Add ImFontLoader::FontAddFallbackSrc for loading fallback fonts dynamically

docs/FONTS.md

@@ -147,7 +147,7 @@ ImFont* font = io.Fonts->AddFontFromFileTTF("font.ttf", size_pixels, &config);
 // Load a first font
 ImFont* font = io.Fonts->AddFontDefault();
 ImFontConfig config;
-config.MergeMode = true;
+config.MergeTarget = font;
 io.Fonts->AddFontFromFileTTF("DroidSans.ttf", 0.0f, &config);           // Merge into first font to add e.g. Asian characters
 io.Fonts->AddFontFromFileTTF("fontawesome-webfont.ttf", 0.0f, &config); // Merge into first font to add Icons
 io.Fonts->Build();
@@ -162,7 +162,7 @@ ImFont* font = io.Fonts->AddFontDefault();
 // so ensure it is available at the time of building or calling GetTexDataAsRGBA32().
 static const ImWchar icons_ranges[] = { 0xf000, 0xf3ff, 0 }; // Will not be copied by AddFont* so keep in scope.
 ImFontConfig config;
-config.MergeMode = true;
+config.MergeTarget = font;
 io.Fonts->AddFontFromFileTTF("DroidSans.ttf", 18.0f, &config, io.Fonts->GetGlyphRangesJapanese()); // Merge into first font
 io.Fonts->AddFontFromFileTTF("fontawesome-webfont.ttf", 18.0f, &config, icons_ranges);             // Merge into first font
 io.Fonts->Build();
@@ -269,9 +269,9 @@ Example Setup:
 // Merge icons into default tool font
 #include "IconsFontAwesome.h"
 ImGuiIO& io = ImGui::GetIO();
-io.Fonts->AddFontDefault();
+ImFont *font = io.Fonts->AddFontDefault();
 ImFontConfig config;
-config.MergeMode = true;
+config.MergeTarget = font;
 config.GlyphMinAdvanceX = 13.0f; // Use if you want to make the icon monospaced
 io.Fonts->AddFontFromFileTTF("fonts/fontawesome-webfont.ttf", 13.0f, &config);
 ```
@@ -280,10 +280,10 @@ io.Fonts->AddFontFromFileTTF("fonts/fontawesome-webfont.ttf", 13.0f, &config);
 // Merge icons into default tool font
 #include "IconsFontAwesome.h"
 ImGuiIO& io = ImGui::GetIO();
-io.Fonts->AddFontDefault();
+ImFont *font = io.Fonts->AddFontDefault();
 
 ImFontConfig config;
-config.MergeMode = true;
+config.MergeTarget = font;
 config.GlyphMinAdvanceX = 13.0f; // Use if you want to make the icon monospaced
 static const ImWchar icon_ranges[] = { ICON_MIN_FA, ICON_MAX_FA, 0 };
 io.Fonts->AddFontFromFileTTF("fonts/fontawesome-webfont.ttf", 13.0f, &config, icon_ranges);
@@ -322,11 +322,11 @@ Here's an application using icons ("Avoyd", https://www.avoyd.com):
 static ImWchar exclude_ranges[] = { ICON_MIN_FA, ICON_MAX_FA, 0 };
 ImFontConfig cfg1;
 cfg1.GlyphExcludeRanges = exclude_ranges;
-io.Fonts->AddFontFromFileTTF("segoeui.ttf", 0.0f, &cfg1);
+ImFont *font1 = io.Fonts->AddFontFromFileTTF("segoeui.ttf", 0.0f, &cfg1);
 
 // Add Font Source 2, which expects to use the range above
 ImFontConfig cfg2;
-cfg2.MergeMode = true;
+cfg2.MergeTarget = font1;
 io.Fonts->AddFontFromFileTTF("FontAwesome4.ttf", 0.0f, &cfg2);
 ```
 Another (silly) example:
@@ -335,11 +335,11 @@ Another (silly) example:
 static ImWchar exclude_ranges[] = { 'A', 'Z', 0 };
 ImFontConfig cfg1;
 cfg1.GlyphExcludeRanges = exclude_ranges;
-io.Fonts->AddFontFromFileTTF("segoeui.ttf", 0.0f, &cfg1);
+ImFont *font1 = io.Fonts->AddFontFromFileTTF("segoeui.ttf", 0.0f, &cfg1);
 
 // Load another font to fill the gaps
 ImFontConfig cfg2;
-cfg2.MergeMode = true;
+cfg2.MergeTarget = font1;
 io.Fonts->AddFontFromFileTTF("Roboto-Medium.ttf", 0.0f, &cfg2);
 ```
