ImDrawList: Added ImDrawCmd::VtxOffset value to support large meshes (64k+ vertices) using 16-bits indices. To enable the feature, the renderer back-end needs to set 'io.BackendFlags |= ImGuiBackendFlags_HasVtxOffset' and honor the ImDrawCmd::VtxOffset field. Otherwise the value will always be zero. This has the advantage of preserving smaller index buffers and allowing to execute on hardware that do not support 32-bits indices.

ImDrawList: Added ImDrawCmd::IdxOffset value, equivalent to summing element count for each draw command. This is provided for convenience and consistency with VtxOffset. (#2591)

docs/CHANGELOG.txt

@@ -56,6 +56,13 @@ Other Changes:
 - Style: Added style.WindowMenuButtonPosition (left/right, defaults to ImGuiDir_Left) to move the
   collapsing/docking button to the other side of the title bar.
 - Style: Made window close button cross slightly smaller.
+- ImDrawList: Added ImDrawCmd::VtxOffset value to support large meshes (64k+ vertices) using 16-bits indices.
+  To enable the feature, the renderer back-end needs to set 'io.BackendFlags |= ImGuiBackendFlags_HasVtxOffset'
+  and honor the ImDrawCmd::VtxOffset field. Otherwise the value will always be zero.
+  This has the advantage of preserving smaller index buffers and allowing to execute on hardware that do not
+  support 32-bits indices. Most examples back-ends have been modified to support the VtxOffset field.
+- ImDrawList: Added ImDrawCmd::IdxOffset value, equivalent to summing element count for each draw command.
+  This is provided for convenience and consistency with VtxOffset.
 - ImFontAtlas: FreeType: Added RasterizerFlags::Monochrome flag to disable font anti-aliasing. (#2545)
   Combine with RasterizerFlags::MonoHinting for best results.
 - ImFontGlyphRangesBuilder: Fixed unnecessarily over-sized buffer, which incidentally was also not

imconfig.h

@@ -62,7 +62,10 @@
         operator MyVec4() const { return MyVec4(x,y,z,w); }
 */
 
-//---- Use 32-bit vertex indices (default is 16-bit) to allow meshes with more than 64K vertices. Render function needs to support it.
+//---- Using 32-bits vertex indices (default is 16-bits) is one way to allow large meshes with more than 64K vertices. 
+// Your renderer back-end will need to support it (most example renderer back-ends support both 16/32-bits indices).
+// Another way to allow large meshes while keeping 16-bits indices is to handle ImDrawCmd::VtxOffset in your renderer. 
+// Read about ImGuiBackendFlags_HasVtxOffset for details.
 //#define ImDrawIdx unsigned int
 
 //---- Tip: You can add extra functions within the ImGui:: namespace, here or in your own headers files.

imgui.cpp

@@ -3502,7 +3502,13 @@ void ImGui::NewFrame()
     IM_ASSERT(g.Font->IsLoaded());
     g.DrawListSharedData.ClipRectFullscreen = ImVec4(0.0f, 0.0f, g.IO.DisplaySize.x, g.IO.DisplaySize.y);
     g.DrawListSharedData.CurveTessellationTol = g.Style.CurveTessellationTol;
-    g.DrawListSharedData.InitialFlags = (g.Style.AntiAliasedLines ? ImDrawListFlags_AntiAliasedLines : 0) | (g.Style.AntiAliasedFill ? ImDrawListFlags_AntiAliasedFill : 0);
+    g.DrawListSharedData.InitialFlags = ImDrawListFlags_None;
+    if (g.Style.AntiAliasedLines)
+        g.DrawListSharedData.InitialFlags |= ImDrawListFlags_AntiAliasedLines;
+    if (g.Style.AntiAliasedFill)
+        g.DrawListSharedData.InitialFlags |= ImDrawListFlags_AntiAliasedFill;
+    if (g.IO.BackendFlags & ImGuiBackendFlags_HasVtxOffset)
+        g.DrawListSharedData.InitialFlags |= ImDrawListFlags_AllowVtxOffset;
 
     g.BackgroundDrawList.Clear();
     g.BackgroundDrawList.PushTextureID(g.IO.Fonts->TexID);
@@ -3774,19 +3780,28 @@ static void AddDrawListToDrawData(ImVector<ImDrawList*>* out_list, ImDrawList* d
             return;
     }
 
