FreeType: select size for fixed sized, unscalable bitmaps and correct for it with render density

Author: Green-Sky

misc/freetype/imgui_freetype.cpp

@@ -108,7 +108,7 @@ static FT_Error ImGuiLunasvgPortPresetSlot(FT_GlyphSlot slot, FT_Bool cache, FT_
 //-------------------------------------------------------------------------
 
 #define FT_CEIL(X)      (((X + 63) & -64) / 64) // From SDL_ttf: Handy routines for converting from fixed point
-#define FT_SCALEFACTOR  64.0f
+#define FT_SCALEFACTOR  64.0f                   // For converting from/to 26.6 factionals
 
 // Glyph metrics:
 // --------------
@@ -435,19 +435,58 @@ static bool ImGui_ImplFreeType_FontBakedInit(ImFontAtlas* atlas, ImFontConfig* s
 
     FT_New_Size(bd_font_data->FtFace, &bd_baked_data->FtSize);
     FT_Activate_Size(bd_baked_data->FtSize);
-
-    // Vuhdo 2017: "I'm not sure how to deal with font sizes properly. As far as I understand, currently ImGui assumes that the 'pixel_height'
-    // is a maximum height of an any given glyph, i.e. it's the sum of font's ascender and descender. Seems strange to me.
-    // FT_Set_Pixel_Sizes() doesn't seem to get us the same result."
-    // (FT_Set_Pixel_Sizes() essentially calls FT_Request_Size() with FT_SIZE_REQUEST_TYPE_NOMINAL)
     const float rasterizer_density = src->RasterizerDensity * baked->RasterizerDensity;
-    FT_Size_RequestRec req;
-    req.type = (bd_font_data->UserFlags & ImGuiFreeTypeLoaderFlags_Bitmap) ? FT_SIZE_REQUEST_TYPE_NOMINAL : FT_SIZE_REQUEST_TYPE_REAL_DIM;
-    req.width = 0;
-    req.height = (uint32_t)(size * 64 * rasterizer_density);
-    req.horiResolution = 0;
-    req.vertResolution = 0;
-    FT_Request_Size(bd_font_data->FtFace, &req);
+    if (((bd_font_data->FtFace->face_flags & FT_FACE_FLAG_FIXED_SIZES) != 0) && ((bd_font_data->FtFace->face_flags & FT_FACE_FLAG_SCALABLE) == 0) && ((bd_font_data->UserFlags & ImGuiFreeTypeLoaderFlags_Bitmap) != 0))
+    {
+        IM_ASSERT(bd_font_data->FtFace->num_fixed_sizes > 0);
+
+        // Loop over sizes and pick the closest, larger (or better equal) size.
+        int best_index = 0;
+        float best_height = bd_font_data->FtFace->available_sizes[best_index].y_ppem / FT_SCALEFACTOR;
+        for (int i = 1; i < bd_font_data->FtFace->num_fixed_sizes; i++)
+        {
+            const float cur_height = bd_font_data->FtFace->available_sizes[i].y_ppem / FT_SCALEFACTOR;
+            // TODO: is this overkill?
+            // TODO: is-float-close with epsilon param would be nice, maybe
+            if (ImFabs(cur_height - size) < 0.001f)
+            {
+                best_index = i;
+                break;
+            }
+            else if (cur_height < size)
+            {
+                if (best_height < cur_height)
+                {
+                    best_index = i;
+                    best_height = cur_height;
+                }
+            }
+            else
+            {
+                if (best_height > cur_height)
+                {
+                    best_index = i;
+                    best_height = cur_height;
+                }
+            }
+        }
+        FT_Select_Size(bd_font_data->FtFace, best_index);
+    }
+    else
+    {
+        // Vuhdo 2017: "I'm not sure how to deal with font sizes properly. As far as I understand, currently ImGui assumes that the 'pixel_height'
+        // is a maximum height of an any given glyph, i.e. it's the sum of font's ascender and descender. Seems strange to me.
+        // FT_Set_Pixel_Sizes() doesn't seem to get us the same result."
+        // (FT_Set_Pixel_Sizes() essentially calls FT_Request_Size() with FT_SIZE_REQUEST_TYPE_NOMINAL)
+
+        FT_Size_RequestRec req;
+        req.type = (bd_font_data->UserFlags & ImGuiFreeTypeLoaderFlags_Bitmap) ? FT_SIZE_REQUEST_TYPE_NOMINAL : FT_SIZE_REQUEST_TYPE_REAL_DIM;
+        req.width = 0;
+        req.height = (uint32_t)(size * FT_SCALEFACTOR * rasterizer_density);
+        req.horiResolution = 0;
+        req.vertResolution = 0;
+        FT_Request_Size(bd_font_data->FtFace, &req);
+    }
 
     // Output
     if (src->MergeMode == false)
@@ -496,7 +535,13 @@ static bool ImGui_ImplFreeType_FontBakedLoadGlyph(ImFontAtlas* atlas, ImFontConf
 
     FT_Face face = bd_font_data->FtFace;
     FT_GlyphSlot slot = face->glyph;
-    const float rasterizer_density = src->RasterizerDensity * baked->RasterizerDensity;
+    float bitmap_density = 1.f;
+    if (((face->face_flags & FT_FACE_FLAG_FIXED_SIZES) != 0) && ((face->face_flags & FT_FACE_FLAG_SCALABLE) == 0) && ((bd_font_data->UserFlags & ImGuiFreeTypeLoaderFlags_Bitmap) != 0))
+    {
+        // scale fixed size bitmap to target size
+        bitmap_density = (float)bd_font_data->FtFace->size->metrics.y_ppem / baked->Size;
+    }
+    const float rasterizer_density = src->RasterizerDensity * baked->RasterizerDensity * bitmap_density;
 
     // Load metrics only mode
     const float advance_x = (slot->advance.x / FT_SCALEFACTOR) / rasterizer_density;