 ![image](https://github.com/user-attachments/assets/f3d751d3-1aee-4698-bd9b-f511902f56bb)
@@ -373,10 +373,10 @@ You can use `Metrics/Debugger->Fonts->Font->Input Glyphs Overlap Detection Tool`
 ![colored glyphs](https://user-images.githubusercontent.com/8225057/106171241-9dc4ba80-6191-11eb-8a69-ca1467b206d1.png)
 
 ```cpp
-io.Fonts->AddFontFromFileTTF("../../../imgui_dev/data/fonts/NotoSans-Regular.ttf", 16.0f);
+ImFont *font = io.Fonts->AddFontFromFileTTF("../../../imgui_dev/data/fonts/NotoSans-Regular.ttf", 16.0f);
 static ImWchar ranges[] = { 0x1, 0x1FFFF, 0 };
 static ImFontConfig cfg;
-cfg.MergeMode = true;
+cfg.MergeTarget = font;
 cfg.FontLoaderFlags |= ImGuiFreeTypeLoaderFlags_LoadColor;
 io.Fonts->AddFontFromFileTTF("C:\\Windows\\Fonts\\seguiemj.ttf", 16.0f, &cfg);
 ```

imgui.h

@@ -3474,7 +3474,7 @@ struct ImFontConfig
     bool            FontDataOwnedByAtlas;   // true     // TTF/OTF data ownership taken by the container ImFontAtlas (will delete memory itself).
 
     // Options
-    bool            MergeMode;              // false    // Merge into previous ImFont, so you can combine multiple inputs font into one ImFont (e.g. ASCII font + icons + Japanese glyphs). You may want to use GlyphOffset.y when merge font of different heights.
+    ImFont*         MergeTarget;            // NULL     // Merge into specified ImFont, so you can combine multiple inputs font into one ImFont (e.g. ASCII font + icons + Japanese glyphs). You may want to use GlyphOffset.y when merge font of different heights.
     bool            PixelSnapH;             // false    // Align every glyph AdvanceX to pixel boundaries. Useful e.g. if you are merging a non-pixel aligned font with the default font. If enabled, you can set OversampleH/V to 1.
     bool            PixelSnapV;             // true     // Align Scaled GlyphOffset.y to pixel boundaries.
     ImS8            OversampleH;            // 0 (2)    // Rasterize at higher quality for sub-pixel positioning. 0 == auto == 1 or 2 depending on size. Note the difference between 2 and 3 is minimal. You can reduce this to 1 for large glyphs save memory. Read https://github.com/nothings/stb/blob/master/tests/oversample/README.md for details.
@@ -3499,6 +3499,7 @@ struct ImFontConfig
     ImFont*         DstFont;                // Target font (as we merging fonts, multiple ImFontConfig may target the same font)
     const ImFontLoader* FontLoader;         // Custom font backend for this source (default source is the one stored in ImFontAtlas)
     void*           FontLoaderData;         // Font loader opaque storage (per font config)
+    void*           UserData;
 
     IMGUI_API ImFontConfig();
 };

imgui_draw.cpp

@@ -3012,7 +3012,7 @@ ImFont* ImFontAtlas::AddFont(const ImFontConfig* font_cfg_in)
 
     // Create new font
     ImFont* font;
-    if (!font_cfg_in->MergeMode)
+    if (!font_cfg_in->MergeTarget)
     {
         font = IM_NEW(ImFont)();
         font->FontId = FontNextUniqueID++;
@@ -3023,8 +3023,8 @@ ImFont* ImFontAtlas::AddFont(const ImFontConfig* font_cfg_in)
     }
     else
     {
-        IM_ASSERT(Fonts.Size > 0 && "Cannot use MergeMode for the first font"); // When using MergeMode make sure that a font has already been added before. You can use ImGui::GetIO().Fonts->AddFontDefault() to add the default imgui font.
