Named palettes yield unexpected result

[banesullivan]

I would assume the following two styles to be equivalent

A	B
{ bands: [ {band: 1, palette: ['#000', '#f00']}, // red {band: 2, palette: ['#000', '#0f0']}, // green {band: 3, palette: ['#000', '#00f']} // blue ] }	{ bands: [ {band: 1, palette: 'red'}, {band: 2, palette: 'green'}, {band: 3, palette: 'blue'} ] }

However, they are not and produce totally different thumbnails:

A	B

Is this expected? Should "red" be interpreted as equivalent to ['#000', '#f00']

[banesullivan]

Notably, C (below) yields the same result as A:

{
 bands: [
    {band: 1, palette: '#f00'},
    {band: 2, palette: '#0f0'},
    {band: 3, palette: '#00f'}
  ]
}

C

[manthey]

Does

{
 bands: [
    {band: 1, palette: 'red'},
    {band: 2, palette: '#0f0'},
    {band: 3, palette: 'blue'}
  ]
}

yield the same? green is shorthand for #008000, not #0f0 or #00ff00. Blame css standards for that one...

[banesullivan]

🤦🏻‍♂️

Would it make sense to override this in large image having the following in getPaletteColors()?:

    if value == 'green':
        # see https://github.com/girder/large_image/issues/855#issuecomment-
        value = '#0f0'

[manthey]

I don't think there is a good answer here -- if we muck with what green means compared to css it will cause surprise. If we don't, it will cause surprise. Maybe we should define R, G, B and RED, GREEN, BLUE to be #f00, #0f0, #00f, since the capitalized versions aren't used.

[banesullivan]

if we muck with what green means compared to css it will cause surprise.

Since this is an image-focused library, I think "green" has a specific meaning in this context to refer to the Green channel of an RGB image which should be mapped to #00ff00 and not #008000

IMO, it is more surprising to follow a CSS convention than the convention of the RGB color model here

Maybe we should define R, G, B and RED, GREEN, BLUE to be #f00, #0f0, #00f`, since the capitalized versions aren't used.

That's a fair compromise, but I think the following would be clunky:

{
 bands: [
    {band: 1, palette: 'red'},
    {band: 2, palette: 'GREEN'},
    {band: 3, palette: 'blue'}
  ]
}

Though the following looks nice to me and we could document that R, G, and B should be used when wanting to map like the channels of an RGB image

{
 bands: [
    {band: 1, palette: 'R'},
    {band: 2, palette: 'G'},
    {band: 3, palette: 'B'}
  ]
}

[banesullivan]

No matter what, this should be documented in docs/tilesource_options.rst

[manthey]

Since this is an image-focused library, I think "green" has a specific meaning in this context to refer to the Green channel of an RGB image which should be mapped to #00ff00 and not #008000

IMO, it is more surprising to follow a CSS convention than the convention of the RGB color model here

Except that a lot of the names are from css (e.g., purple is half-intensity magenta, violet seems to have little to do with the spectral color).

Maybe we should define R, G, B and RED, GREEN, BLUE to be #f00, #0f0, #00f, since the capitalized versions aren't used.

That's a fair compromise, but I think the following would be clunky:

Yes -- that's why using all caps would be mapped for red and blue, too. r, g, b are synonyms to red, green, blue, so we can't reuse those.

[jeffbaumes]

IMO the palette spec could be designed better. The priority order and all the things a string value could represent seems potentially confusing to users.

def getPaletteColors(value):
    """
    Given a list or a name, return a list of colors in the form of a numpy
    array of RGBA.  If a list, each entry is a color name resolvable by either
    PIL.ImageColor.getcolor, by matplotlib.colors, or a 3 or 4 element list or
    tuple of RGB(A) values on a scale of 0-1.  If this is NOT a list, then, if
    it can be parsed as a color, it is treated as ['#000', <value>].  If that
    cannot be parsed, then it is assumed to be a named palette in palettable
    (such as viridis.Viridis_12) or a named palette in matplotlib (including
    plugins).
    :param value: Either a list, a single color name, or a palette name.  See
        above.
    :returns: a numpy array of RGBA value on the scale of [0-255].
    """

As one idea, I'd suggest new specification modes for the different conditionals so the function does not need to interpret opaque strings in so many ways, something like this:

{
 bands: [
    {band: 1, palette: {channel: 'green'}}, // #000 to #0f0
    {band: 2, palette: {matplotlib: 'jet'}},
    {band: 3, palette: {palettable: 'Earth_7'}},
    {band: 4, palette: ['red', '#112233', 'purple', 'green']}, // interpreted as CSS colors
  ]
}

Single-string palettes like palette: 'green' or palette: '#0f0' would not be allowed (or we allow them for now with existing behavior but deprecate it in the future).

This would still let someone do ['#000', 'green'] and get something perhaps unexpected, but I can't see a way around that because you probably want to support CSS colors in palette arrays.

[manthey]

This was resolved in #858.