Add support for subpixel rendering on Linux

kitty/cell_fragment.glsl

@@ -1,8 +1,9 @@
 #version GLSL_VERSION
 #define WHICH_PROGRAM
 #define NOT_TRANSPARENT
+#define NOT_SUBPIXEL
 
-#if defined(SIMPLE) || defined(BACKGROUND) || defined(SPECIAL)
+#if defined(SIMPLE) || defined(BACKGROUND) || defined(SPECIAL) || defined(SUBPIXEL)
 #define NEEDS_BACKROUND
 #endif
 
@@ -28,6 +29,7 @@ in vec3 foreground;
 in vec4 cursor_color_vec;
 in vec3 decoration_fg;
 in float colored_sprite;
+in float subpixel;
 #endif
 
 out vec4 final_color;
@@ -94,8 +96,23 @@ vec4 blend_onto_opaque_premul(vec3 over, float over_alpha, vec3 under) {
 vec4 calculate_foreground() {
     // returns the effective foreground color in pre-multiplied form
     vec4 text_fg = texture(sprites, sprite_pos);
+#ifdef SUBPIXEL
+    vec3 unblended_fg = mix(foreground, text_fg.rgb, colored_sprite);
+#ifdef TRANSPARENT
+    float alpha = text_fg.g; // Cairo uses green channel to convert FreeType's subpixel buffer to ARGB
+    float scale_coeff = mix(1, alpha, alpha > 0);
+    vec3 scaled_mask = text_fg.rgb / scale_coeff;
+    vec3 blended_fg = foreground * scaled_mask;
+    float text_alpha = mix(text_fg.a, alpha, subpixel);
+#else
+    vec3 blended_fg = mix(background, foreground, text_fg.rgb);
+    float text_alpha = text_fg.a;
+#endif
+    vec3 fg = mix(unblended_fg, blended_fg, subpixel);
+#else
     vec3 fg = mix(foreground, text_fg.rgb, colored_sprite);
     float text_alpha = text_fg.a;
+#endif
     float underline_alpha = texture(sprites, underline_pos).a;
     float strike_alpha = texture(sprites, strike_pos).a;
     float cursor_alpha = texture(sprites, cursor_pos).a;

kitty/cell_vertex.glsl

@@ -6,6 +6,7 @@
 #define STRIKE_SHIFT {STRIKE_SHIFT}
 #define DIM_SHIFT {DIM_SHIFT}
 #define USE_SELECTION_FG
+#define NOT_SUBPIXEL
 
 // Inputs {{{
 layout(std140) uniform CellRenderData {
@@ -35,7 +36,7 @@ const uvec2 cell_pos_map[] = uvec2[4](
 // }}}
 
 
-#if defined(SIMPLE) || defined(BACKGROUND) || defined(SPECIAL)
+#if defined(SIMPLE) || defined(BACKGROUND) || defined(SPECIAL) || defined(SUBPIXEL)
 #define NEEDS_BACKROUND
 #endif
 
@@ -61,6 +62,7 @@ out vec3 strike_pos;
 out vec3 foreground;
 out vec3 decoration_fg;
 out float colored_sprite;
+out float subpixel;
 out float effective_text_alpha;
 #endif
 
@@ -69,6 +71,7 @@ out float effective_text_alpha;
 const uint BYTE_MASK = uint(0xFF);
 const uint Z_MASK = uint(0xFFF);
 const uint COLOR_MASK = uint(0x4000);
+const uint SUBPIXEL_MASK = uint(0x8000);
 const uint ZERO = uint(0);
 const uint ONE = uint(1);
 const uint TWO = uint(2);
@@ -166,6 +169,7 @@ void main() {
     // The character sprite being rendered
     sprite_pos = to_sprite_pos(pos, sprite_coords.x, sprite_coords.y, sprite_coords.z & Z_MASK);
     colored_sprite = float((sprite_coords.z & COLOR_MASK) >> 14);
+    subpixel = float((sprite_coords.z & SUBPIXEL_MASK) >> 15);
 
