Error when rendering to two OpenGL3 / SDL2 Windows

[hires]

Hi, I'm new to Dear Imgui and so far it's very cool.

But I'm trying to make a modular API for my application that can open multiple SDL2 / OpenGL3 windows. Imgui works fine in a single window, but when I make two Imgui contexts and two GL contexts, I get the following error when running calling: ImplSDL2_NewFrame()

Assertion io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer back-end. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame()."' failed.

I didn't do anything with the fonts so I don't understand this error.

[ocornut]

Hello,

1. You didn’t fill issue template. Please fill requested issue template when creating an issue.

2. which is the reason you want to create two windows, wouldn’t the Multi-viewport feature solve your problem? (See Multi-viewports / virtual viewports #1542)

3. you are essentially hitting on the fact that most backends assume 1 instance of each. There are discussions here about this (see eg Nesting multiple imgui contexts (glfw+opengl3) #2004).

[hires]

Okay thanks I will try follow those links.

[ocornut]

It's an open problem, even though I think most users would be better off using multi-viewports with a single context, I still appreciate it can be useful or required in some cases you use multiple contexts, and I hoping to rework the backends accordingly.

Essentially the issue is we need to either change the backends API to use explicit an backend context structure, either we can magically attempt to have the backend use a key>map where the key is the imgui context itself. The earlier would be more sane and explicit (and faster too) so I feel this is the better solution but that's a breaking change, so if we do it we should do it right and once, so maybe that's something we can schedule for 1.80.