ImFontAtlas: Rewrote stb_truetype based builder.

- Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth).
- Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233)
- Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas.

docs/CHANGELOG.txt

@@ -72,6 +72,9 @@ Other Changes:
   Missing calls to End(), past the assert, should not lead to crashes or to the fallback Debug window appearing on screen.
   Those changes makes it easier to integrate dear imgui with a scripting language allowing, given asserts are redirected
   into e.g. an error log and stopping the script execution.
+- ImFontAtlas: Stb: Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth).
+- ImFontAtlas: Stb: Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233)
+- ImFontAtlas: Stb: Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas.
 - ImDrawList: Optimized some of the functions for performance of debug builds where non-inline function call cost are non-negligible.
   (Our test UI scene on VS2015 Debug Win64 with /RTC1 went ~5.9 ms -> ~4.9 ms. In Release same scene stays at ~0.3 ms.)
 - IO: Added BackendPlatformUserData, BackendRendererUserData, BackendLanguageUserData void* for storage use by back-ends.

docs/TODO.txt

@@ -235,9 +235,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - pie menus patterns (#434)
  - markup: simple markup language for color change? (#902)
 
-!- font: need handling of missing glyphs by not packing/rasterizing glyph 0 of a given font.
- - font: MergeMode: flags to select overwriting or not.
- - font: MergeMode: duplicate glyphs are stored in the atlas texture which is suboptimal.
+ - font: MergeMode: flags to select overwriting or not (this is now very easy with refactored ImFontAtlasBuildWithStbTruetype)
  - font: free the Alpha buffer if user only requested RGBA.
 !- font: better CalcTextSizeA() API, at least for simple use cases. current one is horrible (perhaps have simple vs extended versions).
  - font: a CalcTextHeight() helper could run faster than CalcTextSize().y
@@ -252,7 +250,6 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - font/draw: vertical and/or rotated text renderer (#705) - vertical is easier clipping wise
  - font/draw: need to be able to specify wrap start position.
  - font/draw: better reserve policy for large horizontal block of text (shouldn't reserve for all clipped lines)
- - font: imgui_freetype.h alternative renderer (#618)
  - font: optimization: for monospace font (like the default one) we can trim IndexXAdvance as long as trailing value is == FallbackXAdvance (need to make sure TAB is still correct).
  - font: add support for kerning, probably optional. A) perhaps default to (32..128)^2 matrix ~ 9K entries = 36KB, then hash for non-ascii?. B) or sparse lookup into per-char list?
  - font: add a simpler CalcTextSizeA() api? current one ok but not welcome if user needs to call it directly (without going through ImGui::CalcTextSize)

imgui.h

@@ -2047,7 +2047,7 @@ struct ImFontAtlas
     ImFontAtlasFlags            Flags;              // Build flags (see ImFontAtlasFlags_)
     ImTextureID                 TexID;              // User data to refer to the texture once it has been uploaded to user's graphic systems. It is passed back to you during rendering via the ImDrawCmd structure.
     int                         TexDesiredWidth;    // Texture width desired by user before Build(). Must be a power-of-two. If have many glyphs your graphics API have texture size restrictions you may want to increase texture width to decrease height.
-    int                         TexGlyphPadding;    // Padding between glyphs within texture in pixels. Defaults to 1.
+    int                         TexGlyphPadding;    // Padding between glyphs within texture in pixels. Defaults to 1. If your rendering method doesn't rely on bilinear filtering you may set this to 0.
 
     // [Internal]
     // NB: Access texture data via GetTexData*() calls! Which will setup a default font for you.

imgui_draw.cpp

@@ -1541,11 +1541,11 @@ ImFont* ImFontAtlas::AddFont(const ImFontConfig* font_cfg)
     if (!font_cfg->MergeMode)
         Fonts.push_back(IM_NEW(ImFont));
     else
-        IM_ASSERT(!Fonts.empty()); // 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.
+        IM_ASSERT(!Fonts.empty() && "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.
 
     ConfigData.push_back(*font_cfg);
     ImFontConfig& new_font_cfg = ConfigData.back();
-    if (!new_font_cfg.DstFont)
+    if (new_font_cfg.DstFont == NULL)
         new_font_cfg.DstFont = Fonts.back();
     if (!new_font_cfg.FontDataOwnedByAtlas)
     {
@@ -1733,139 +1733,220 @@ void    ImFontAtlasBuildMultiplyRectAlpha8(const unsigned char table[256], unsig
             data[i] = table[data[i]];
 }
 
