ImGui 学习笔记（四）—— 实现每窗口背景色

ImGui 的窗口背景仅通过全局的 style 控制，这一点不方便于我们设置特定窗口的背景透明度（一般不用于调整颜色），分析代码，我们可以找到 ImGui::RenderWindowDecorations 函数：

void ImGui::RenderWindowDecorations(ImGuiWindow* window, const ImRect& title_bar_rect, bool title_bar_is_highlight, bool handle_borders_and_resize_grips, int resize_grip_count, const ImU32 resize_grip_col[4], float resize_grip_draw_size)
{
    ImGuiContext& g = *GImGui;
    ImGuiStyle& style = g.Style;
    ImGuiWindowFlags flags = window->Flags;

    // Ensure that Scrollbar() doesn't read last frame's SkipItems
    // 确保 Scrollbar() 不读取上一帧的 SkipItems
    IM_ASSERT(window->BeginCount == 0);
    window->SkipItems = false;
    window->DC.NavLayerCurrent = ImGuiNavLayer_Menu;

    // Draw window + handle manual resize
    // 绘制窗口 + 手动调整大小
    // As we highlight the title bar when want_focus is set, multiple reappearing windows will have their title bar highlighted on their reappearing frame.
    // 当设置了 want_focus 后，在我们突出显示标题栏时，多个重新出现的窗口将在其重新出现的框架上突出显示其标题栏。
    const float window_rounding = window->WindowRounding;
    const float window_border_size = window->WindowBorderSize;
    if (window->Collapsed)
    {
        // Title bar only
        // 仅标题栏
        const float backup_border_size = style.FrameBorderSize;
        g.Style.FrameBorderSize = window->WindowBorderSize;
        ImU32 title_bar_col = GetColorU32((title_bar_is_highlight && g.NavCursorVisible) ? ImGuiCol_TitleBgActive : ImGuiCol_TitleBgCollapsed);
        if (window->ViewportOwned)
            title_bar_col |= IM_COL32_A_MASK; // No alpha (we don't support is_docking_transparent_payload here because simpler and less meaningful, but could with a bit of code shuffle/reuse)
        RenderFrame(title_bar_rect.Min, title_bar_rect.Max, title_bar_col, true, window_rounding);
        g.Style.FrameBorderSize = backup_border_size;
    }
    else
    {
        // Window background
        // 窗口背景
        if (!(flags & ImGuiWindowFlags_NoBackground))
        {
            bool is_docking_transparent_payload = false;
            if (g.DragDropActive && (g.FrameCount - g.DragDropAcceptFrameCount) <= 1 && g.IO.ConfigDockingTransparentPayload)
                if (g.DragDropPayload.IsDataType(IMGUI_PAYLOAD_TYPE_WINDOW) && *(ImGuiWindow**)g.DragDropPayload.Data == window)
                    is_docking_transparent_payload = true;

            ImU32 bg_col = GetColorU32(GetWindowBgColorIdx(window));
            if (window->ViewportOwned)
            {
                //bg_col |= IM_COL32_A_MASK; // No alpha
                if (!(g.ConfigFlagsCurrFrame & ImGuiConfigFlags_TransparentBackbuffers)) {
                    bg_col = (bg_col | IM_COL32_A_MASK);
                }
                if (is_docking_transparent_payload)
                    window->Viewport->Alpha *= DOCKING_TRANSPARENT_PAYLOAD_ALPHA;
            }
            else
            {
                // Adjust alpha. For docking
                // 用于 dock 模式下调整 Alpha 
                bool override_alpha = false;
                float alpha = 1.0f;
                if (g.NextWindowData.Flags & ImGuiNextWindowDataFlags_HasBgAlpha)
                {
                    alpha = g.NextWindowData.BgAlphaVal;
                    override_alpha = true;
                }
                if (is_docking_transparent_payload)
                {
                    alpha *= DOCKING_TRANSPARENT_PAYLOAD_ALPHA; // FIXME-DOCK: Should that be an override?
                    override_alpha = true;
                }
                if (override_alpha)
                    bg_col = (bg_col & ~IM_COL32_A_MASK) | (IM_F32_TO_INT8_SAT(alpha) << IM_COL32_A_SHIFT);
            }
......
}

这里的 ImU32 bg_col = GetColorU32(GetWindowBgColorIdx(window)); 就是窗口要设置的背景颜色，想办法改成跟窗口 id，name 或者 hash 绑定的数值即可。