feat: Add hard-coded invisible color 254 (#94)

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #94

doc/teapot.lyx

@@ -4018,6 +4018,18 @@ teapot
 fteapot
 \family default
  set up a default palette appropriate for their respective capabilities.
+ Currently, the color number 254 is treated specially: when the foreground
+ is set to this color, the contents of the cell are not drawn into the cell
+ at all.
+ So color 254 is a hard-coded 
+\begin_inset Quotes eld
+\end_inset
+
+invisible
+\begin_inset Quotes erd
+\end_inset
+
+ or zero-alpha color.
  Hopefully, a future enhancement will allow one to display, edit, save,
  and restore the palette.
 \end_layout
@@ -10467,7 +10479,15 @@ noprefix "false"
 
 \end_inset
 
-) or making its foreground and background the same color, etc.
+) or using the hard-coded 
+\begin_inset Quotes eld
+\end_inset
+
+invisible
+\begin_inset Quotes erd
+\end_inset
+
+ color number 254, etc.
 \end_layout
 
 \begin_layout Subsection

src/common/style.h

@@ -30,6 +30,7 @@ extern const char FloatFormat_Char[];
 typedef unsigned char ColorNum;
 #define NO_COLOR_SET 0
 #define MAX_MAX_COLORS UCHAR_MAX
+#define INVISIBLE_COLOR (UCHAR_MAX - 1)
 #define COLOR_FORMAT "%hhu"
 
 typedef enum

src/display.c

@@ -606,19 +606,22 @@ void redraw_sheet(Sheet *sheet)
 	  if (sc.bold) wattron(stdscr, A_BOLD);
 	  if (sc.underline) wattron(stdscr, A_UNDERLINE);
 	  short nextcp = hashcp;
-	  if (hashcp == usecp) {
+          chtype blank = (chtype)' ';
+	  if (hashcp == usecp && fg != INVISIBLE_COLOR) {
 	    (void)mvwaddstr(stdscr, (int)(height + sheet->oriy),
 	                            (int)(width  + sheet->orix),
 	                    buf+cutoff);
 	  } else { // Alternate colors to simulate hash
+            chtype next_char = (chtype)buf[cutoff];
+            if (fg == INVISIBLE_COLOR) next_char = blank;
 	    (void)mvwaddch(stdscr, (int)(height + sheet->oriy),
-	                   (int)(width  + sheet->orix),
+	                   (int)(width  + sheet->orix), next_char);
-	                   (chtype)buf[cutoff]);
 	    for (size_t ix = cutoff+1; buf[ix]; ++ix) {
 	      wcolor_set(stdscr, nextcp, NULL);
 	      if (nextcp == hashcp) nextcp = usecp;
 	      else nextcp = hashcp;
-	      (void)waddch(stdscr, (chtype)buf[ix]);
+              next_char = fg == INVISIBLE_COLOR ? blank : (chtype)buf[ix];
+	      (void)waddch(stdscr, next_char);
 	    }
 	  }
 	  for (fill=mbslen(buf+cutoff); fill<realsize; ++fill) {

src/fteapot.fl

@@ -194,42 +194,44 @@ class TeapotTable {open : {public Fl_Table}}
 	    fl_push_clip(xx+3, yy+3, W-6, H-6);
 	    ColorNum fgcn = sc.aspect[FOREGROUND];
 	    if (fgcn == NO_COLOR_SET) fgcn = DefaultCN[FOREGROUND];
-	    Fl_Color cellfg = ((Fl_Color *)(cursheet->palette))[fgcn];
+	    if (fgcn != INVISIBLE_COLOR) {
-	    if (sc.dim) cellfg = fl_color_average(cellfg, cellbg, 0.6f);
+	      Fl_Color cellfg = ((Fl_Color *)(cursheet->palette))[fgcn];
-	    fl_color(cellfg);
+	      if (sc.dim) cellfg = fl_color_average(cellfg, cellbg, 0.6f);
-	    Fl_Font cellfont = FL_HELVETICA;
+	      fl_color(cellfg);
-	    if (sc.bold) cellfont |= FL_BOLD;
+	      Fl_Font cellfont = FL_HELVETICA;
-	    if (sc.italic) cellfont |= FL_ITALIC;
+	      if (sc.bold) cellfont |= FL_BOLD;
-	    fl_font(cellfont, fontsize);
+	      if (sc.italic) cellfont |= FL_ITALIC;
+	      fl_font(cellfont, fontsize);
 
-	    size_t ulen = printvalue(s, sizeof(s), 0, quote, sc.fform,
+	      size_t ulen = printvalue(s, sizeof(s), 0, quote, sc.fform,
 					sc.precision, cursheet, test);
-	    ptrdiff_t len = ulen;
+	      ptrdiff_t len = ulen;
-	    ptrdiff_t nlen = len;
+	      ptrdiff_t nlen = len;
-	    int ww = 0, hh = 0;
+	      int ww = 0, hh = 0;
-	    fl_measure(s, ww, hh, 0);
+	      fl_measure(s, ww, hh, 0);
-	    while (ww > W-6) {
+	      while (ww > W-6) {
-		char *dc = mbspos(s+nlen, -1);
+		  char *dc = mbspos(s+nlen, -1);
-		nlen = dc - s;
+		  nlen = dc - s;
-	        s[nlen] = (char)0;
+	          s[nlen] = (char)0;
-		ww = 0;
+		  ww = 0;
-		fl_measure(s, ww, hh, 0);
+		  fl_measure(s, ww, hh, 0);
+	      }
+	      if (nlen < len) {
+	          if (nlen < 1) { s[0] = '\#'; s[1] = (char)0; }
+	          else if (nlen < 6) for (int i = 0; s[i]; i++) s[i] = '\#';
+	          else {
+	              char *ov = mbspos(s+nlen-1, -5);
+		      if (ov < s) ov = s;
+		      strncpy(ov, "...\#\#", sizeof(s) - nlen);
+	          }
+	      }
+	      Adjust adj = sc.adjust;
+	      fl_draw(s, xx+3, yy+3, W-6, H-6,
+	              adj == RIGHT ? FL_ALIGN_RIGHT :
+		      adj == LEFT ? FL_ALIGN_LEFT : FL_ALIGN_CENTER,
+		      NULL, 0);
+	      if (sc.underline) fl_xyline(xx, yy+H-7, xx+W);
 	    }
-	    if (nlen < len) {
-	        if (nlen < 1) { s[0] = '\#'; s[1] = (char)0; }
-	        else if (nlen < 6) for (int i = 0; s[i]; i++) s[i] = '\#';
-	        else {
-	            char *ov = mbspos(s+nlen-1, -5);
-		    if (ov < s) ov = s;
-		    strncpy(ov, "...\#\#", sizeof(s) - nlen);
-	        }
-	    }
-	    Adjust adj = sc.adjust;
-	    fl_draw(s, xx+3, yy+3, W-6, H-6,
-	            adj == RIGHT ? FL_ALIGN_RIGHT :
-		    adj == LEFT ? FL_ALIGN_LEFT : FL_ALIGN_CENTER,
-		    NULL, 0);
-	    if (sc.underline) fl_xyline(xx, yy+H-7, xx+W);
 	    fl_pop_clip();
 
 	    return;