-    // Draw list sanity check. Detect mismatch between PrimReserve() calls and incrementing _VtxCurrentIdx, _VtxWritePtr etc. May trigger for you if you are using PrimXXX functions incorrectly.
+    // Draw list sanity check. Detect mismatch between PrimReserve() calls and incrementing _VtxCurrentIdx, _VtxWritePtr etc. 
+    // May trigger for you if you are using PrimXXX functions incorrectly.
     IM_ASSERT(draw_list->VtxBuffer.Size == 0 || draw_list->_VtxWritePtr == draw_list->VtxBuffer.Data + draw_list->VtxBuffer.Size);
     IM_ASSERT(draw_list->IdxBuffer.Size == 0 || draw_list->_IdxWritePtr == draw_list->IdxBuffer.Data + draw_list->IdxBuffer.Size);
+    if (!(draw_list->Flags & ImDrawListFlags_AllowVtxOffset))
         IM_ASSERT((int)draw_list->_VtxCurrentIdx == draw_list->VtxBuffer.Size);
 
     // Check that draw_list doesn't use more vertices than indexable (default ImDrawIdx = unsigned short = 2 bytes = 64K vertices per ImDrawList = per window)
     // If this assert triggers because you are drawing lots of stuff manually:
-    // A) Make sure you are coarse clipping, because ImDrawList let all your vertices pass. You can use the Metrics window to inspect draw list contents.
+    // - First, make sure you are coarse clipping yourself and not trying to draw many things outside visible bounds. 
-    // B) If you need/want meshes with more than 64K vertices, uncomment the '#define ImDrawIdx unsigned int' line in imconfig.h to set the index size to 4 bytes.
+    //   Be mindful that the ImDrawList API doesn't filter vertices. Use the Metrics window to inspect draw list contents.
-    //    You'll need to handle the 4-bytes indices to your renderer. For example, the OpenGL example code detect index size at compile-time by doing:
+    // - If you want large meshes with more than 64K vertices, you can either:
+    //   (A) Handle the ImDrawCmd::VtxOffset value in your renderer back-end, and set 'io.BackendFlags |= ImGuiBackendFlags_HasVtxOffset'.
+    //       Most example back-ends already support this from 1.71. Pre-1.71 back-ends won't.
+    //       Some graphics API such as GL ES 1/2 don't have a way to offset the starting vertex so it is not supported for them.
+    //   (B) Or handle 32-bits indices in your renderer back-end, and uncomment '#define ImDrawIdx unsigned int' line in imconfig.h.
+    //       Most example back-ends already support this. For example, the OpenGL example code detect index size at compile-time:
     //         glDrawElements(GL_TRIANGLES, (GLsizei)pcmd->ElemCount, sizeof(ImDrawIdx) == 2 ? GL_UNSIGNED_SHORT : GL_UNSIGNED_INT, idx_buffer_offset);
-    //    Your own engine or render API may use different parameters or function calls to specify index sizes. 2 and 4 bytes indices are generally supported by most API.
+    //       Your own engine or render API may use different parameters or function calls to specify index sizes. 
-    // C) If for some reason you cannot use 4 bytes indices or don't want to, a workaround is to call BeginChild()/EndChild() before reaching the 64K limit to split your draw commands in multiple draw lists.
+    //       2 and 4 bytes indices are generally supported by most graphics API.
+    // - If for some reason neither of those solutions works for you, a workaround is to call BeginChild()/EndChild() before reaching 
+    //   the 64K limit to split your draw commands in multiple draw lists.
     if (sizeof(ImDrawIdx) == 2)
         IM_ASSERT(draw_list->_VtxCurrentIdx < (1 << 16) && "Too many vertices in ImDrawList using 16-bit indices. Read comment above");
 
