Option to print debug information about font fallback

kitty/cli.py

@@ -107,6 +107,11 @@ Path to file in which to store the raw bytes received from the child process
 --debug-gl
 type=bool-set
 Debug OpenGL commands. This will cause all OpenGL calls to check for errors instead of ignoring them. Useful when debugging rendering problems
+
+
+--debug-font-fallback
+type=bool-set
+Print out information about the selection of fallback fonts for characters not present in the main font.
 '''
 
 

kitty/fonts.c

@@ -339,6 +339,19 @@ has_cell_text(Font *self, Cell *cell) {
     return true;
 }
 
+static inline void
+output_cell_fallback_data(Cell *cell, bool bold, bool italic, bool emoji_presentation, PyObject *face, bool new_face) {
+    printf("U+%x ", cell->ch);
+    for (unsigned i = 0; i < arraysz(cell->cc_idx) && cell->cc_idx[i]; i++) {
+        printf("U+%x ", codepoint_for_mark(cell->cc_idx[i]));
+    }
+    if (bold) printf("bold ");
+    if (italic) printf("italic ");
+    if (emoji_presentation) printf("emoji_presentation ");
+    PyObject_Print(face, stdout, 0);
+    if (new_face) printf(" (new face)");
+    printf("\n");
+}
 
 static inline ssize_t
 load_fallback_font(Cell *cell, bool bold, bool italic, bool emoji_presentation) {
@@ -352,6 +365,7 @@ load_fallback_font(Cell *cell, bool bold, bool italic, bool emoji_presentation)
     PyObject *face = create_fallback_face(fonts.fonts[f].face, cell, bold, italic, emoji_presentation);
     if (face == NULL) { PyErr_Print(); return MISSING_FONT; }
     if (face == Py_None) { Py_DECREF(face); return MISSING_FONT; }
+    if (global_state.debug_font_fallback) output_cell_fallback_data(cell, bold, italic, emoji_presentation, face, true);
     set_size_for_face(face, cell_height, true);
 
     ensure_space_for(&fonts, fonts, Font, fonts.fonts_count + 1, fonts_capacity, 5, true);
@@ -364,7 +378,6 @@ load_fallback_font(Cell *cell, bool bold, bool italic, bool emoji_presentation)
     return ans;
 }
 
-
 static inline ssize_t
 fallback_font(Cell *cell) {
     bool bold = (cell->attrs >> BOLD_SHIFT) & 1;
@@ -375,6 +388,7 @@ fallback_font(Cell *cell) {
     for (size_t i = 0, j = fonts.first_fallback_font_idx; i < fonts.fallback_fonts_count; i++, j++)  {
         Font *ff = fonts.fonts +j;
         if (ff->bold == bold && ff->italic == italic && ff->emoji_presentation == emoji_presentation && has_cell_text(ff, cell)) {
+            if (global_state.debug_font_fallback) output_cell_fallback_data(cell, bold, italic, emoji_presentation, ff->face, false);
             return j;
         }
     }

kitty/main.py

@@ -39,7 +39,7 @@ def init_graphics():
 
 def run_app(opts, args):
     set_scale(opts.box_drawing_scale)
-    set_options(opts, is_wayland, args.debug_gl)
+    set_options(opts, is_wayland, args.debug_gl, args.debug_font_fallback)
     if is_macos:
         from .fast_data_types import macos_change_titlebar_color
         macos_change_titlebar_color(opts.macos_titlebar_color)

kitty/state.c

@@ -323,10 +323,11 @@ PYWRAP1(handle_for_window_id) {
 
 PYWRAP1(set_options) {
     PyObject *ret, *opts;
-    int is_wayland = 0, debug_gl = 0;
-    PA("O|pp", &opts, &is_wayland, &debug_gl);
+    int is_wayland = 0, debug_gl = 0, debug_font_fallback = 0;
+    PA("O|ppp", &opts, &is_wayland, &debug_gl, &debug_font_fallback);
     global_state.is_wayland = is_wayland ? true : false;
     global_state.debug_gl = debug_gl ? true : false;
+    global_state.debug_font_fallback = debug_font_fallback ? true : false;
 #define GA(name) ret = PyObject_GetAttrString(opts, #name); if (ret == NULL) return NULL;
 #define S(name, convert) { GA(name); global_state.opts.name = convert(ret); Py_DECREF(ret); if (PyErr_Occurred()) return NULL; }
     S(visual_bell_duration, PyFloat_AsDouble);

kitty/state.h

@@ -130,7 +130,7 @@ typedef struct {
     OSWindow *callback_os_window;
     bool close_all_windows;
     bool is_wayland;
-    bool debug_gl;
+    bool debug_gl, debug_font_fallback;
     bool has_pending_resizes;
 } GlobalState;