The result is a Number, which is the byte index of the character in window {winid} at buffer line {lnum} and virtual column {col}. If {col} is greater than the last virtual column in line {lnum}, then the byte index of the character at the last virtual column is returned. The {winid} argument can be the window number or the |window-ID|. If this is zero, then the current window is used. Returns -1 if the window {winid} doesn't exist or the buffer line {lnum} or virtual column {col} is invalid. See also |screenpos()|, |virtcol()| and |col()|. Can also be used as a |method|: GetWinid()->virtcol2col(lnum, col)