terminal background color detection algorithm

[habamax]

Hi, does anyone remembers or knows about the algorithm being used to detect dark/light background of terminal?

vim/src/term.c

Lines 6287 to 6295 in fef6630

	// Check for fore/background color response from the terminal,
	// starting} with <Esc>] or OSC
	else if ((*T_RBG != NUL || *T_RFG != NUL)
	&& ((tp[0] == ESC && len >= 2 && tp[1] == ']')
	|| tp[0] == OSC))
	{
	if (handle_osc(tp, argp, len, key_name, &slen) == FAIL)
	return -1;
	}

vim/src/term.c

Line 5776 in fef6630

char *new_bg_val = (3 * '6' < *tp_r + *tp_g +

It looks like nowadays it "almost always" incorrectly detects light background for terminals with black background color.

[habamax]

• https://stackoverflow.com/questions/2507337/how-to-determine-a-terminals-background-color
• https://unix.stackexchange.com/questions/245378/common-environment-variable-to-set-dark-or-light-terminal-background

[habamax]

On windows terminal using WSL debian handle_osc is never called.

There is however term_bg_default function, I wonder if it is feasible to invert condition to default to dark background, assuming most of the terminals are dark?

vim/src/term.c

Lines 3212 to 3245 in de7f5bd

	/*
	* Return "dark" or "light" depending on the kind of terminal.
	* This is just guessing! Recognized are:
	* "linux" Linux console
	* "screen.linux" Linux console with screen
	* "cygwin.*" Cygwin shell
	* "putty.*" Putty program
	* We also check the COLORFGBG environment variable, which is set by
	* rxvt and derivatives. This variable contains either two or three
	* values separated by semicolons; we want the last value in either
	* case. If this value is 0-6 or 8, our background is dark.
	*/
	char_u *
	term_bg_default(void)
	{
	#if defined(MSWIN)
	// DOS console is nearly always black
	return (char_u *)"dark";
	#else
	char_u *p;
	if (STRCMP(T_NAME, "linux") == 0
	|| STRCMP(T_NAME, "screen.linux") == 0
	|| STRNCMP(T_NAME, "cygwin", 6) == 0
	|| STRNCMP(T_NAME, "putty", 5) == 0
	|| ((p = mch_getenv((char_u *)"COLORFGBG")) != NULL
	&& (p = vim_strrchr(p, ';')) != NULL
	&& ((p[1] >= '0' && p[1] <= '6') || p[1] == '8')
	&& p[2] == NUL))
	return (char_u *)"dark";
	return (char_u *)"light";
	#endif
	}

[habamax]

I have inverted it, and this might be a BREAKING change: #13977

[saccarosium]

I was quite interested in this myself. I found this commit from neovim. There is an escape sequence that lets you support this. Should be called OSC. Also you might be interested in this issue that tracked some issues related to the feature.

[habamax]

Should be called OSC

Vim has it. The thing is, vim defaults to 'light' background and checks for the terminal OSC response which might fail/be incorrect.

To add to it, more than 1000 screendump tests are relying on background=light, and a lot of people got used to it.

[saccarosium]

and a lot of people got used to it.

Who exactly? I see one person complaining in the PR but thats it.

[habamax]

There were on libera #vim

[habamax]

But PR is there for anyone to continue the journey.