config: add underline-offset option

This option allows the user to configure a custom underline
offset. That is, use the user provided offset instead of the font
provided one.

Closes #490

CHANGELOG.md

@@ -65,6 +65,8 @@
   (https://codeberg.org/dnkl/foot/issues/581).
 * Support for overriding configuration options on the command line
   (https://codeberg.org/dnkl/foot/issues/554).
+* `underline-offset` option to `foot.ini`
+  (https://codeberg.org/dnkl/foot/issues/490).
 
 
 ### Changed

config.c

@@ -793,6 +793,14 @@ parse_section_main(const char *key, const char *value, struct config *conf,
             return false;
     }
 
+    else if (strcmp(key, "underline-offset") == 0) {
+        if (!str_to_pt_or_px(
+                value, &conf->underline_offset,
+                conf, path, lineno, "default", "underline-offset"))
+            return false;
+        conf->use_custom_underline_offset = true;
+    }
+
     else if (strcmp(key, "dpi-aware") == 0) {
         if (strcmp(value, "auto") == 0)
             conf->dpi_aware = DPI_AWARE_AUTO;
@@ -2514,10 +2522,11 @@ config_load(struct config *conf, const char *conf_path,
         },
         .startup_mode = STARTUP_WINDOWED,
         .fonts = {tll_init(), tll_init(), tll_init(), tll_init()},
-        .line_height = { .pt = 0, .px = -1, },
-        .letter_spacing = { .pt = 0, .px = 0, },
-        .horizontal_letter_offset = {.pt = 0, .px = 0, },
-        .vertical_letter_offset = {.pt = 0, .px = 0, },
+        .line_height = {.pt = 0, .px = -1},
+        .letter_spacing = {.pt = 0, .px = 0},
+        .horizontal_letter_offset = {.pt = 0, .px = 0},
+        .vertical_letter_offset = {.pt = 0, .px = 0},
+        .use_custom_underline_offset = false,
         .box_drawings_uses_font_glyphs = false,
         .dpi_aware = DPI_AWARE_AUTO, /* DPI-aware when scaling-factor == 1 */
         .bell = {

config.h

@@ -99,6 +99,9 @@ struct config {
     struct pt_or_px horizontal_letter_offset;
     struct pt_or_px vertical_letter_offset;
 
+    bool use_custom_underline_offset;
+    struct pt_or_px underline_offset;
+
     bool box_drawings_uses_font_glyphs;
 
     struct {

doc/foot.ini.5.scd

@@ -112,10 +112,24 @@ in this order:
 	
 	Default: _0_.
 
-*box-drawings-uses-font-glyphs*
-	Boolean. When disabled, foot generates box/line drawing characters
-	itself. The are several advantages to doing this instead of using
-	font glyphs:
+*underline-offset*
+	Use a custom offset for underlines. The offset is, by default, in
+	_points_ and relative the font's baseline. A positive value
+	positions the underline under the baseline, while a negative value
+	positions it above the baseline.
+	
+	To specify an offset in _pixels_, append *px*:
+	*underline-offset=2px*.
+	
+	If left unset (the default), the offset specified in the font is
+	used, or estimated by foot if the font lacks underline positioning
+	information.
+	
+	Default: _unset_.
+
+*box-drawings-uses-font-glyphs* Boolean. When disabled, foot generates
+	box/line drawing characters itself. The are several advantages to
+	doing this instead of using font glyphs:
 	
 		- No antialiasing effects where e.g. line endpoints appear
 		  dimmed down, or blurred.

foot.ini

@@ -12,6 +12,7 @@
 # letter-spacing=0
 # horizontal-letter-offset=0
 # vertical-letter-offset=0
+# underline-offset=<font metrics>
 # box-drawings-uses-font-glyphs=no
 # dpi-aware=yes
 

render.c

@@ -319,7 +319,11 @@ draw_underline_with_thickness(
     const pixman_color_t *color, int x, int y, int cols, int thickness)
 {
     /* Make sure the line isn't positioned below the cell */
-    int y_ofs = font_baseline(term) - font->underline.position;
+    int y_ofs = font_baseline(term) -
+        (term->conf->use_custom_underline_offset
+         ? -term_pt_or_px_as_pixels(term, &term->conf->underline_offset)
+         : font->underline.position);
+
     y_ofs = min(y_ofs, term->cell_height - thickness);
 
     pixman_image_fill_rectangles(