Fonts: Prefer using U+FFFD character for fallback instead of '?'. Use U+FF0E dot character to construct an ellipsis if U+002E '.' is not available. (#4269)

docs/CHANGELOG.txt

@@ -49,6 +49,8 @@ Other Changes:
 - TabBar: Fixed using more than 32 KB-worth of tab names. (#4176)
 - Fixed printf-style format checks on non-MinGW flavors. (#4183, #3592)
 - Fonts: Functions with a 'float size_pixels' parameter can accept zero if it is set in ImFontSize::SizePixels.
+- Fonts: Prefer using U+FFFD character for fallback instead of '?', if available. (#4269)
+- Fonts: Use U+FF0E dot character to construct an ellipsis if U+002E '.' is not available. (#4269)
 - Demo: Fixed requirement in 1.83 to link with imgui_demo.cpp if IMGUI_DISABLE_METRICS_WINDOW is not set. (#4171)
   Normally the right way to disable compiling the demo is to set IMGUI_DISABLE_DEMO_WINDOWS, but we want to avoid
   implying that the file is required.

imgui.cpp

@@ -2786,7 +2786,7 @@ void ImGui::RenderTextEllipsis(ImDrawList* draw_list, const ImVec2& pos_min, con
         int ellipsis_char_count = 1;
         if (ellipsis_char == (ImWchar)-1)
         {
-            ellipsis_char = (ImWchar)'.';
+            ellipsis_char = font->DotChar;
             ellipsis_char_count = 3;
         }
         const ImFontGlyph* glyph = font->FindGlyph(ellipsis_char);

imgui.h

@@ -2709,7 +2709,8 @@ struct ImFont
     const ImFontConfig*         ConfigData;         // 4-8   // in  //            // Pointer within ContainerAtlas->ConfigData
     short                       ConfigDataCount;    // 2     // in  // ~ 1        // Number of ImFontConfig involved in creating this font. Bigger than 1 when merging multiple font sources into one ImFont.
     ImWchar                     FallbackChar;       // 2     // in  // = '?'      // Replacement character if a glyph isn't found. Only set via SetFallbackChar()
-    ImWchar                     EllipsisChar;       // 2     // out // = -1       // Character used for ellipsis rendering.
+    ImWchar                     EllipsisChar;       // 2     // out // = '...'    // Character used for ellipsis rendering.
+    ImWchar                     DotChar;            // 2     // out // = '.'      // Dot character used for fallback ellipsis rendering.
     bool                        DirtyLookupTables;  // 1     // out //
     float                       Scale;              // 4     // in  // = 1.f      // Base font scale, multiplied by the per-window font scale which you can adjust with SetWindowFontScale()
     float                       Ascent, Descent;    // 4+4   // out //            // Ascent: distance from top to bottom of e.g. 'A' [0..FontSize]

imgui_draw.cpp

@@ -2795,23 +2795,6 @@ void ImFontAtlasBuildFinish(ImFontAtlas* atlas)
     for (int i = 0; i < atlas->Fonts.Size; i++)
         if (atlas->Fonts[i]->DirtyLookupTables)
             atlas->Fonts[i]->BuildLookupTable();
-
-    // Ellipsis character is required for rendering elided text. We prefer using U+2026 (horizontal ellipsis).
-    // However some old fonts may contain ellipsis at U+0085. Here we auto-detect most suitable ellipsis character.
-    // FIXME: Also note that 0x2026 is currently seldom included in our font ranges. Because of this we are more likely to use three individual dots.
-    for (int i = 0; i < atlas->Fonts.size(); i++)
-    {
-        ImFont* font = atlas->Fonts[i];
-        if (font->EllipsisChar != (ImWchar)-1)
-            continue;
-        const ImWchar ellipsis_variants[] = { (ImWchar)0x2026, (ImWchar)0x0085 };
-        for (int j = 0; j < IM_ARRAYSIZE(ellipsis_variants); j++)
-            if (font->FindGlyphNoFallback(ellipsis_variants[j]) != NULL) // Verify glyph exists
-            {
-                font->EllipsisChar = ellipsis_variants[j];
-                break;
-            }
-    }
 }
 
 // Retrieve list of range (2 int per range, values are inclusive)
@@ -3110,8 +3093,9 @@ ImFont::ImFont()
 {
     FontSize = 0.0f;
     FallbackAdvanceX = 0.0f;
-    FallbackChar = (ImWchar)'?';
+    FallbackChar = (ImWchar)-1;
     EllipsisChar = (ImWchar)-1;
+    DotChar = (ImWchar)-1;
     FallbackGlyph = NULL;
     ContainerAtlas = NULL;
     ConfigData = NULL;
@@ -3142,6 +3126,14 @@ void    ImFont::ClearOutputData()
     MetricsTotalSurface = 0;
 }
 
+static ImWchar FindFirstExistingGlyph(ImFont* font, const ImWchar* candidate_chars, int candidate_chars_count)
+{
+    for (int n = 0; n < candidate_chars_count; n++)
+        if (font->FindGlyphNoFallback(candidate_chars[n]) != NULL)
+            return candidate_chars[n];
+    return (ImWchar)-1;
+}
+
 void ImFont::BuildLookupTable()
 {
     int max_codepoint = 0;
@@ -3184,9 +3176,31 @@ void ImFont::BuildLookupTable()
     SetGlyphVisible((ImWchar)' ', false);
     SetGlyphVisible((ImWchar)'\t', false);
 
-    // Setup fall-backs
+    // Ellipsis character is required for rendering elided text. We prefer using U+2026 (horizontal ellipsis).
+    // However some old fonts may contain ellipsis at U+0085. Here we auto-detect most suitable ellipsis character.
+    // FIXME: Note that 0x2026 is rarely included in our font ranges. Because of this we are more likely to use three individual dots.
+    const ImWchar ellipsis_chars[] = { (ImWchar)0x2026, (ImWchar)0x0085 };
+    const ImWchar dots_chars[] = { (ImWchar)'.', (ImWchar)0xFF0E };
+    if (EllipsisChar == (ImWchar)-1)
+        EllipsisChar = FindFirstExistingGlyph(this, ellipsis_chars, IM_ARRAYSIZE(ellipsis_chars));
+    if (DotChar == (ImWchar)-1)
+        DotChar = FindFirstExistingGlyph(this, dots_chars, IM_ARRAYSIZE(dots_chars));
+
+    // Setup fallback character
+    const ImWchar fallback_chars[] = { (ImWchar)IM_UNICODE_CODEPOINT_INVALID, (ImWchar)'?', (ImWchar)' ' };
     FallbackGlyph = FindGlyphNoFallback(FallbackChar);
-    FallbackAdvanceX = FallbackGlyph ? FallbackGlyph->AdvanceX : 0.0f;
+    if (FallbackGlyph == NULL)
+    {
+        FallbackChar = FindFirstExistingGlyph(this, fallback_chars, IM_ARRAYSIZE(fallback_chars));
+        FallbackGlyph = FindGlyphNoFallback(FallbackChar);
+        if (FallbackGlyph == NULL)
+        {
+            FallbackGlyph = &Glyphs.back();
+            FallbackChar = FallbackGlyph->Codepoint;
+        }
+    }
+
+    FallbackAdvanceX = FallbackGlyph->AdvanceX;
     for (int i = 0; i < max_codepoint + 1; i++)
         if (IndexAdvanceX[i] < 0.0f)
             IndexAdvanceX[i] = FallbackAdvanceX;