@@ -9790,9 +9805,10 @@ void ImGui::ShowMetricsWindow(bool* p_open)
                     continue;
                 }
                 ImDrawIdx* idx_buffer = (draw_list->IdxBuffer.Size > 0) ? draw_list->IdxBuffer.Data : NULL;
-                bool pcmd_node_open = ImGui::TreeNode((void*)(pcmd - draw_list->CmdBuffer.begin()), 
+                char buf[300];
-                    "Draw %4d %s vtx, tex 0x%p, clip_rect (%4.0f,%4.0f)-(%4.0f,%4.0f)", 
+                ImFormatString(buf, IM_ARRAYSIZE(buf), "Draw %4d triangles, tex 0x%p, clip_rect (%4.0f,%4.0f)-(%4.0f,%4.0f)",
-                    pcmd->ElemCount, draw_list->IdxBuffer.Size > 0 ? "indexed" : "non-indexed", (void*)(intptr_t)pcmd->TextureId, pcmd->ClipRect.x, pcmd->ClipRect.y, pcmd->ClipRect.z, pcmd->ClipRect.w);
+                    pcmd->ElemCount/3, (void*)(intptr_t)pcmd->TextureId, pcmd->ClipRect.x, pcmd->ClipRect.y, pcmd->ClipRect.z, pcmd->ClipRect.w);
+                bool pcmd_node_open = ImGui::TreeNode((void*)(pcmd - draw_list->CmdBuffer.begin()), "%s", buf);
                 if (show_drawcmd_clip_rects && fg_draw_list && ImGui::IsItemHovered())
                 {
                     ImRect clip_rect = pcmd->ClipRect;
@@ -9806,11 +9822,11 @@ void ImGui::ShowMetricsWindow(bool* p_open)
                     continue;
 
                 // Display individual triangles/vertices. Hover on to get the corresponding triangle highlighted.
+                ImGui::Text("ElemCount: %d, ElemCount/3: %d, VtxOffset: +%d, IdxOffset: +%d", pcmd->ElemCount, pcmd->ElemCount/3, pcmd->VtxOffset, pcmd->IdxOffset);
                 ImGuiListClipper clipper(pcmd->ElemCount/3); // Manually coarse clip our print out of individual vertices to save CPU, only items that may be visible.
                 while (clipper.Step())
                     for (int prim = clipper.DisplayStart, idx_i = elem_offset + clipper.DisplayStart*3; prim < clipper.DisplayEnd; prim++)
                     {
-                        char buf[300];
                         char *buf_p = buf, *buf_end = buf + IM_ARRAYSIZE(buf);
                         ImVec2 triangles_pos[3];
                         for (int n = 0; n < 3; n++, idx_i++)
@@ -9819,7 +9835,7 @@ void ImGui::ShowMetricsWindow(bool* p_open)
                             ImDrawVert& v = draw_list->VtxBuffer[vtx_i];
                             triangles_pos[n] = v.pos;
                             buf_p += ImFormatString(buf_p, buf_end - buf_p, "%s %04d: pos (%8.2f,%8.2f), uv (%.6f,%.6f), col %08X\n",
-                                (n == 0) ? "idx" : "   ", idx_i, v.pos.x, v.pos.y, v.uv.x, v.uv.y, v.col);
+                                (n == 0) ? "elem" : "    ", idx_i, v.pos.x, v.pos.y, v.uv.x, v.uv.y, v.col);
                         }
                         ImGui::Selectable(buf, false);
                         if (fg_draw_list && ImGui::IsItemHovered())

imgui.h

@@ -47,7 +47,7 @@ Index of this file:
 // Version
 // (Integer encoded as XYYZZ for use in #if preprocessor conditionals. Work in progress versions typically starts at XYY99 then bounce up to XYY00, XYY01 etc. when release tagging happens)
 #define IMGUI_VERSION               "1.71 WIP"
-#define IMGUI_VERSION_NUM           17002
+#define IMGUI_VERSION_NUM           17003
 #define IMGUI_CHECKVERSION()        ImGui::DebugCheckVersionAndDataLayout(IMGUI_VERSION, sizeof(ImGuiIO), sizeof(ImGuiStyle), sizeof(ImVec2), sizeof(ImVec4), sizeof(ImDrawVert), sizeof(ImDrawIdx))
 