     // Foreground
     uint resolved_fg = resolve_color(colors[fg_index], default_colors[fg_index]);

kitty/config_data.py

@@ -295,6 +295,12 @@ def box_drawing_scale(x):
 and very thick lines.
 '''))
 
+o('subpixel_rendering', False, long_text=_('''
+Use subpixel rendering instead of grayscale in freetype. Impacts performance,
+but may look better on low DPI screens. Possible values are :code:`none`,
+:code:`lcd`, and :code:`lcd_v`.
+'''))
+
 # }}}
 
 g('cursor')  # {{{

kitty/core_text.m

@@ -456,7 +456,7 @@
 }
 
 bool
-render_glyphs_in_cells(PyObject *s, bool bold, bool italic, hb_glyph_info_t *info, hb_glyph_position_t *hb_positions, unsigned int num_glyphs, pixel *canvas, unsigned int cell_width, unsigned int cell_height, unsigned int num_cells, unsigned int baseline, bool *was_colored, FONTS_DATA_HANDLE fg, bool center_glyph) {
+render_glyphs_in_cells(PyObject *s, bool bold, bool italic, hb_glyph_info_t *info, hb_glyph_position_t *hb_positions, unsigned int num_glyphs, pixel *canvas, unsigned int cell_width, unsigned int cell_height, unsigned int num_cells, unsigned int baseline, bool *was_colored, bool UNUSED *was_subpixel, FONTS_DATA_HANDLE fg, bool center_glyph) {
     CTFace *self = (CTFace*)s;
     for (unsigned i=0; i < num_glyphs; i++) glyphs[i] = info[i].codepoint;
     return do_render(self->ct_font, bold, italic, info, hb_positions, num_glyphs, canvas, cell_width, cell_height, num_cells, baseline, was_colored, true, fg, center_glyph);

kitty/fontconfig.c

@@ -50,7 +50,7 @@ pattern_as_dict(FcPattern *pat) {
     I(FC_SLANT, slant);
     I(FC_HINT_STYLE, hint_style);
     I(FC_INDEX, index);
-    I(FC_RGBA, subpixel);
+    I(FC_RGBA, rgba);
     I(FC_LCD_FILTER, lcdfilter);
     B(FC_HINTING, hinting);
     B(FC_SCALABLE, scalable);

kitty/fonts.c

@@ -32,7 +32,7 @@ typedef struct {
 typedef struct SpritePosition SpritePosition;
 struct SpritePosition {
     SpritePosition *next;
-    bool filled, rendered, colored;
+    bool filled, rendered, colored, subpixel;
     sprite_index x, y, z;
     uint8_t ligature_index;
     glyph_index glyph;
@@ -247,6 +247,7 @@ sprite_position_for(FontGroup *fg, Font *font, glyph_index glyph, ExtraGlyphs *e
     s->filled = true;
     s->rendered = false;
     s->colored = false;
+    s->subpixel = false;
     s->x = fg->sprite_tracker.x; s->y = fg->sprite_tracker.y; s->z = fg->sprite_tracker.z;
     do_increment(fg, error);
     return s;
@@ -301,7 +302,7 @@ free_maps(Font *font) {
 
 void
 clear_sprite_map(Font *font) {
-#define CLEAR(s) s->filled = false; s->rendered = false; s->colored = false; s->glyph = 0; memset(&s->extra_glyphs, 0, sizeof(ExtraGlyphs)); s->x = 0; s->y = 0; s->z = 0; s->ligature_index = 0;
+#define CLEAR(s) s->filled = false; s->rendered = false; s->colored = false; s->subpixel = false; s->glyph = 0; memset(&s->extra_glyphs, 0, sizeof(ExtraGlyphs)); s->x = 0; s->y = 0; s->z = 0; s->ligature_index = 0;
     SpritePosition *s;
     for (size_t i = 0; i < sizeof(font->sprite_map)/sizeof(font->sprite_map[0]); i++) {
         s = font->sprite_map + i;
@@ -606,6 +607,7 @@ render_box_cell(FontGroup *fg, CPUCell *cpu_cell, GPUCell *gpu_cell) {
     if (sp->rendered) return;
     sp->rendered = true;
     sp->colored = false;
+    sp->subpixel = false;
     PyObject *ret = PyObject_CallFunction(box_drawing_function, "IIId", cpu_cell->ch, fg->cell_width, fg->cell_height, (fg->logical_dpi_x + fg->logical_dpi_y) / 2.0);
     if (ret == NULL) { PyErr_Print(); return; }
     uint8_t *alpha_mask = PyLong_AsVoidPtr(PyTuple_GET_ITEM(ret, 0));
@@ -640,6 +642,7 @@ static inline void
 set_cell_sprite(GPUCell *cell, SpritePosition *sp) {
     cell->sprite_x = sp->x; cell->sprite_y = sp->y; cell->sprite_z = sp->z;
     if (sp->colored) cell->sprite_z |= 0x4000;
+    if (sp->subpixel) cell->sprite_z |= 0x8000;
 }
 
