Add support for transparent window backgrounds

[ThiconZ]

Currently it does not seem possible to create ImGui viewports/windows with a transparent background for the platform. This seems to stem from a core ImGui issue that requires the ImGui_ImpWin32 class to support the ImGui_ImplWin32_EnableAlphaCompositing function located in the ImGui source code at https://github.com/ocornut/imgui/blob/master/backends/imgui_impl_win32.cpp#L955.

Would it be possible to have this function exposed and either have the patch applied, or provide a way for users to enable/supply the patch without needing to maintain a fork of this project?

ImGui Issue: ocornut/imgui#5988

Note: I am on Hexa.NET.ImGui version 2.2.9 based on ImGui 1.92.2b as NuGet doesn't appear to have been updated yet.

[Ian-McCleary]

What have you tried? I dont have any issues using the ImGuiWIndowFlags.NoBackground on Hexa.NET.ImGui version 2.2.9 / ImGui 1.92.2b, Silk.NET for GLFW windowing and OpenGL bindings.

internal sealed class TestWindow()
{
  private ImGuiWindowFlags _flags = ImGuiWindowFlags.NoBackground;
  public void Render()
  {
    if (ImGui.Begin("Test Window", _flags))
    {
      ImGui.Text("Hello World");
    }
    ImGui.End();
  }
}

I'm also able to set the opacity to 0 which has a slightly different effect as you still see the edge of the frame.

internal sealed class TestWindow()
{
  public void Render()
  {
    // set color to black, 0 opacity
    ImGui.PushStyleColor(ImGuiCol.WindowBg, new Vector4(0, 0, 0, 0));
    if (ImGui.Begin("Test Window"))
    {
      ImGui.Text("Hello World");
    }
    ImGui.End();
    ImGui.PopStyleColor();
  }
}

[JunaMeinhold]

Hi, 2.2.10 is currently blocked by #106. But i think there should be another workaround if that from @Ian-McCleary isn't working out for you, by enumerating the windows manually and then modifying the state of the win32 HWNDs, you can get them over PlatformHandleRaw.

[ThiconZ]

Hi @Ian-McCleary and @JunaMeinhold, I think there may be some confusion about what kind of window I am talking about. I am trying to have the ImGui window background transparent in a platform window. This is not an ImGui window floating inside of another ImGui window which would work when setting the flag to remove the background as it composites the ImGui contents with it. This would be compositing the other non-ImGui content with the removed background - what the ImGui_ImplWin32_EnableAlphaCompositing and linked Imgui issue talk about.

I am using GLFW and DX11 on Windows.

For example, here is a window where I would try to set the window background to transparent. However, that currently does not work. Instead, the background will just become solid black when using ImGuiWindowFlags.NoBackground.

The example window with ImGuiWindowFlags.NoBackground set, showing the background turns solid black instead of being transparent.

[JunaMeinhold]

@ThiconZ yes that's why i mentioned the manual workaround, with GLFW it's the same thing the PlatformHandleRaw inside of ImGuiViewport is a GLFW window pointer which allows you to modify it freely, you can access all the viewports over the imgui context.

[JunaMeinhold]

And btw the function ImGui_ImplWin32_EnableAlphaCompositing is exposed, you just need to add the nuget package Hexa.NET.ImGui.Backends and from there ImGuiImplWin32.EnableAlphaCompositing

Hexa.NET.ImGui/Hexa.NET.ImGui.Backends/Generated/Windows/Functions/Functions.000.cs

Line 234 in 03963b9

public static void EnableAlphaCompositing(void* hwnd)

[ThiconZ]

From what I understand of GLFW_TRANSPARENT_FRAMEBUFFER, it must be set via WindowHint as SetWindowAttrib will result in no effect. So being able to iterate through the viewports won't be of help here - unless I misunderstand something (which is entirely possible).

Calling ImGuiImplWin32.EnableAlphaCompositing on the window unfortunately also does not appear to have an impact on the results. (Thank you for pointing out where it was, I somehow completely missed it before.)

I've switch over to the ExampleGLFWD3D11 project here to try and reproduce my issue and have a standard example you can hopefully see the issue with as well. Using this example, if I add GLFW.WindowHint(GLFW.GLFW_TRANSPARENT_FRAMEBUFFER, 1);, change the manager.Clear to be transparent, and create a new ImGui window with ImGuiWindowFlags.NoBackground, the ImGui window will end up with a black background instead of a transparent one when it's pulled out from the original GLFW window.