Implement subpixel font positioning

Subpixel positioning is only by 1/3 of pixel

lib/font_renderer/font_renderer.cpp

@@ -61,6 +61,10 @@ void FR_Renderer_Free(FR_Renderer *font_renderer) {
     delete font_renderer;
 }
 
+int FR_Subpixel_Scale(FR_Renderer *font_renderer) {
+    return font_renderer->subpixel_scale();
+}
+
 int FR_Load_Font(FR_Renderer *font_renderer, const char *filename) {
     bool success = font_renderer->renderer_alpha().load_font(filename);
     return (success ? 0 : 1);
@@ -283,7 +287,9 @@ FR_Bitmap *FR_Bake_Font_Bitmap(FR_Renderer *font_renderer, int font_height,
 
         glyph_info.xoff = 0;
         glyph_info.yoff = -pad_y - gbounds.y2 + ascender_px;
-        glyph_info.xadvance = (x_next - x) / subpixel_scale;
+        // Note that below the xadvance is in pixels times the subpixel_scale.
+        // This is meant for subpixel positioning.
+        glyph_info.xadvance = roundf(x_next - x);
 
         if (subpixel_scale != 1 && glyph_info.x1 > glyph_info.x0) {
             glyph_lut_convolution(ren_buf, lcd_lut, cover_swap_buffer, glyph_info);
@@ -346,15 +352,18 @@ void blend_solid_hspan_subpixel(agg::rendering_buffer& rbuf, agg::lcd_distributi
 
 // destination implicitly BGRA32. Source implictly single-byte renderer_alpha coverage with subpixel scale = 3.
 // FIXME: consider using something like RenColor* instead of uint8_t * for dst.
-void FR_Blend_Glyph(FR_Renderer *font_renderer, FR_Clip_Area *clip, int x, int y, uint8_t *dst, int dst_width, const FR_Bitmap *glyphs_bitmap, const FR_Bitmap_Glyph_Metrics *glyph, FR_Color color) {
+void FR_Blend_Glyph(FR_Renderer *font_renderer, FR_Clip_Area *clip, int x_mult, int y, uint8_t *dst, int dst_width, const FR_Bitmap *glyphs_bitmap, const FR_Bitmap_Glyph_Metrics *glyph, FR_Color color) {
     agg::lcd_distribution_lut& lcd_lut = font_renderer->lcd_distribution_lut();
     const int subpixel_scale = font_renderer->subpixel_scale();
     const int pixel_size = 4; // Pixel size for BGRA32 format.
 
+    int x = x_mult / subpixel_scale;
+
     x += glyph->xoff;
     y += glyph->yoff;
 
     int glyph_x = glyph->x0, glyph_y = glyph->y0;
+    int glyph_x_subpixel = -(x_mult % subpixel_scale);
     int glyph_width  = glyph->x1 - glyph->x0;
     int glyph_height = glyph->y1 - glyph->y0;
 
@@ -371,7 +380,7 @@ void FR_Blend_Glyph(FR_Renderer *font_renderer, FR_Clip_Area *clip, int x, int y
     dst += (x + y * dst_width) * pixel_size;
     agg::rendering_buffer dst_ren_buf(dst, glyph_width, glyph_height, dst_width * pixel_size);
 
-    uint8_t *src = glyphs_bitmap->pixels + (glyph_x + glyph_y * glyphs_bitmap->width) * subpixel_scale;
+    uint8_t *src = glyphs_bitmap->pixels + (glyph_x + glyph_y * glyphs_bitmap->width) * subpixel_scale + glyph_x_subpixel;
     int src_stride = glyphs_bitmap->width * subpixel_scale;
 
     const agg::rgba8 color_a(color.r, color.g, color.b);

lib/font_renderer/font_renderer.h

@@ -49,6 +49,9 @@ void            FR_Blend_Glyph(FR_Renderer *font_renderer,
                     uint8_t *dst, int dst_width,
                     const FR_Bitmap *glyphs_bitmap,
                     const FR_Bitmap_Glyph_Metrics *glyph, FR_Color color);
+int             FR_Subpixel_Scale(FR_Renderer *);
+
+#define FR_XADVANCE_TO_PIXELS(x, scale) (((x) + (scale) / 2) / (scale))
 
 #ifdef __cplusplus
 }

src/renderer.c

@@ -109,11 +109,6 @@ static GlyphSet* load_glyphset(RenFont *font, int idx) {
   set->image = FR_Bake_Font_Bitmap(font->renderer, font->height, idx << 8, 256, set->glyphs);
   check_alloc(set->image);
 
-  /* adjust glyph's xadvance */
-  for (int i = 0; i < 256; i++) {
-    set->glyphs[i].xadvance = floor(set->glyphs[i].xadvance + 0.5);
-  }
-
   return set;
 }
 
@@ -188,13 +183,14 @@ int ren_get_font_width(RenFont *font, const char *text) {
   int x = 0;
   const char *p = text;
   unsigned codepoint;
+  const int subpixel_scale = FR_Subpixel_Scale(font->renderer);
   while (*p) {
     p = utf8_to_codepoint(p, &codepoint);
     GlyphSet *set = get_glyphset(font, codepoint);
     FR_Bitmap_Glyph_Metrics *g = &set->glyphs[codepoint & 0xff];
     x += g->xadvance;
   }
-  return x;
+  return FR_XADVANCE_TO_PIXELS(x, subpixel_scale);
 }
 
 
@@ -291,15 +287,17 @@ int ren_draw_text(RenFont *font, const char *text, int x, int y, RenColor color)
   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, y, (uint8_t *) surf->pixels, surf->w, set->image, g, color_fr);
+        x_mult, y, (uint8_t *) surf->pixels, surf->w, set->image, g, color_fr);
     }
-    x += g->xadvance;
+    x_mult += g->xadvance;
   }
-  return x;
+  return FR_XADVANCE_TO_PIXELS(x_mult, subpixel_scale);
 }