 static inline pixel*
@@ -671,12 +674,14 @@ render_group(FontGroup *fg, unsigned int num_cells, unsigned int num_glyphs, CPU
 
     clear_canvas(fg);
     bool was_colored = (gpu_cells->attrs & WIDTH_MASK) == 2 && is_emoji(cpu_cells->ch);
-    render_glyphs_in_cells(font->face, font->bold, font->italic, info, positions, num_glyphs, fg->canvas, fg->cell_width, fg->cell_height, num_cells, fg->baseline, &was_colored, (FONTS_DATA_HANDLE)fg, center_glyph);
+    bool was_subpixel = false;
+    render_glyphs_in_cells(font->face, font->bold, font->italic, info, positions, num_glyphs, fg->canvas, fg->cell_width, fg->cell_height, num_cells, fg->baseline, &was_colored, &was_subpixel, (FONTS_DATA_HANDLE)fg, center_glyph);
     if (PyErr_Occurred()) PyErr_Print();
 
     for (unsigned int i = 0; i < num_cells; i++) {
         sprite_position[i]->rendered = true;
         sprite_position[i]->colored = was_colored;
+        sprite_position[i]->subpixel = was_subpixel;
         set_cell_sprite(gpu_cells + i, sprite_position[i]);
         pixel *buf = num_cells == 1 ? fg->canvas : extract_cell_from_canvas(fg, i, num_cells);
         current_send_sprite_to_gpu((FONTS_DATA_HANDLE)fg, sprite_position[i]->x, sprite_position[i]->y, sprite_position[i]->z, buf);

kitty/fonts.h

@@ -26,7 +26,7 @@ bool is_glyph_empty(PyObject *, glyph_index);
 hb_font_t* harfbuzz_font_for_face(PyObject*);
 bool set_size_for_face(PyObject*, unsigned int, bool, FONTS_DATA_HANDLE);
 void cell_metrics(PyObject*, unsigned int*, unsigned int*, unsigned int*, unsigned int*, unsigned int*);
-bool render_glyphs_in_cells(PyObject *f, bool bold, bool italic, hb_glyph_info_t *info, hb_glyph_position_t *positions, unsigned int num_glyphs, pixel *canvas, unsigned int cell_width, unsigned int cell_height, unsigned int num_cells, unsigned int baseline, bool *was_colored, FONTS_DATA_HANDLE, bool center_glyph);
+bool render_glyphs_in_cells(PyObject *f, bool bold, bool italic, hb_glyph_info_t *info, hb_glyph_position_t *positions, unsigned int num_glyphs, pixel *canvas, unsigned int cell_width, unsigned int cell_height, unsigned int num_cells, unsigned int baseline, bool *was_colored, bool *was_subpixel, FONTS_DATA_HANDLE, bool center_glyph);
 PyObject* create_fallback_face(PyObject *base_face, CPUCell* cell, bool bold, bool italic, bool emoji_presentation, FONTS_DATA_HANDLE fg);
 PyObject* specialize_font_descriptor(PyObject *base_descriptor, FONTS_DATA_HANDLE);
 PyObject* face_from_path(const char *path, int index, FONTS_DATA_HANDLE);