+// Temporary data for one source font (multiple source fonts can be merged into one destination ImFont)
+// (C++03 doesn't allow instancing ImVector<> with function-local types so we declare the type here.)
+struct ImFontBuildSrcData
+{
+    stbtt_fontinfo      FontInfo;
+    stbtt_pack_range    PackRange;          // Hold the list of codepoints to pack (essentially points to Codepoints.Data)
+    stbrp_rect*         Rects;              // Rectangle to pack. We first fill in their size and the packer will give us their position.
+    stbtt_packedchar*   PackedChars;        // Output glyphs
+    const ImWchar*      SrcRanges;          // Ranges as requested by user (user is allowed to request too much, e.g. 0x0020..0xFFFF)
+    int                 DstIndex;           // Index into atlas->Fonts[] and dst_tmp_array[]
+    int                 GlyphsHighest;      // Highest requested codepoint
+    int                 GlyphsCount;        // Glyph count (excluding missing glyphs and glyphs already set by an earlier source font)
+    ImBoolVector        GlyphsSet;          // Glyph bit map (random access, 1-bit per codepoint. This will be a maximum of 8KB)
+    ImVector<int>       GlyphsList;         // Glyph codepoints list (flattened version of GlyphsMap)
+};
+
+// Temporary data for one destination ImFont* (multiple source fonts can be merged into one destination ImFont)
+struct ImFontBuildDstData
+{
+    int                 SrcCount;           // Number of source fonts targeting this destination font.
+    int                 GlyphsHighest;
+    int                 GlyphsCount;
+    ImBoolVector        GlyphsSet;          // This is used to resolve collision when multiple sources are merged into a same destination font.
+};
+
+static void UnpackBoolVectorToFlatIndexList(const ImBoolVector* in, ImVector<int>* out)
+{
+    IM_ASSERT(sizeof(in->Storage.Data[0]) == sizeof(int));
+    const int* it_begin = in->Storage.begin();
+    const int* it_end = in->Storage.end();
+    for (const int* it = it_begin; it < it_end; it++)
+        if (int entries_32 = *it)
+            for (int bit_n = 0; bit_n < 32; bit_n++)
+                if (entries_32 & (1 << bit_n))
+                    out->push_back((int)((it - it_begin) << 5) + bit_n);
+}
+
 bool    ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas)
 {
     IM_ASSERT(atlas->ConfigData.Size > 0);
 
     ImFontAtlasBuildRegisterDefaultCustomRects(atlas);
 
+    // Clear atlas
     atlas->TexID = (ImTextureID)NULL;
     atlas->TexWidth = atlas->TexHeight = 0;
     atlas->TexUvScale = ImVec2(0.0f, 0.0f);
     atlas->TexUvWhitePixel = ImVec2(0.0f, 0.0f);
     atlas->ClearTexData();
 
-    // Count glyphs/ranges
-    int total_glyphs_count = 0;
-    int total_ranges_count = 0;
-    for (int input_i = 0; input_i < atlas->ConfigData.Size; input_i++)
+    // Temporary storage for building
+    ImVector<ImFontBuildSrcData> src_tmp_array;
+    ImVector<ImFontBuildDstData> dst_tmp_array;
+    src_tmp_array.resize(atlas->ConfigData.Size);
+    dst_tmp_array.resize(atlas->Fonts.Size);
+    memset(src_tmp_array.Data, 0, (size_t)src_tmp_array.size_in_bytes());
+    memset(dst_tmp_array.Data, 0, (size_t)dst_tmp_array.size_in_bytes());
+
+    // 1. Initialize font loading structure, check font data validity
+    for (int src_i = 0; src_i < atlas->ConfigData.Size; src_i++)
     {
-        ImFontConfig& cfg = atlas->ConfigData[input_i];
-        if (!cfg.GlyphRanges)
-            cfg.GlyphRanges = atlas->GetGlyphRangesDefault();
-        for (const ImWchar* in_range = cfg.GlyphRanges; in_range[0] && in_range[1]; in_range += 2, total_ranges_count++)
-            total_glyphs_count += (in_range[1] - in_range[0]) + 1;
-    }
-
-    // We need a width for the skyline algorithm. Using a dumb heuristic here to decide of width. User can override TexDesiredWidth and TexGlyphPadding if they wish.
-    // Width doesn't really matter much, but some API/GPU have texture size limitations and increasing width can decrease height.
-    atlas->TexWidth = (atlas->TexDesiredWidth > 0) ? atlas->TexDesiredWidth : (total_glyphs_count > 4000) ? 4096 : (total_glyphs_count > 2000) ? 2048 : (total_glyphs_count > 1000) ? 1024 : 512;
-    atlas->TexHeight = 0;
-
-    // Start packing
-    const int max_tex_height = 1024*32;
-    stbtt_pack_context spc = {};
-    if (!stbtt_PackBegin(&spc, NULL, atlas->TexWidth, max_tex_height, 0, atlas->TexGlyphPadding, NULL))
-        return false;
-    stbtt_PackSetOversampling(&spc, 1, 1);
-
-    // Pack our extra data rectangles first, so it will be on the upper-left corner of our texture (UV will have small values).
-    ImFontAtlasBuildPackCustomRects(atlas, spc.pack_info);
-
-    // Initialize font information (so we can error without any cleanup)
-    struct ImFontTempBuildData
-    {
-        stbtt_fontinfo      FontInfo;
-        stbrp_rect*         Rects;
-        int                 RectsCount;
-        stbtt_pack_range*   Ranges;
-        int                 RangesCount;
-    };
-    ImFontTempBuildData* tmp_array = (ImFontTempBuildData*)ImGui::MemAlloc((size_t)atlas->ConfigData.Size * sizeof(ImFontTempBuildData));
-    for (int input_i = 0; input_i < atlas->ConfigData.Size; input_i++)
-    {
-        ImFontConfig& cfg = atlas->ConfigData[input_i];
-        ImFontTempBuildData& tmp = tmp_array[input_i];
+        ImFontBuildSrcData& src_tmp = src_tmp_array[src_i];
+        ImFontConfig& cfg = atlas->ConfigData[src_i];
         IM_ASSERT(cfg.DstFont && (!cfg.DstFont->IsLoaded() || cfg.DstFont->ContainerAtlas == atlas));
 
