`--invert` does not produce equivalent output with 2-color block symbol sets

[badele]

I am trying to work with ANSI block characters compatible with CP437, for example:

[ ░▒▓█]

In some situations, I prefer the background to be rendered with spaces " ", and in others with the full block "█". That is why I find the --invert option very useful.

However, based on the output, --invert does not seem to reproduce the same image with inverted colors. Visually, the result looks different, not just color-inverted.

In theory, in strict 2-color mode, using a symbol set like:

[ █]

should produce the same image when inverted (except that foreground and background colors are swapped).

Here is a minimal example that shows the issue:

test -f Flower_yellow_and_green.png || wget https://upload.wikimedia.org/wikipedia/commons/e/e4/Flower_yellow_and_green.png
chafa -f symbols --symbols "[ █]" --fill "[ █]" --colors 2 --color-space din99d --font-ratio 0.5 -w 5 --size 60 Flower_yellow_and_green.png
chafa -f symbols --symbols "[ █]" --fill "[ █]" --invert --colors 2 --color-space din99d --font-ratio 0.5 -w 5 --size 60 Flower_yellow_and_green.png
chafa -f symbols --symbols "[ █]" --fill "[ █]" --colors 16/8 --color-space din99d --font-ratio 0.5 -w 5 --size 60 Flower_yellow_and_green.png