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 SkipItemsIM_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 alphaif (!(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。