+        // Find index from cfg.DstFont (we allow the user to set cfg.DstFont. Also it makes casual debugging nicer than when storing indices)
+        src_tmp.DstIndex = -1;
+        for (int output_i = 0; output_i < atlas->Fonts.Size && src_tmp.DstIndex == -1; output_i++)
+            if (cfg.DstFont == atlas->Fonts[output_i])
+                src_tmp.DstIndex = output_i;
+        IM_ASSERT(src_tmp.DstIndex != -1); // cfg.DstFont not pointing within atlas->Fonts[] array?
+        if (src_tmp.DstIndex == -1)
+            return false;
+
+        // Initialize helper structure for font loading and verify that the TTF/OTF data is correct
         const int font_offset = stbtt_GetFontOffsetForIndex((unsigned char*)cfg.FontData, cfg.FontNo);
         IM_ASSERT(font_offset >= 0 && "FontData is incorrect, or FontNo cannot be found.");
-        if (!stbtt_InitFont(&tmp.FontInfo, (unsigned char*)cfg.FontData, font_offset))
-        {
-            atlas->TexWidth = atlas->TexHeight = 0; // Reset output on failure
-            ImGui::MemFree(tmp_array);
+        if (!stbtt_InitFont(&src_tmp.FontInfo, (unsigned char*)cfg.FontData, font_offset))
             return false;
-        }
+
+        // Measure highest codepoints
+        ImFontBuildDstData& dst_tmp = dst_tmp_array[src_tmp.DstIndex];
+        src_tmp.SrcRanges = cfg.GlyphRanges ? cfg.GlyphRanges : atlas->GetGlyphRangesDefault();
+        for (const ImWchar* src_range = src_tmp.SrcRanges; src_range[0] && src_range[1]; src_range += 2)
+            src_tmp.GlyphsHighest = ImMax(src_tmp.GlyphsHighest, (int)src_range[1]);
+        dst_tmp.SrcCount++;
+        dst_tmp.GlyphsHighest = ImMax(dst_tmp.GlyphsHighest, src_tmp.GlyphsHighest);
     }
 