-        font = Fonts.back();
+        IM_ASSERT(font_cfg_in->MergeTarget->ContainerAtlas == this);
+        font = font_cfg_in->MergeTarget;
     }
 
     // Add to list
@@ -3064,7 +3064,7 @@ ImFont* ImFontAtlas::AddFont(const ImFontConfig* font_cfg_in)
         ImFontAtlasFontDestroySourceData(this, font_cfg);
         Sources.pop_back();
         font->Sources.pop_back();
-        if (!font_cfg->MergeMode)
+        if (!font_cfg->MergeTarget)
         {
             IM_DELETE(font);
             Fonts.pop_back();
@@ -3610,7 +3610,7 @@ bool ImFontAtlasFontSourceInit(ImFontAtlas* atlas, ImFontConfig* src)
 
 void ImFontAtlasFontSourceAddToFont(ImFontAtlas* atlas, ImFont* font, ImFontConfig* src)
 {
-    if (src->MergeMode == false)
+    if (!src->MergeTarget)
     {
         font->ClearOutputData();
         //font->FontSize = src->SizePixels;
@@ -3718,7 +3718,7 @@ static void ImFontAtlasBuildSetupFontBakedBlanks(ImFontAtlas* atlas, ImFontBaked
 }
 
 // Load/identify special glyphs
-// (note that this is called again for fonts with MergeMode)
+// (note that this is called again for fonts with MergeTarget)
 void ImFontAtlasBuildSetupFontSpecialGlyphs(ImFontAtlas* atlas, ImFont* font, ImFontConfig* src)
 {
     IM_UNUSED(atlas);
@@ -4436,11 +4436,14 @@ static ImFontGlyph* ImFontBaked_BuildLoadGlyph(ImFontBaked* baked, ImWchar codep
             return glyph;
 
     // Call backend
-    char* loader_user_data_p = (char*)baked->FontLoaderDatas;
-    int src_n = 0;
-    for (ImFontConfig* src : font->Sources)
+    bool no_fallback = false;
+    size_t loader_user_data_n = 0;
+    auto loadGlyph = [=, &no_fallback, &loader_user_data_n](int src_n) -> ImFontGlyph*
     {
+        ImFontConfig* src = font->Sources[src_n];
         const ImFontLoader* loader = src->FontLoader ? src->FontLoader : atlas->FontLoader;
+        void *loader_user_data_p = (char*)baked->FontLoaderDatas + loader_user_data_n;
+        loader_user_data_n += loader->FontBakedSrcLoaderDataSize;
         if (!src->GlyphExcludeRanges || ImFontAtlasBuildAcceptCodepointForSource(src, codepoint))
         {
             if (only_load_advance_x == NULL)
@@ -4460,12 +4463,57 @@ static ImFontGlyph* ImFontBaked_BuildLoadGlyph(ImFontBaked* baked, ImWchar codep
                 if (loader->FontBakedLoadGlyph(atlas, src, baked, loader_user_data_p, codepoint, NULL, only_load_advance_x))
                 {
                     ImFontAtlasBakedAddFontGlyphAdvancedX(atlas, baked, src, codepoint, *only_load_advance_x);
+                    no_fallback = true;
                     return NULL;
                 }
             }
         }
-        loader_user_data_p += loader->FontBakedSrcLoaderDataSize;
-        src_n++;
+
+        return NULL;
+    };
+
+    int src_n = 0;
+    for (; src_n < font->Sources.Size; ++src_n)
+    {
+        ImFontGlyph* glyph = loadGlyph(src_n);
+        if (glyph || no_fallback)
+            return glyph;
+    }
+
+    if (atlas->FontLoader->FontAddFallbackSrc)
+    {
+        atlas->FontLoader->FontAddFallbackSrc(atlas, font, codepoint);
+
+        for (; src_n < font->Sources.Size; ++src_n)
+        {
+            ImFontConfig* src = font->Sources[src_n];
+            const ImFontLoader* loader = src->FontLoader ? src->FontLoader : atlas->FontLoader;
+
+            for (int baked_n = 0; baked_n < atlas->Builder->BakedPool.Size; baked_n++)
+            {
+                ImFontBaked* baked_sibling = &atlas->Builder->BakedPool[baked_n];
+                if (baked_sibling->ContainerFont != font || baked_sibling->WantDestroy)
+                    continue;
+
+                if (loader->FontBakedSrcLoaderDataSize > 0)
+                {
+                    void *new_data = IM_ALLOC(loader_user_data_n + loader->FontBakedSrcLoaderDataSize);
