Add a user-accessible BackgroundDrawList

[esquellington]

A simple feature request: having an API method ImGui::GetBackgroundDrawList() that returns an internal ImDrawList that is always rendered first at full display size, without any window overhead (context, scrollbars, alpha, etc...) just a list of primitives guaranteed to be rendered before any ImGui window.

The BackgroundDrawList would act as a global overlay on the user-drawn scene, but always remain behind ImGui windows and mouse cursor (the internal OverlayDrawList). There's a few use cases for this:

• Draw text labels anywhere in the window using existing ImGui font load/render functionality, avoiding the need for a different font load/render code path in simple applications.
• Draw primitives anywhere in the window using existing ImGui functionality, for example, to display rubber-banding selection rectangles overlayed on a 3D scene, or even using ImGui as a simple 2D immediate rendering API.

It was pointed out to me by @ocornut that the same effect can be achieved with a "naked window" (code below), however, this requires draw commands to be issued inside the specific Begin/End block, or switching to/from the "naked window" in the middle of user code that may want to issue both global background text/primitives and normal ImGui windows.
bool bOpened(true);
ImGui::PushStyleColor( ImGuiCol_WindowBg, ImVec4(0,0,0,0) );
ImGui::Begin("BCKGND",&bOpened, ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoScrollWithMouse | ImGuiWindowFlags_NoCollapse | ImGuiWindowFlags_NoSavedSettings | ImGuiWindowFlags_NoInputs | ImGuiWindowFlags_NoFocusOnAppearing | ImGuiWindowFlags_NoBringToFrontOnFocus );
ImGui::SetWindowPos( ImVec2(0,0), ImGuiSetCond_FirstUseEver );
ImGui::SetWindowSize( ImGui::GetIO().DisplaySize );
ImGui::SetWindowCollapsed( false );
ImGui::GetWindowDrawList()->AddText( ImVec2(400,400), ImColor(1.0f,1.0f,1.0f,1.0f), "Text in Background Layer" );
ImGui::End();
ImGui::PopStyleColor();

As a generalization, instead of a single BackgroundDrawList, it might be better to have an array of ordered background layers 0...N, retrieved with ImGui::GetLayerDrawList( int layer_id ), to which draw commands can be directly sent regardless of the current window stack state.

I have implemented basic ImGui::GetBackgroundDrawList() feature successfully in my local branch, and could work on the ImGui::GetLayerDrawList() generalization if it's considered better, but first I'd like to know if there's any interest/drawbacks on this becoming part of the ImGui API.

[ocornut]

Not answering the full question, FYI your code above for a naked window can be shortened:

ImGui::SetNextWindowPos( ImVec2(0,0) );
ImGui::Begin("BCKGND", NULL, ImGui::GetIO().DisplaySize, 0,0f, ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoScrollWithMouse | ImGuiWindowFlags_NoCollapse | ImGuiWindowFlags_NoSavedSettings | ImGuiWindowFlags_NoInputs | ImGuiWindowFlags_NoFocusOnAppearing | ImGuiWindowFlags_NoBringToFrontOnFocus );
ImGui::GetWindowDrawList()->AddText( ImVec2(400,400), ImColor(1.0f,1.0f,1.0f,1.0f), "Text in Background Layer" );
ImGui::End();

I wouldn't mind some helper combination flag, e.g ImGuiWindowFlags_Naked the problem is to decide on a set of flags that makes definitive sense under a given label. And they are actually harder to find than you'd expect.

[esquellington]

Thanks, the (...)_Naked flags combination would be helpful but could just be done on the user side I guess.
My main suggestion/question is if it would make sense to have naked layers with no window context at all, outside Begin/End blocks. I understand it may not fit as a "core" feature of ImGui or may break some existing or future abstractions, but would allow users to reuse existing immediate 2D renderer, specially for text.

[ocornut]

@esquellington GetBackgroundDrawList() has been added now, see #2391
Closing this.

Also note that 1.66 also introduced helper flags ImGuiWindowFlags_NoDecoration to facilitate custom versions of this.

[esquellington]

That's awesome, thanks!