ImDrawListSplitter::Merge - corrupted data in buffer (64k+ vertices) with 16-bits indices #3129

draw_list_splitter_bug_repro.cpp

static bool randomize = false;
static int vtx_count = 64790;
static int map[100000 / 4];
static int rev_map[100000 / 4];

# include <random>
ImDrawList* ShowBackendCheckerWindow(bool* p_open, int* p_vertex_delta)
{
    ImDrawList* draw_list = nullptr;

    int dummy_vertex_delta = 0;
    int& vertex_delta = p_vertex_delta ? *p_vertex_delta : dummy_vertex_delta;
    vertex_delta = 0;

    if (!ImGui::Begin("Dear ImGui Backend Checker##1", p_open))
    {
        ImGui::End();
        return draw_list;
    }

    ImGuiIO& io = ImGui::GetIO();
    ImGui::Text("Dear ImGui %s Backend Checker", ImGui::GetVersion());
    ImGui::Text("io.BackendPlatformName: %s", io.BackendPlatformName ? io.BackendPlatformName : "NULL");
    ImGui::Text("io.BackendRendererName: %s", io.BackendRendererName ? io.BackendRendererName : "NULL");
    ImGui::Separator();

    if (ImGui::TreeNodeEx("0001: Renderer: Large Mesh Support", ImGuiTreeNodeFlags_DefaultOpen))
    {
        draw_list = ImGui::GetWindowDrawList();
        {
            auto* draw_list = ImGui::GetWindowDrawList();

            int start_vertex_count = draw_list->VtxBuffer.Size;

            ImDrawListSplitter splitter;
            splitter.Split(draw_list, 100000 / 4);

            ImVec2 p = ImGui::GetCursorScreenPos();
            for (int n = 0; n < vtx_count / 4; n++)
            {
                splitter.SetCurrentChannel(draw_list, map[n]);

                float off_x = (float)(n % 100) * 3.0f;
                float off_y = (float)(n % 100) * 1.0f;
                ImU32 col = IM_COL32(((n * 17) & 255), ((n * 59) & 255), ((n * 83) & 255), 255);
                draw_list->AddRectFilled(ImVec2(p.x + off_x, p.y + off_y), ImVec2(p.x + off_x + 50, p.y + off_y + 50), col);
            }

            splitter.Merge(draw_list);

            int end_vertex_count = draw_list->VtxBuffer.Size;

            vertex_delta = end_vertex_count - start_vertex_count;

            ImGui::Dummy(ImVec2(300 + 50, 100 + 50));
            ImGui::Text("VtxBuffer.Size = %d", draw_list->VtxBuffer.Size);
        }

        ImGui::TreePop();
    }

    ImGui::End();

    return draw_list;
}

ImDrawList* ShowBackendCheckerWindow_NoSplitter(bool* p_open, int* p_vertex_delta)
{
    ImDrawList* draw_list = nullptr;

    int dummy_vertex_delta = 0;
    int& vertex_delta = p_vertex_delta ? *p_vertex_delta : dummy_vertex_delta;
    vertex_delta = 0;

    if (!ImGui::Begin("Dear ImGui Backend Checker##2", p_open))
    {
        ImGui::End();
        return draw_list;
    }

    ImGuiIO& io = ImGui::GetIO();
    ImGui::Text("Dear ImGui %s Backend Checker", ImGui::GetVersion());
    ImGui::Text("io.BackendPlatformName: %s", io.BackendPlatformName ? io.BackendPlatformName : "NULL");
    ImGui::Text("io.BackendRendererName: %s", io.BackendRendererName ? io.BackendRendererName : "NULL");
    ImGui::Separator();

    if (ImGui::TreeNodeEx("0001: Renderer: Large Mesh Support", ImGuiTreeNodeFlags_DefaultOpen))
    {
        draw_list = ImGui::GetWindowDrawList();
        {
            auto* draw_list = ImGui::GetWindowDrawList();

            int start_vertex_count = draw_list->VtxBuffer.Size;

            ImVec2 p = ImGui::GetCursorScreenPos();
            for (int i = 0; i < vtx_count / 4; i++)
            {
                int n = randomize ? rev_map[i] : i;

                float off_x = (float)(n % 100) * 3.0f;
                float off_y = (float)(n % 100) * 1.0f;
                ImU32 col = IM_COL32(((n * 17) & 255), ((n * 59) & 255), ((n * 83) & 255), 255);
                draw_list->AddRectFilled(ImVec2(p.x + off_x, p.y + off_y), ImVec2(p.x + off_x + 50, p.y + off_y + 50), col);
            }

            int end_vertex_count = draw_list->VtxBuffer.Size;

            vertex_delta = end_vertex_count - start_vertex_count;

            ImGui::Dummy(ImVec2(300 + 50, 100 + 50));
            ImGui::Text("VtxBuffer.Size = %d", draw_list->VtxBuffer.Size);
        }