 // Define attributes of all API symbols declarations (e.g. for DLL under Windows)
@@ -1003,9 +1003,10 @@ enum ImGuiConfigFlags_
 enum ImGuiBackendFlags_
 {
     ImGuiBackendFlags_None                  = 0,
-    ImGuiBackendFlags_HasGamepad            = 1 << 0,   // Back-end supports gamepad and currently has one connected.
+    ImGuiBackendFlags_HasGamepad            = 1 << 0,   // Platform Back-end supports gamepad and currently has one connected.
-    ImGuiBackendFlags_HasMouseCursors       = 1 << 1,   // Back-end supports honoring GetMouseCursor() value to change the OS cursor shape.
+    ImGuiBackendFlags_HasMouseCursors       = 1 << 1,   // Platform Back-end supports honoring GetMouseCursor() value to change the OS cursor shape.
-    ImGuiBackendFlags_HasSetMousePos        = 1 << 2    // Back-end supports io.WantSetMousePos requests to reposition the OS mouse position (only used if ImGuiConfigFlags_NavEnableSetMousePos is set).
+    ImGuiBackendFlags_HasSetMousePos        = 1 << 2,   // Platform Back-end supports io.WantSetMousePos requests to reposition the OS mouse position (only used if ImGuiConfigFlags_NavEnableSetMousePos is set).
+    ImGuiBackendFlags_HasVtxOffset          = 1 << 3    // Renderer Back-end supports ImDrawCmd::VtxOffset. This enables output of large meshes (64K+ vertices) while still using 16-bits indices.
 };
 
 // Enumeration for PushStyleColor() / PopStyleColor()
@@ -1771,25 +1772,31 @@ struct ImColor
 // The expected behavior from your rendering function is 'if (cmd.UserCallback != NULL) { cmd.UserCallback(parent_list, cmd); } else { RenderTriangles() }'
 typedef void (*ImDrawCallback)(const ImDrawList* parent_list, const ImDrawCmd* cmd);
 
-// Special Draw Callback value to request renderer back-end to reset the graphics/render state.
+// Special Draw callback value to request renderer back-end to reset the graphics/render state.
 // The renderer back-end needs to handle this special value, otherwise it will crash trying to call a function at this address.
 // This is useful for example if you submitted callbacks which you know have altered the render state and you want it to be restored.
 // It is not done by default because they are many perfectly useful way of altering render state for imgui contents (e.g. changing shader/blending settings before an Image call).
 #define ImDrawCallback_ResetRenderState     (ImDrawCallback)(-1)
 
 // Typically, 1 command = 1 GPU draw call (unless command is a callback)
+// Pre 1.71 back-ends will typically ignore the VtxOffset/IdxOffset fields. When (io.BackendFlags & ImGuiBackendFlags_HasVtxOffset) 
+// is enabled, those fields allow us to render meshes larger than 64K vertices while keeping 16-bits indices.
 struct ImDrawCmd
 {
     unsigned int    ElemCount;              // Number of indices (multiple of 3) to be rendered as triangles. Vertices are stored in the callee ImDrawList's vtx_buffer[] array, indices in idx_buffer[].
     ImVec4          ClipRect;               // Clipping rectangle (x1, y1, x2, y2). Subtract ImDrawData->DisplayPos to get clipping rectangle in "viewport" coordinates
     ImTextureID     TextureId;              // User-provided texture ID. Set by user in ImfontAtlas::SetTexID() for fonts or passed to Image*() functions. Ignore if never using images or multiple fonts atlas.
+    unsigned int    VtxOffset;              // Start offset in vertex buffer. Pre-1.71 or without ImGuiBackendFlags_HasVtxOffset: always 0. With ImGuiBackendFlags_HasVtxOffset: may be >0 to support meshes larger than 64K vertices with 16-bits indices.
+    unsigned int    IdxOffset;              // Start offset in index buffer. Always equal to sum of ElemCount drawn so far.
     ImDrawCallback  UserCallback;           // If != NULL, call the function instead of rendering the vertices. clip_rect and texture_id will be set normally.
     void*           UserCallbackData;       // The draw callback code can access this.
 
