Introduce subpixel text positioning within rencache

In order to fix the issue with cursor positioning a subpixel-aware draw
text operation within rencache was required.

With this modification the cursor positioning problem is completely
resolved.

A new function renderer.draw_text_subpixel is introduced to perform
consecutive, inline, text drawings with subpixel accuracy.

data/core/docview.lua

@@ -309,11 +309,12 @@ end
 
 
 function DocView:draw_line_text(idx, x, y)
-  local tx, ty = x, y + self:get_line_text_y_offset()
   local font = self:get_font()
+  local subpixel_scale = font:subpixel_scale()
+  local tx, ty = subpixel_scale * x, y + self:get_line_text_y_offset()
   for _, type, text in self.doc.highlighter:each_token(idx) do
     local color = style.syntax[type]
-    tx = renderer.draw_text(font, text, tx, ty, color)
+    tx = renderer.draw_text_subpixel(font, text, tx, ty, color)
   end
 end
 

src/api/renderer.c

@@ -78,20 +78,33 @@ static int f_draw_text(lua_State *L) {
   int x = luaL_checknumber(L, 3);
   int y = luaL_checknumber(L, 4);
   RenColor color = checkcolor(L, 5, 255);
-  x = rencache_draw_text(*font, text, x, y, color);
+  x = rencache_draw_text(*font, text, x, y, color, false);
   lua_pushnumber(L, x);
   return 1;
 }
 
 
+static int f_draw_text_subpixel(lua_State *L) {
+  RenFont **font = luaL_checkudata(L, 1, API_TYPE_FONT);
+  const char *text = luaL_checkstring(L, 2);
+  int x_subpixel = luaL_checknumber(L, 3);
+  int y = luaL_checknumber(L, 4);
+  RenColor color = checkcolor(L, 5, 255);
+  x_subpixel = rencache_draw_text(*font, text, x_subpixel, y, color, true);
+  lua_pushnumber(L, x_subpixel);
+  return 1;
+}
+
+
 static const luaL_Reg lib[] = {
-  { "show_debug",    f_show_debug    },
-  { "get_size",      f_get_size      },
-  { "begin_frame",   f_begin_frame   },
-  { "end_frame",     f_end_frame     },
-  { "set_clip_rect", f_set_clip_rect },
-  { "draw_rect",     f_draw_rect     },
-  { "draw_text",     f_draw_text     },
+  { "show_debug",         f_show_debug         },
+  { "get_size",           f_get_size           },
+  { "begin_frame",        f_begin_frame        },
+  { "end_frame",          f_end_frame          },
+  { "set_clip_rect",      f_set_clip_rect      },
+  { "draw_rect",          f_draw_rect          },
+  { "draw_text",          f_draw_text          },
+  { "draw_text_subpixel", f_draw_text_subpixel },
   { NULL,            NULL            }
 };
 

src/rencache.c

@@ -11,13 +11,15 @@
 #define CELL_SIZE 96
 #define COMMAND_BUF_SIZE (1024 * 512)
 
-enum { FREE_FONT, SET_CLIP, DRAW_TEXT, DRAW_RECT };
+enum { FREE_FONT, SET_CLIP, DRAW_TEXT, DRAW_RECT, DRAW_TEXT_SUBPIXEL };
 
 typedef struct {
   int type, size;
   RenRect rect;
   RenColor color;
   RenFont *font;
+  short int subpixel_scale;
+  short int x_subpixel_offset;
   int tab_width;
   char text[0];
 } Command;
@@ -128,29 +130,30 @@ void rencache_draw_rect(RenRect rect, RenColor color) {
   }
 }
 
-
-int rencache_draw_text(RenFont *font, const char *text, int x, int y, RenColor color) {
+int rencache_draw_text(RenFont *font, const char *text, int x, int y, RenColor color, bool draw_subpixel) {
   int subpixel_scale;
+  int w_subpixel = ren_get_font_width(font, text, &subpixel_scale);
   RenRect rect;
-  rect.x = x;
+  rect.x = (draw_subpixel ? ren_font_subpixel_round(x, subpixel_scale, -1) : x);
   rect.y = y;
-  int w = ren_get_font_width(font, text, &subpixel_scale);
-  rect.width = ren_font_subpixel_round(w, subpixel_scale, 0);
+  rect.width = ren_font_subpixel_round(w_subpixel, subpixel_scale, 0);
   rect.height = ren_get_font_height(font);
 