        ImGui::TreePop();
    }

    ImGui::End();

    return draw_list;
}



void TestSplitterBug()
{
    auto& io = ImGui::GetIO();

    ImGui::SliderInt("VtxCount##1", &vtx_count, 0, 100000);
    if (ImGui::Button("-10", ImVec2(50.0f, 0.0f)))
        vtx_count -= 10;
    ImGui::SameLine();
    if (ImGui::Button("-1", ImVec2(50.0f, 0.0f)))
        vtx_count -= 1;
    ImGui::SameLine();
    if (ImGui::Button("+1", ImVec2(50.0f, 0.0f)))
        vtx_count += 1;
    ImGui::SameLine();
    if (ImGui::Button("+10", ImVec2(50.0f, 0.0f)))
        vtx_count += 10;
    vtx_count = ImMax(ImMin(vtx_count, 100000), 0);
    ImGui::SameLine();
    if (ImGui::Button("Copy", ImVec2(50.0f, 0.0f)))
    {
        char buffer[64];
        _itoa_s(vtx_count, buffer, 10);
        ImGui::SetClipboardText(buffer);
    }
    ImGui::SameLine();
    ImGui::Checkbox("Randomize", &randomize);
    for (int n = 0; n < vtx_count / 4; n++)
        map[n] = n;

    if (randomize)
    {
        std::srand(13);
        std::random_shuffle(&map[0], &map[vtx_count / 4], [](auto n) { return std::rand() % n; });

        for (int i = 0; i < vtx_count / 4; ++i)
            rev_map[map[i]] = i;
    }

    ImGui::Spacing();
    ImGui::Spacing();
    ImGui::Spacing();

    int vertex_delta_a = 0;
    int vertex_delta_b = 0;
    ImGui::SetNextWindowPos(ImVec2(700, 100));
    auto draw_list_a = ShowBackendCheckerWindow(nullptr, &vertex_delta_a);
    ImGui::SetNextWindowPos(ImVec2(1200, 100));
    auto draw_list_b = ShowBackendCheckerWindow_NoSplitter(nullptr, &vertex_delta_b);

    if (!draw_list_a || !draw_list_b)
        return;

    ImGui::Spacing();
    ImGui::Spacing();
    ImGui::Spacing();

    const int table_width = 600.0f;

    ImGui::PushItemWidth(table_width);
    auto work_rect_max_x = ImGui::GetCurrentWindow()->WorkRect.Max.x;
    ImGui::GetCurrentWindow()->WorkRect.Max.x = table_width;
    ImGui::GetCurrentWindow()->Size.x -= work_rect_max_x - table_width;
    ImGui::Columns(3);
    ImGui::NextColumn();
    ImGui::Text("Splitter");
    ImGui::NextColumn();
    ImGui::Text("No Splitter");
    ImGui::Separator(); ImGui::NextColumn();

    ImGui::Text("VtxCount");
    ImGui::NextColumn();
    ImGui::Text("%d", draw_list_a->VtxBuffer.Size);
    ImGui::Text("%d (delta)", vertex_delta_a);
    ImGui::NextColumn();
    ImGui::Text("%d", draw_list_b->VtxBuffer.Size);
    ImGui::Text("%d (delta)", vertex_delta_b);
    ImGui::Separator(); ImGui::NextColumn();

    ImGui::Text("Commands");
    ImGui::NextColumn();
    ImGui::Text("%d", draw_list_a->CmdBuffer.Size);
    ImGui::NextColumn();
    ImGui::Text("%d", draw_list_b->CmdBuffer.Size);
    ImGui::Separator(); ImGui::NextColumn();

    auto max_cmd_buffers = ImMax(draw_list_a->CmdBuffer.Size, draw_list_b->CmdBuffer.Size);
    for (int i = 0; i < max_cmd_buffers; ++i)
    {
        ImGui::Text("Command Buffer %d", i);
        ImGui::NextColumn();

        if (i < draw_list_a->CmdBuffer.Size)
        {
            auto& cmd_buffer = draw_list_a->CmdBuffer[i];
            ImGui::Text("ElemCount %d", cmd_buffer.ElemCount);
            ImGui::Text("VtxOffset %d", cmd_buffer.VtxOffset);
        }
        ImGui::NextColumn();

        if (i < draw_list_b->CmdBuffer.Size)
        {
            auto& cmd_buffer = draw_list_b->CmdBuffer[i];
            ImGui::Text("ElemCount %d", cmd_buffer.ElemCount);
            ImGui::Text("VtxOffset %d", cmd_buffer.VtxOffset);
        }
        ImGui::Separator(); ImGui::NextColumn();
    }

    ImGui::GetCurrentWindow()->WorkRect.Max.x = work_rect_max_x;
    ImGui::GetCurrentWindow()->Size.x += work_rect_max_x - table_width;
    ImGui::Columns();

    ImGui::PopItemWidth();
}