[3ds] Load custom font in ImGui backend

[marcie1000]

I'm not sure if it's the right place to ask this, so I apologize if it's not.
I'm using the 3DS ImGui backend from FTPD for a little homebrew game project. I'd like to use a custom font with it but couldn't figure how to make it work.

I first tried to follow the official ImGui instructions :

// main.cpp

// (...)
if (!imgui::ctru::init()) 
    return false;
imgui::citro3d::init();

auto &io = ImGui::GetIO();
// (...)
ImFont* font = io.Fonts->AddFontFromFileTTF("romfs:/Roboto-Regular.ttf", 13.0f);

while (aptMainLoop()) {

    hidScanInput ();
    auto const kDown = hidKeysDown ();
    if (kDown & KEY_START)
        break;

    imgui::ctru::newFrame();
    ImGui::NewFrame();
    
    // (...)
}

But in imgui::ctru::newFrame() the following assertion fails :

imgui_ctru.cpp:246: void imgui::ctru::newFrame(): Assertion `io.Fonts->IsBuilt () && "Font atlas not built! It is generally built by the renderer back-end. Missing call to renderer _NewFrame() function?"' failed.

I suppose it means that I should call something like imgui::citro3d::newFrame() but (unless I'm mistaken) there's no such thing in the citro3d backend.
I then tried to build the font directly after loading TTF :

ImFont* font = io.Fonts->AddFontFromFileTTF("romfs:/Roboto-Regular.ttf", 13.0f);
io.Fonts->Build();

but the following assertion fails:

imgui_draw.cpp:2483: static bool ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas): Assertion `font_offset >= 0 && "FontData is incorrect, or FontNo cannot be found."' failed.

I don't understand what it means or if I'm doing something wrong.

In an attempt to understand how the citro3d backend loads fonts, I tried to modify imgui_citro3d.cpp to make it load my font instead of the 3ds system font, taking inspiration from this example code from devkitPro that loads a font with citro2d library. However it seams to have no effect, the rendered font is still the 3ds system font.

// imgui_citro3d.cpp
// (...)
#include <citro2d.h>
// (...)
// Redefine C2D_Font_s to avoid compiler error "invalid use of incomplete type 'struct C2D_Font_s'"
// from https://github.com/devkitPro/citro2d/blob/master/source/internal.h
struct C2D_Font_s
{
	CFNT_s* cfnt;
	C3D_Tex* glyphSheets;
	float textScale;
};
// (...)
void imgui::citro3d::init ()
{
    // (...)
    // load the glyph texture sheets
	// auto const font      = fontGetSystemFont ();
    auto const c2dfont = C2D_FontLoad("romfs:/Roboto-Regular.bcfnt");
    auto const font = c2dfont->cfnt;
    auto const fontInfo  = fontGetInfo (font);
	auto const glyphInfo = fontGetGlyphInfo (font);
	// (...)
}

So my question is: is there a way to load a custom (TTF) font in imgui with citro3d / ctru backends? Am I missing something?

Thanks in advance