-    ImDrawCmd() { ElemCount = 0; ClipRect.x = ClipRect.y = ClipRect.z = ClipRect.w = 0.0f; TextureId = (ImTextureID)NULL; UserCallback = NULL; UserCallbackData = NULL; }
+    ImDrawCmd() { ElemCount = 0; ClipRect.x = ClipRect.y = ClipRect.z = ClipRect.w = 0.0f; TextureId = (ImTextureID)NULL; VtxOffset = IdxOffset = 0;  UserCallback = NULL; UserCallbackData = NULL; }
 };
 
-// Vertex index (override with '#define ImDrawIdx unsigned int' in imconfig.h)
+// Vertex index 
+// (to allow large meshes with 16-bits indices: set 'io.BackendFlags |= ImGuiBackendFlags_HasVtxOffset' and handle ImDrawCmd::VtxOffset in the renderer back-end)
+// (to use 32-bits indices: override with '#define ImDrawIdx unsigned int' in imconfig.h)
 #ifndef ImDrawIdx
 typedef unsigned short ImDrawIdx;
 #endif
@@ -1835,7 +1842,8 @@ enum ImDrawListFlags_
 {
     ImDrawListFlags_None             = 0,
     ImDrawListFlags_AntiAliasedLines = 1 << 0,  // Lines are anti-aliased (*2 the number of triangles for 1.0f wide line, otherwise *3 the number of triangles)
-    ImDrawListFlags_AntiAliasedFill  = 1 << 1   // Filled shapes have anti-aliased edges (*2 the number of vertices)
+    ImDrawListFlags_AntiAliasedFill  = 1 << 1,  // Filled shapes have anti-aliased edges (*2 the number of vertices)
+    ImDrawListFlags_AllowVtxOffset   = 1 << 2   // Can emit 'VtxOffset > 0' to allow large meshes. Set when 'io.BackendFlags & ImGuiBackendFlags_HasVtxOffset' is enabled.
 };
 
 // Draw command list
@@ -1855,7 +1863,8 @@ struct ImDrawList
     // [Internal, used while building lists]
     const ImDrawListSharedData* _Data;          // Pointer to shared draw data (you can use ImGui::GetDrawListSharedData() to get the one from current ImGui context)
     const char*             _OwnerName;         // Pointer to owner window's name for debugging
-    unsigned int            _VtxCurrentIdx;     // [Internal] == VtxBuffer.Size
+    unsigned int            _VtxCurrentOffset;  // [Internal] Always 0 unless 'Flags & ImDrawListFlags_AllowVtxOffset'.
+    unsigned int            _VtxCurrentIdx;     // [Internal] Generally == VtxBuffer.Size unless we are past 64K vertices, in which case this gets reset to 0.
     ImDrawVert*             _VtxWritePtr;       // [Internal] point within VtxBuffer.Data after each add command (to avoid using the ImVector<> operators too much)
     ImDrawIdx*              _IdxWritePtr;       // [Internal] point within IdxBuffer.Data after each add command (to avoid using the ImVector<> operators too much)
     ImVector<ImVec4>        _ClipRectStack;     // [Internal]

