ImFontAtlas: heuristic increase texture width up to 4096 with 4000+ glyphs. Various comments (#491)

extra_fonts/README.txt

@@ -2,6 +2,8 @@
  The code in imgui.cpp embeds a copy of 'ProggyClean.ttf' that you can use without any external files.
  Those are only provided as a convenience, you can load your own .TTF files.
 
+ Fonts are rasterized in a single texture at the time of calling either of io.Fonts.GetTexDataAsAlpha8()/GetTexDataAsRGBA32()/Build().
+
 ---------------------------------
  LOADING INSTRUCTIONS
 ---------------------------------
@@ -20,10 +22,17 @@
 
    ImFontConfig config;
    config.OversampleH = 3;
-   config.OversampleV = 3;
+   config.OversampleV = 1;
    config.GlyphExtraSpacing.x = 1.0f;
    io.Fonts->AddFontFromFileTTF("font.ttf", size_pixels, &config);
 
+ If you have very large number of glyphs or multiple fonts:
+
+  - Mind the fact that some graphics drivers have texture size limitation.
+  - Set io.Fonts.TexDesiredWidth to specify a texture width to minimize texture height (see comment in ImFontAtlas::Build function).
+  - You may reduce oversampling, e.g. config.OversampleH = 2 or 1.
+  - Reduce glyphs ranges, consider calculating them based on your source data if this is possible.
+
  Combine two fonts into one:
 
    // Load main font

imgui.cpp

@@ -359,6 +359,7 @@
 
  Q: How can I load multiple fonts?
  A: Use the font atlas to pack them into a single texture:
+    (Read extra_fonts/README.txt and the code in ImFontAtlas for more details.)
 
       ImGuiIO& io = ImGui::GetIO();
       ImFont* font0 = io.Fonts->AddFontDefault();
@@ -371,7 +372,7 @@
       // Options
       ImFontConfig config;
       config.OversampleH = 3;
-      config.OversampleV = 3;
+      config.OversampleV = 1;
       config.GlyphExtraSpacing.x = 1.0f;
       io.Fonts->LoadFromFileTTF("myfontfile.ttf", size_pixels, &config);
 
@@ -383,8 +384,6 @@
       io.Fonts->LoadFromFileTTF("fontawesome-webfont.ttf", 16.0f, &config, ranges);
       io.Fonts->LoadFromFileTTF("myfontfile.ttf", size_pixels, NULL, &config, io.Fonts->GetGlyphRangesJapanese());
 
-    Read extra_fonts/README.txt or ImFontAtlas class for more details.
-
  Q: How can I display and input non-Latin characters such as Chinese, Japanese, Korean, Cyrillic?
  A: When loading a font, pass custom Unicode ranges to specify the glyphs to load. ImGui will support UTF-8 encoding across the board.
     Character input depends on you passing the right character code to io.AddInputCharacter(). The example applications do that.

imgui_draw.cpp

@@ -1244,8 +1244,9 @@ bool    ImFontAtlas::Build()
         }
     }
 
-    // Start packing
+    // Start packing. We need a known width for the skyline algorithm. Using a cheap heuristic here to decide of width. User can override TexDesiredWidth if they wish.
-    TexWidth = (TexDesiredWidth > 0) ? TexDesiredWidth : (total_glyph_count > 2000) ? 2048 : (total_glyph_count > 1000) ? 1024 : 512;  // Width doesn't actually matters much but some API/GPU have texture size limitations, and increasing width can decrease height.
+    // After packing is done, width shouldn't matter much, but some API/GPU have texture size limitations and increasing width can decrease height.
+    TexWidth = (TexDesiredWidth > 0) ? TexDesiredWidth : (total_glyph_count > 4000) ? 4096 : (total_glyph_count > 2000) ? 2048 : (total_glyph_count > 1000) ? 1024 : 512;
     TexHeight = 0;
     const int max_tex_height = 1024*32;
     stbtt_pack_context spc;