   if (rects_overlap(screen_rect, rect)) {
     int sz = strlen(text) + 1;
-    Command *cmd = push_command(DRAW_TEXT, sizeof(Command) + sz);
+    Command *cmd = push_command(draw_subpixel ? DRAW_TEXT_SUBPIXEL : DRAW_TEXT, sizeof(Command) + sz);
     if (cmd) {
       memcpy(cmd->text, text, sz);
       cmd->color = color;
       cmd->font = font;
       cmd->rect = rect;
+      cmd->subpixel_scale = (draw_subpixel ? subpixel_scale : 1);
+      cmd->x_subpixel_offset = x - subpixel_scale * rect.x;
       cmd->tab_width = ren_get_font_tab_width(font);
     }
   }
 
-  return x + rect.width;
+  return x + (draw_subpixel ? w_subpixel : rect.width);
 }
 
 
@@ -261,6 +264,10 @@ void rencache_end_frame(void) {
           ren_set_font_tab_width(cmd->font, cmd->tab_width);
           ren_draw_text(cmd->font, cmd->text, cmd->rect.x, cmd->rect.y, cmd->color);
           break;
+        case DRAW_TEXT_SUBPIXEL:
+          ren_set_font_tab_width(cmd->font, cmd->tab_width);
+          ren_draw_text_subpixel(cmd->font, cmd->text, cmd->subpixel_scale * cmd->rect.x + cmd->x_subpixel_offset, cmd->rect.y, cmd->color);
+          break;
       }
     }
 

src/rencache.h

@@ -8,7 +8,7 @@ void rencache_show_debug(bool enable);
 void rencache_free_font(RenFont *font);
 void rencache_set_clip_rect(RenRect rect);
 void rencache_draw_rect(RenRect rect, RenColor color);
-int  rencache_draw_text(RenFont *font, const char *text, int x, int y, RenColor color);
+int  rencache_draw_text(RenFont *font, const char *text, int x, int y, RenColor color, bool draw_subpixel);
 void rencache_invalidate(void);
 void rencache_begin_frame(void);
 void rencache_end_frame(void);

src/renderer.c

@@ -284,26 +284,31 @@ void ren_draw_image(RenImage *image, RenRect *sub, int x, int y, RenColor color)
   }
 }
 
-void ren_draw_text(RenFont *font, const char *text, int x, int y, RenColor color) {
+
+void ren_draw_text_subpixel(RenFont *font, const char *text, int x_subpixel, int y, RenColor color) {
   const char *p = text;
   unsigned codepoint;
   SDL_Surface *surf = SDL_GetWindowSurface(window);
   FR_Color color_fr = { .r = color.r, .g = color.g, .b = color.b };
-  const int subpixel_scale = FR_Subpixel_Scale(font->renderer);
-  int x_mult = subpixel_scale * x;
   while (*p) {
     p = utf8_to_codepoint(p, &codepoint);
     GlyphSet *set = get_glyphset(font, codepoint);
     FR_Bitmap_Glyph_Metrics *g = &set->glyphs[codepoint & 0xff];
     if (color.a != 0) {
       FR_Blend_Glyph(font->renderer, &clip,
-        x_mult, y, (uint8_t *) surf->pixels, surf->w, set->image, g, color_fr);
+        x_subpixel, y, (uint8_t *) surf->pixels, surf->w, set->image, g, color_fr);
     }
-    x_mult += g->xadvance;
+    x_subpixel += g->xadvance;
   }
 }
 
 
+void ren_draw_text(RenFont *font, const char *text, int x, int y, RenColor color) {
+  const int subpixel_scale = FR_Subpixel_Scale(font->renderer);
+  ren_draw_text_subpixel(font, text, subpixel_scale * x, y, color);
+}
+
+
 int ren_font_subpixel_round(int width, int subpixel_scale, int orientation) {
   int w_mult;
   if (orientation < 0) {

src/renderer.h

@@ -42,5 +42,6 @@ int ren_font_subpixel_round(int width, int subpixel_scale, int orientation);
 void ren_draw_rect(RenRect rect, RenColor color);
 void ren_draw_image(RenImage *image, RenRect *sub, int x, int y, RenColor color);
 void ren_draw_text(RenFont *font, const char *text, int x, int y, RenColor color);
+void ren_draw_text_subpixel(RenFont *font, const char *text, int x_subpixel, int y, RenColor color);
 
 #endif