+                    memcpy(new_data, baked_sibling->FontLoaderDatas, loader_user_data_n);
+                    IM_FREE(baked_sibling->FontLoaderDatas);
+                    baked_sibling->FontLoaderDatas = new_data;
+                }
+
+                if (loader->FontBakedInit)
+                {
+                    void *loader_user_data_p = (char*)baked_sibling->FontLoaderDatas + loader_user_data_n;
+                    loader->FontBakedInit(atlas, src, baked_sibling, loader_user_data_p);
+                }
+            }
+
+            ImFontGlyph* glyph = loadGlyph(src_n);
+            if (glyph || no_fallback)
+                return glyph;
+        }
     }
 
     // Lazily load fallback glyph
@@ -4572,14 +4620,14 @@ static bool ImGui_ImplStbTrueType_FontSrcInit(ImFontAtlas* atlas, ImFontConfig*
     }
     src->FontLoaderData = bd_font_data;
 
-    if (src->MergeMode && src->SizePixels == 0.0f)
+    if (src->MergeTarget && src->SizePixels == 0.0f)
         src->SizePixels = src->DstFont->Sources[0]->SizePixels;
 
     if (src->SizePixels >= 0.0f)
         bd_font_data->ScaleFactor = stbtt_ScaleForPixelHeight(&bd_font_data->FontInfo, 1.0f);
     else
         bd_font_data->ScaleFactor = stbtt_ScaleForMappingEmToPixels(&bd_font_data->FontInfo, 1.0f);
-    if (src->MergeMode && src->SizePixels != 0.0f)
+    if (src->MergeTarget && src->SizePixels != 0.0f)
         bd_font_data->ScaleFactor *= src->SizePixels / src->DstFont->Sources[0]->SizePixels; // FIXME-NEWATLAS: Should tidy up that a bit
 
     return true;
@@ -4609,7 +4657,7 @@ static bool ImGui_ImplStbTrueType_FontBakedInit(ImFontAtlas* atlas, ImFontConfig
     IM_UNUSED(atlas);
 
     ImGui_ImplStbTrueType_FontSrcData* bd_font_data = (ImGui_ImplStbTrueType_FontSrcData*)src->FontLoaderData;
-    if (src->MergeMode == false)
+    if (!src->MergeTarget)
     {
         // FIXME-NEWFONTS: reevaluate how to use sizing metrics
         // FIXME-NEWFONTS: make use of line gap value

imgui_internal.h

@@ -3700,6 +3700,7 @@ struct ImFontLoader
     bool            (*FontBakedInit)(ImFontAtlas* atlas, ImFontConfig* src, ImFontBaked* baked, void* loader_data_for_baked_src);
     void            (*FontBakedDestroy)(ImFontAtlas* atlas, ImFontConfig* src, ImFontBaked* baked, void* loader_data_for_baked_src);
     bool            (*FontBakedLoadGlyph)(ImFontAtlas* atlas, ImFontConfig* src, ImFontBaked* baked, void* loader_data_for_baked_src, ImWchar codepoint, ImFontGlyph* out_glyph, float* out_advance_x);
+    void            (*FontAddFallbackSrc)(ImFontAtlas* atlas, ImFont* parent, ImWchar codepoint); // merges zero or more new font sources into the parent ImFont which should contain a glyph for the codepoint
 
     // Size of backend data, Per Baked * Per Source. Buffers are managed by core to avoid excessive allocations.
     // FIXME: At this point the two other types of buffers may be managed by core to be consistent?

misc/freetype/imgui_freetype.cpp

@@ -422,7 +422,7 @@ bool ImGui_ImplFreeType_FontBakedInit(ImFontAtlas* atlas, ImFontConfig* src, ImF
 {
     IM_UNUSED(atlas);
     float size = baked->Size;
-    if (src->MergeMode && src->SizePixels != 0.0f)
+    if (src->MergeTarget && src->SizePixels != 0.0f)
         size *= (src->SizePixels / baked->ContainerFont->Sources[0]->SizePixels);
 
     ImGui_ImplFreeType_FontSrcData* bd_font_data = (ImGui_ImplFreeType_FontSrcData*)src->FontLoaderData;
@@ -450,7 +450,7 @@ bool ImGui_ImplFreeType_FontBakedInit(ImFontAtlas* atlas, ImFontConfig* src, ImF
     FT_Request_Size(bd_font_data->FtFace, &req);
 
     // Output
-    if (src->MergeMode == false)
+    if (!src->MergeTarget)
     {
         // Read metrics
         FT_Size_Metrics metrics = bd_baked_data->FtSize->metrics;