+    // 2. For every requested codepoint, check for their presence in the font data, and handle redundancy or overlaps between source fonts to avoid unused glyphs.
+    int total_glyphs_count = 0;
+    for (int src_i = 0; src_i < src_tmp_array.Size; src_i++)
+    {
+        ImFontBuildSrcData& src_tmp = src_tmp_array[src_i];
+        ImFontBuildDstData& dst_tmp = dst_tmp_array[src_tmp.DstIndex];
+        ImFontConfig& cfg = atlas->ConfigData[src_i];
+        src_tmp.GlyphsSet.Resize(src_tmp.GlyphsHighest);
+        if (dst_tmp.SrcCount > 1 && dst_tmp.GlyphsSet.Storage.empty())
+            dst_tmp.GlyphsSet.Resize(dst_tmp.GlyphsHighest);
+
+        for (const ImWchar* src_range = src_tmp.SrcRanges; src_range[0] && src_range[1]; src_range += 2)
+            for (int codepoint = src_range[0]; codepoint <= src_range[1]; codepoint++)
+            {
+                if (cfg.MergeMode && dst_tmp.GlyphsSet.GetBit(codepoint))   // Don't overwrite existing glyphs. We could make this an option (e.g. MergeOverwrite)
+                    continue;
+                if (!stbtt_FindGlyphIndex(&src_tmp.FontInfo, codepoint))    // It is actually in the font?
+                    continue;
+                
+                // Add to avail set/counters
+                src_tmp.GlyphsCount++;
+                dst_tmp.GlyphsCount++;
+                src_tmp.GlyphsSet.SetBit(codepoint, true);
+                if (dst_tmp.SrcCount > 1)
+                    dst_tmp.GlyphsSet.SetBit(codepoint, true);
+                total_glyphs_count++;
+            }
+    }
+
+    // 3. Unpack our bit map into a flat list (we now have all the Unicode points that we know are requested _and_ available _and_ not overlapping another)
+    for (int src_i = 0; src_i < src_tmp_array.Size; src_i++)
+    {
+        ImFontBuildSrcData& src_tmp = src_tmp_array[src_i];
+        src_tmp.GlyphsList.reserve(src_tmp.GlyphsCount);
+        UnpackBoolVectorToFlatIndexList(&src_tmp.GlyphsSet, &src_tmp.GlyphsList);
+        src_tmp.GlyphsSet.Clear();
+        IM_ASSERT(src_tmp.GlyphsList.Size == src_tmp.GlyphsCount);
+    }
+    for (int dst_i = 0; dst_i < dst_tmp_array.Size; dst_i++)
+        dst_tmp_array[dst_i].GlyphsSet.Clear();
+    dst_tmp_array.clear();
+
     // Allocate packing character data and flag packed characters buffer as non-packed (x0=y0=x1=y1=0)
-    int buf_packedchars_n = 0, buf_rects_n = 0, buf_ranges_n = 0;
-    stbtt_packedchar* buf_packedchars = (stbtt_packedchar*)ImGui::MemAlloc(total_glyphs_count * sizeof(stbtt_packedchar));
-    stbrp_rect* buf_rects = (stbrp_rect*)ImGui::MemAlloc(total_glyphs_count * sizeof(stbrp_rect));
-    stbtt_pack_range* buf_ranges = (stbtt_pack_range*)ImGui::MemAlloc(total_ranges_count * sizeof(stbtt_pack_range));
-    memset(buf_packedchars, 0, total_glyphs_count * sizeof(stbtt_packedchar));
-    memset(buf_rects, 0, total_glyphs_count * sizeof(stbrp_rect));              // Unnecessary but let's clear this for the sake of sanity.
-    memset(buf_ranges, 0, total_ranges_count * sizeof(stbtt_pack_range));
+    // (We technically don't need to zero-clear buf_rects, but let's do it for the sake of sanity)
+    ImVector<stbrp_rect> buf_rects;
+    ImVector<stbtt_packedchar> buf_packedchars;
+    buf_rects.resize(total_glyphs_count);
+    buf_packedchars.resize(total_glyphs_count);
+    memset(buf_rects.Data, 0, (size_t)buf_rects.size_in_bytes());
+    memset(buf_packedchars.Data, 0, (size_t)buf_packedchars.size_in_bytes());
 
-    // First font pass: pack all glyphs (no rendering at this point, we are working with rectangles in an infinitely tall texture at this point)
-    for (int input_i = 0; input_i < atlas->ConfigData.Size; input_i++)
+    // 4. Gather glyphs sizes so we can pack them in our virtual canvas.
+    int total_surface = 0;
+    int buf_rects_out_n = 0;
+    int buf_packedchars_out_n = 0;
+    for (int src_i = 0; src_i < src_tmp_array.Size; src_i++)
     {
-        ImFontConfig& cfg = atlas->ConfigData[input_i];
-        ImFontTempBuildData& tmp = tmp_array[input_i];
+        ImFontBuildSrcData& src_tmp = src_tmp_array[src_i];
+        if (src_tmp.GlyphsCount == 0)
+            continue;
 
