to R, then G, then B, independently of which label is set in the query.

This means that the following queries are equivalent:

for c in (rgb) return encode({
  red:   c.red;
  green: c.green;
  blue:  c.blue}, "png")

for c in (rgb) return encode({
  green: c.red;
  red:   c.green;
  blue:  c.blue}, "png")

[Dimitar Misev]

yes there's no semantics attached to the band labels as far as I know, i.e. it's just some random name for query documentation I guess. So I'd close this ticket as invalid.

[abeccati]

This is at least quite confusing from an user perspective and should be addressed somehow (with "trivial" priority) e.g. at least clearly documented.

Labeling determines the name of the range field in the created coverage and they are clearly not interpreted by the PNG formatter (which is not part of the WCPS standard itself).

Something that would nicely fit into the user manual in something like "supported formats"

[Piero Campalani]

I agree, it should be addressed: do we want to specify this in the user manual, or do we want to enable assignment by label in the WCPS?

[Dimitar Misev]

Which user manual? I only know of the WCPS standard 08-068r2

What matters is the order of the range components → PNG obviously expects them in RGB(A) order, so if you want to switch red and green you'd do

for c in (rgb) return encode({
  R: c.green;
  G: c.red;
  B: c.blue},
"png")