imgui_demo.cpp

@@ -350,6 +350,7 @@ void ImGui::ShowDemoWindow(bool* p_open)
             ImGui::CheckboxFlags("io.BackendFlags: HasGamepad", (unsigned int *)&backend_flags, ImGuiBackendFlags_HasGamepad);
             ImGui::CheckboxFlags("io.BackendFlags: HasMouseCursors", (unsigned int *)&backend_flags, ImGuiBackendFlags_HasMouseCursors);
             ImGui::CheckboxFlags("io.BackendFlags: HasSetMousePos", (unsigned int *)&backend_flags, ImGuiBackendFlags_HasSetMousePos);
+            ImGui::CheckboxFlags("io.BackendFlags: HasVtxOffset", (unsigned int *)&backend_flags, ImGuiBackendFlags_HasVtxOffset);
             ImGui::TreePop();
             ImGui::Separator();
         }
@@ -2862,6 +2863,7 @@ void ImGui::ShowAboutWindow(bool* p_open)
         if (io.BackendFlags & ImGuiBackendFlags_HasGamepad)             ImGui::Text(" HasGamepad");
         if (io.BackendFlags & ImGuiBackendFlags_HasMouseCursors)        ImGui::Text(" HasMouseCursors");
         if (io.BackendFlags & ImGuiBackendFlags_HasSetMousePos)         ImGui::Text(" HasSetMousePos");
+        if (io.BackendFlags & ImGuiBackendFlags_HasVtxOffset)           ImGui::Text(" HasVtxOffset");
         ImGui::Separator();
         ImGui::Text("io.Fonts: %d fonts, Flags: 0x%08X, TexSize: %d,%d", io.Fonts->Fonts.Size, io.Fonts->Flags, io.Fonts->TexWidth, io.Fonts->TexHeight);
         ImGui::Text("io.DisplaySize: %.2f,%.2f", io.DisplaySize.x, io.DisplaySize.y);

imgui_draw.cpp

@@ -364,6 +364,7 @@ void ImDrawList::Clear()
     IdxBuffer.resize(0);
     VtxBuffer.resize(0);
     Flags = _Data->InitialFlags;
+    _VtxCurrentOffset = 0;
     _VtxCurrentIdx = 0;
     _VtxWritePtr = NULL;
     _IdxWritePtr = NULL;
@@ -416,6 +417,8 @@ void ImDrawList::AddDrawCmd()
     ImDrawCmd draw_cmd;
     draw_cmd.ClipRect = GetCurrentClipRect();
     draw_cmd.TextureId = GetCurrentTextureId();
+    draw_cmd.VtxOffset = _VtxCurrentOffset;
+    draw_cmd.IdxOffset = IdxBuffer.Size;
 
     IM_ASSERT(draw_cmd.ClipRect.x <= draw_cmd.ClipRect.z && draw_cmd.ClipRect.y <= draw_cmd.ClipRect.w);
     CmdBuffer.push_back(draw_cmd);
@@ -604,6 +607,14 @@ void ImDrawList::ChannelsSetCurrent(int idx)
 // NB: this can be called with negative count for removing primitives (as long as the result does not underflow)
 void ImDrawList::PrimReserve(int idx_count, int vtx_count)
 {
+    // Large mesh support (when enabled)
+    if (sizeof(ImDrawIdx) == 2 && (_VtxCurrentIdx + vtx_count >= (1 << 16)) && (Flags & ImDrawListFlags_AllowVtxOffset))
+    {
+        _VtxCurrentOffset = VtxBuffer.Size;
+        _VtxCurrentIdx = 0;
+        AddDrawCmd();
+    }
+
     ImDrawCmd& draw_cmd = CmdBuffer.Data[CmdBuffer.Size-1];
     draw_cmd.ElemCount += idx_count;
 
@@ -2950,7 +2961,7 @@ void ImFont::RenderText(ImDrawList* draw_list, float size, ImVec2 pos, ImU32 col
     draw_list->CmdBuffer[draw_list->CmdBuffer.Size-1].ElemCount -= (idx_expected_size - draw_list->IdxBuffer.Size);
     draw_list->_VtxWritePtr = vtx_write;
     draw_list->_IdxWritePtr = idx_write;
-    draw_list->_VtxCurrentIdx = (unsigned int)draw_list->VtxBuffer.Size;
+    draw_list->_VtxCurrentIdx = vtx_current_idx;
 }
 
 //-----------------------------------------------------------------------------