-        // Setup ranges
-        int font_glyphs_count = 0;
-        int font_ranges_count = 0;
-        for (const ImWchar* in_range = cfg.GlyphRanges; in_range[0] && in_range[1]; in_range += 2, font_ranges_count++)
-            font_glyphs_count += (in_range[1] - in_range[0]) + 1;
-        tmp.Ranges = buf_ranges + buf_ranges_n;
-        tmp.RangesCount = font_ranges_count;
-        buf_ranges_n += font_ranges_count;
-        for (int i = 0; i < font_ranges_count; i++)
+        src_tmp.Rects = &buf_rects[buf_rects_out_n];
+        src_tmp.PackedChars = &buf_packedchars[buf_packedchars_out_n];
+        buf_rects_out_n += src_tmp.GlyphsCount;
+        buf_packedchars_out_n += src_tmp.GlyphsCount;
+
+        // Convert our ranges in the format stb_truetype wants
+        ImFontConfig& cfg = atlas->ConfigData[src_i];
+        src_tmp.PackRange.font_size = cfg.SizePixels;
+        src_tmp.PackRange.first_unicode_codepoint_in_range = 0;
+        src_tmp.PackRange.array_of_unicode_codepoints = src_tmp.GlyphsList.Data;
+        src_tmp.PackRange.num_chars = src_tmp.GlyphsList.Size;
+        src_tmp.PackRange.chardata_for_range = src_tmp.PackedChars;
+        src_tmp.PackRange.h_oversample = (unsigned char)cfg.OversampleH;
+        src_tmp.PackRange.v_oversample = (unsigned char)cfg.OversampleV;
+
+        // Gather the sizes of all rectangles we will need to pack (this loop is based on stbtt_PackFontRangesGatherRects)
+        const float scale = (cfg.SizePixels > 0) ? stbtt_ScaleForPixelHeight(&src_tmp.FontInfo, cfg.SizePixels) : stbtt_ScaleForMappingEmToPixels(&src_tmp.FontInfo, -cfg.SizePixels);
+        const int padding = atlas->TexGlyphPadding;
+        for (int glyph_i = 0; glyph_i < src_tmp.GlyphsList.Size; glyph_i++)
         {
-            const ImWchar* in_range = &cfg.GlyphRanges[i * 2];
-            stbtt_pack_range& range = tmp.Ranges[i];
-            range.font_size = cfg.SizePixels;
-            range.first_unicode_codepoint_in_range = in_range[0];
-            range.num_chars = (in_range[1] - in_range[0]) + 1;
-            range.chardata_for_range = buf_packedchars + buf_packedchars_n;
-            buf_packedchars_n += range.num_chars;
-        }
-
-        // Gather the sizes of all rectangle we need
-        tmp.Rects = buf_rects + buf_rects_n;
-        tmp.RectsCount = font_glyphs_count;
-        buf_rects_n += font_glyphs_count;
-        stbtt_PackSetOversampling(&spc, cfg.OversampleH, cfg.OversampleV);
-        int n = stbtt_PackFontRangesGatherRects(&spc, &tmp.FontInfo, tmp.Ranges, tmp.RangesCount, tmp.Rects);
-        IM_ASSERT(n == font_glyphs_count);
-
-        // Detect missing glyphs and replace them with a zero-sized box instead of relying on the default glyphs
-        // This allows us merging overlapping icon fonts more easily.
-        int rect_i = 0;
-        for (int range_i = 0; range_i < tmp.RangesCount; range_i++)
-            for (int char_i = 0; char_i < tmp.Ranges[range_i].num_chars; char_i++, rect_i++)
-                if (stbtt_FindGlyphIndex(&tmp.FontInfo, tmp.Ranges[range_i].first_unicode_codepoint_in_range + char_i) == 0)
-                    tmp.Rects[rect_i].w = tmp.Rects[rect_i].h = 0;
-
-        // Pack
-        stbrp_pack_rects((stbrp_context*)spc.pack_info, tmp.Rects, n);
-
-        // Extend texture height
-        // Also mark missing glyphs as non-packed so we don't attempt to render into them
-        for (int i = 0; i < n; i++)
-        {
-            if (tmp.Rects[i].w == 0 && tmp.Rects[i].h == 0)
-                tmp.Rects[i].was_packed = 0;
-            if (tmp.Rects[i].was_packed)
-                atlas->TexHeight = ImMax(atlas->TexHeight, tmp.Rects[i].y + tmp.Rects[i].h);
+            int x0, y0, x1, y1;
+            const int glyph_index_in_font = stbtt_FindGlyphIndex(&src_tmp.FontInfo, src_tmp.GlyphsList[glyph_i]);
+            IM_ASSERT(glyph_index_in_font != 0);
+            stbtt_GetGlyphBitmapBoxSubpixel(&src_tmp.FontInfo, glyph_index_in_font, scale * cfg.OversampleH, scale * cfg.OversampleV, 0, 0, &x0, &y0, &x1, &y1);
+            src_tmp.Rects[glyph_i].w = (stbrp_coord)(x1 - x0 + padding + cfg.OversampleH - 1);
+            src_tmp.Rects[glyph_i].h = (stbrp_coord)(y1 - y0 + padding + cfg.OversampleV - 1);
+            total_surface += src_tmp.Rects[glyph_i].w * src_tmp.Rects[glyph_i].h;
         }
     }
-    IM_ASSERT(buf_rects_n == total_glyphs_count);
-    IM_ASSERT(buf_packedchars_n == total_glyphs_count);
-    IM_ASSERT(buf_ranges_n == total_ranges_count);
 
-    // Create texture
+    // We need a width for the skyline algorithm, any width!
+    // The exact width doesn't really matter much, but some API/GPU have texture size limitations and increasing width can decrease height.
+    // User can override TexDesiredWidth and TexGlyphPadding if they wish, otherwise we use a simple heuristic to select the width based on expected surface.
+    const int surface_sqrt = (int)ImSqrt((float)total_surface) + 1;
+    atlas->TexHeight = 0;
+    if (atlas->TexDesiredWidth > 0)
+        atlas->TexWidth = atlas->TexDesiredWidth;
+    else
+        atlas->TexWidth = (surface_sqrt >= 4096*0.7f) ? 4096 : (surface_sqrt >= 2048*0.7f) ? 2048 : (surface_sqrt >= 1024*0.7f) ? 1024 : 512;
+
+    // 5. Start packing
+    // Pack our extra data rectangles first, so it will be on the upper-left corner of our texture (UV will have small values).
+    const int TEX_HEIGHT_MAX = 1024 * 32;
+    stbtt_pack_context spc = {};
+    stbtt_PackBegin(&spc, NULL, atlas->TexWidth, TEX_HEIGHT_MAX, 0, atlas->TexGlyphPadding, NULL);
+    ImFontAtlasBuildPackCustomRects(atlas, spc.pack_info);
+
+    // 6. Pack each source font. No rendering yet, we are working with rectangles in an infinitely tall texture at this point.
+    for (int src_i = 0; src_i < src_tmp_array.Size; src_i++)
+    {
+        ImFontBuildSrcData& src_tmp = src_tmp_array[src_i];
+        if (src_tmp.GlyphsCount == 0)
+            continue;
+
+        stbrp_pack_rects((stbrp_context*)spc.pack_info, src_tmp.Rects, src_tmp.GlyphsCount);
+
+        // Extend texture height and mark missing glyphs as non-packed so we won't render them.
+        // FIXME: We are not handling packing failure here (would happen if we got off TEX_HEIGHT_MAX or if a single if larger than TexWidth?)
+        for (int glyph_i = 0; glyph_i < src_tmp.GlyphsCount; glyph_i++)
+            if (src_tmp.Rects[glyph_i].was_packed)
+                atlas->TexHeight = ImMax(atlas->TexHeight, src_tmp.Rects[glyph_i].y + src_tmp.Rects[glyph_i].h);
+    }
+
+    // 7. Allocate texture
     atlas->TexHeight = (atlas->Flags & ImFontAtlasFlags_NoPowerOfTwoHeight) ? (atlas->TexHeight + 1) : ImUpperPowerOfTwo(atlas->TexHeight);
     atlas->TexUvScale = ImVec2(1.0f / atlas->TexWidth, 1.0f / atlas->TexHeight);
     atlas->TexPixelsAlpha8 = (unsigned char*)ImGui::MemAlloc(atlas->TexWidth * atlas->TexHeight);
@@ -1873,41 +1954,46 @@ bool    ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas)
     spc.pixels = atlas->TexPixelsAlpha8;
     spc.height = atlas->TexHeight;
 
-    // Second pass: render font characters
-    for (int input_i = 0; input_i < atlas->ConfigData.Size; input_i++)
+    // 8. Render/rasterize font characters into the texture
+    for (int src_i = 0; src_i < src_tmp_array.Size; src_i++)
     {
-        ImFontConfig& cfg = atlas->ConfigData[input_i];
-        ImFontTempBuildData& tmp = tmp_array[input_i];
-        stbtt_PackSetOversampling(&spc, cfg.OversampleH, cfg.OversampleV);
-        stbtt_PackFontRangesRenderIntoRects(&spc, &tmp.FontInfo, tmp.Ranges, tmp.RangesCount, tmp.Rects);
+        ImFontConfig& cfg = atlas->ConfigData[src_i];
+        ImFontBuildSrcData& src_tmp = src_tmp_array[src_i];
+        if (src_tmp.GlyphsCount == 0)
+            continue;
+
+        stbtt_PackFontRangesRenderIntoRects(&spc, &src_tmp.FontInfo, &src_tmp.PackRange, 1, src_tmp.Rects);
+
+        // Apply multiply operator
         if (cfg.RasterizerMultiply != 1.0f)
         {
             unsigned char multiply_table[256];
             ImFontAtlasBuildMultiplyCalcLookupTable(multiply_table, cfg.RasterizerMultiply);
-            for (const stbrp_rect* r = tmp.Rects; r != tmp.Rects + tmp.RectsCount; r++)
+            stbrp_rect* r = &src_tmp.Rects[0];
+            for (int glyph_i = 0; glyph_i < src_tmp.GlyphsCount; glyph_i++, r++)
                 if (r->was_packed)
-                    ImFontAtlasBuildMultiplyRectAlpha8(multiply_table, spc.pixels, r->x, r->y, r->w, r->h, spc.stride_in_bytes);
+                    ImFontAtlasBuildMultiplyRectAlpha8(multiply_table, atlas->TexPixelsAlpha8, r->x, r->y, r->w, r->h, atlas->TexWidth * 1);
         }
-        tmp.Rects = NULL;
+        src_tmp.Rects = NULL;
     }
 
     // End packing
     stbtt_PackEnd(&spc);
-    ImGui::MemFree(buf_rects);
-    buf_rects = NULL;
+    buf_rects.clear();
 
-    // Third pass: setup ImFont and glyphs for runtime
-    for (int input_i = 0; input_i < atlas->ConfigData.Size; input_i++)
+    // 9. Setup ImFont and glyphs for runtime
+    for (int src_i = 0; src_i < src_tmp_array.Size; src_i++)
     {
-        ImFontConfig& cfg = atlas->ConfigData[input_i];
-        ImFontTempBuildData& tmp = tmp_array[input_i];
-        ImFont* dst_font = cfg.DstFont; // We can have multiple input fonts writing into a same destination font (when using MergeMode=true)
-        if (cfg.MergeMode)
-            dst_font->BuildLookupTable();
+        ImFontBuildSrcData& src_tmp = src_tmp_array[src_i];
+        if (src_tmp.GlyphsCount == 0)
+            continue;
 
-        const float font_scale = stbtt_ScaleForPixelHeight(&tmp.FontInfo, cfg.SizePixels);
+        ImFontConfig& cfg = atlas->ConfigData[src_i];
+        ImFont* dst_font = cfg.DstFont; // We can have multiple input fonts writing into a same destination font (when using MergeMode=true)
+
+        const float font_scale = stbtt_ScaleForPixelHeight(&src_tmp.FontInfo, cfg.SizePixels);
         int unscaled_ascent, unscaled_descent, unscaled_line_gap;
-        stbtt_GetFontVMetrics(&tmp.FontInfo, &unscaled_ascent, &unscaled_descent, &unscaled_line_gap);
+        stbtt_GetFontVMetrics(&src_tmp.FontInfo, &unscaled_ascent, &unscaled_descent, &unscaled_line_gap);
 
         const float ascent = ImFloor(unscaled_ascent * font_scale + ((unscaled_ascent > 0.0f) ? +1 : -1));
         const float descent = ImFloor(unscaled_descent * font_scale + ((unscaled_descent > 0.0f) ? +1 : -1));
@@ -1915,40 +2001,30 @@ bool    ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas)
         const float font_off_x = cfg.GlyphOffset.x;
         const float font_off_y = cfg.GlyphOffset.y + (float)(int)(dst_font->Ascent + 0.5f);
 
-        for (int i = 0; i < tmp.RangesCount; i++)
+        for (int glyph_i = 0; glyph_i < src_tmp.GlyphsCount; glyph_i++)
         {
-            stbtt_pack_range& range = tmp.Ranges[i];
-            for (int char_idx = 0; char_idx < range.num_chars; char_idx += 1)
-            {
-                const stbtt_packedchar& pc = range.chardata_for_range[char_idx];
-                if (!pc.x0 && !pc.x1 && !pc.y0 && !pc.y1)
-                    continue;
+            const int codepoint = src_tmp.GlyphsList[glyph_i];
+            const stbtt_packedchar& pc = src_tmp.PackedChars[glyph_i];
 
-                const int codepoint = range.first_unicode_codepoint_in_range + char_idx;
-                if (cfg.MergeMode && dst_font->FindGlyphNoFallback((ImWchar)codepoint))
-                    continue;
-
-                float char_advance_x_org = pc.xadvance;
-                float char_advance_x_mod = ImClamp(char_advance_x_org, cfg.GlyphMinAdvanceX, cfg.GlyphMaxAdvanceX);
-                float char_off_x = font_off_x;
-                if (char_advance_x_org != char_advance_x_mod)
-                    char_off_x += cfg.PixelSnapH ? (float)(int)((char_advance_x_mod - char_advance_x_org) * 0.5f) : (char_advance_x_mod - char_advance_x_org) * 0.5f;
-
-                stbtt_aligned_quad q;
-                float dummy_x = 0.0f, dummy_y = 0.0f;
-                stbtt_GetPackedQuad(range.chardata_for_range, atlas->TexWidth, atlas->TexHeight, char_idx, &dummy_x, &dummy_y, &q, 0);
-                dst_font->AddGlyph((ImWchar)codepoint, q.x0 + char_off_x, q.y0 + font_off_y, q.x1 + char_off_x, q.y1 + font_off_y, q.s0, q.t0, q.s1, q.t1, char_advance_x_mod);
-            }
+            const float char_advance_x_org = pc.xadvance;
+            const float char_advance_x_mod = ImClamp(char_advance_x_org, cfg.GlyphMinAdvanceX, cfg.GlyphMaxAdvanceX);
+            float char_off_x = font_off_x;
+            if (char_advance_x_org != char_advance_x_mod)
+                char_off_x += cfg.PixelSnapH ? (float)(int)((char_advance_x_mod - char_advance_x_org) * 0.5f) : (char_advance_x_mod - char_advance_x_org) * 0.5f;
+            
+            // Register glyph
+            stbtt_aligned_quad q;
+            float dummy_x = 0.0f, dummy_y = 0.0f;
+            stbtt_GetPackedQuad(src_tmp.PackedChars, atlas->TexWidth, atlas->TexHeight, glyph_i, &dummy_x, &dummy_y, &q, 0);
+            dst_font->AddGlyph((ImWchar)codepoint, q.x0 + char_off_x, q.y0 + font_off_y, q.x1 + char_off_x, q.y1 + font_off_y, q.s0, q.t0, q.s1, q.t1, char_advance_x_mod);
         }
     }
 
-    // Cleanup temporaries
-    ImGui::MemFree(buf_packedchars);
-    ImGui::MemFree(buf_ranges);
-    ImGui::MemFree(tmp_array);
+    // Cleanup temporary (ImVector doesn't honor destructor)
+    for (int src_i = 0; src_i < src_tmp_array.Size; src_i++)
+        src_tmp_array[src_i].~ImFontBuildSrcData();
 
     ImFontAtlasBuildFinish(atlas);
-
     return true;
 }
 
@@ -1976,16 +2052,16 @@ void ImFontAtlasBuildSetupFont(ImFontAtlas* atlas, ImFont* font, ImFontConfig* f
     font->ConfigDataCount++;
 }
 
-void ImFontAtlasBuildPackCustomRects(ImFontAtlas* atlas, void* pack_context_opaque)
+void ImFontAtlasBuildPackCustomRects(ImFontAtlas* atlas, void* stbrp_context_opaque)
 {
-    stbrp_context* pack_context = (stbrp_context*)pack_context_opaque;
+    stbrp_context* pack_context = (stbrp_context*)stbrp_context_opaque;
 
     ImVector<ImFontAtlas::CustomRect>& user_rects = atlas->CustomRects;
     IM_ASSERT(user_rects.Size >= 1); // We expect at least the default custom rects to be registered, else something went wrong.
 
     ImVector<stbrp_rect> pack_rects;
     pack_rects.resize(user_rects.Size);
-    memset(pack_rects.Data, 0, sizeof(stbrp_rect) * user_rects.Size);
+    memset(pack_rects.Data, 0, (size_t)pack_rects.size_in_bytes());
     for (int i = 0; i < user_rects.Size; i++)
     {
         pack_rects[i].w = user_rects[i].Width;

imgui_internal.h

@@ -1452,7 +1452,7 @@ namespace ImGui
 IMGUI_API bool              ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas);
 IMGUI_API void              ImFontAtlasBuildRegisterDefaultCustomRects(ImFontAtlas* atlas);
 IMGUI_API void              ImFontAtlasBuildSetupFont(ImFontAtlas* atlas, ImFont* font, ImFontConfig* font_config, float ascent, float descent);
-IMGUI_API void              ImFontAtlasBuildPackCustomRects(ImFontAtlas* atlas, void* spc);
+IMGUI_API void              ImFontAtlasBuildPackCustomRects(ImFontAtlas* atlas, void* stbrp_context_opaque);
 IMGUI_API void              ImFontAtlasBuildFinish(ImFontAtlas* atlas);
 IMGUI_API void              ImFontAtlasBuildMultiplyCalcLookupTable(unsigned char out_table[256], float in_multiply_factor);
 IMGUI_API void              ImFontAtlasBuildMultiplyRectAlpha8(const unsigned char table[256], unsigned char* pixels, int x, int y, int w, int h, int stride);