Function ImFontAtlasUpdateNewFrame

imgui_tiny_app/imgui/imgui_draw.cpp:2758–2857

Called by NewFrame() for atlases owned by a context. If you manually manage font atlases, you'll need to call this yourself. - 'frame_count' needs to be provided because we can gc/prioritize baked fonts based on their age. - 'frame_count' may not match those of all imgui contexts using this atlas, as contexts may be updated as different frequencies. But generally you can use ImGui::GetFrameCount()

// - 'frame_count' needs to be provided because we can gc/prioritize baked fonts based on their age.
// - 'frame_count' may not match those of all imgui contexts using this atlas, as contexts may be updated as different frequencies. But generally you can use ImGui::GetFrameCount() on one of your context.
void ImFontAtlasUpdateNewFrame(ImFontAtlas* atlas, int frame_count, bool renderer_has_textures)
{
    IM_ASSERT(atlas->Builder == NULL || atlas->Builder->FrameCount < frame_count); // Protection against being called twice.
    atlas->RendererHasTextures = renderer_has_textures;

    // Check that font atlas was built or backend support texture reload in which case we can build now
    if (atlas->RendererHasTextures)
    {
        atlas->TexIsBuilt = true;
        if (atlas->Builder == NULL) // This will only happen if fonts were not already loaded.
            ImFontAtlasBuildMain(atlas);
    }
    // Legacy backend
    if (!atlas->RendererHasTextures)
        IM_ASSERT_USER_ERROR(atlas->TexIsBuilt, "Backend does not support ImGuiBackendFlags_RendererHasTextures, and font atlas is not built! Update backend OR make sure you called ImGui_ImplXXXX_NewFrame() function for renderer backend, which should call io.Fonts->GetTexDataAsRGBA32() / GetTexDataAsAlpha8().");
    if (atlas->TexIsBuilt && atlas->Builder->PreloadedAllGlyphsRanges)
        IM_ASSERT_USER_ERROR(atlas->RendererHasTextures == false, "Called ImFontAtlas::Build() before ImGuiBackendFlags_RendererHasTextures got set! With new backends: you don't need to call Build().");

    // Clear BakedCurrent cache, this is important because it ensure the uncached path gets taken once.
    // We also rely on ImFontBaked* pointers never crossing frames.
    ImFontAtlasBuilder* builder = atlas->Builder;
    builder->FrameCount = frame_count;
    for (ImFont* font : atlas->Fonts)
        font->LastBaked = NULL;

    // Garbage collect BakedPool
    if (builder->BakedDiscardedCount > 0)
    {
        int dst_n = 0, src_n = 0;
        for (; src_n < builder->BakedPool.Size; src_n++)
        {
            ImFontBaked* p_src = &builder->BakedPool[src_n];
            if (p_src->WantDestroy)
                continue;
            ImFontBaked* p_dst = &builder->BakedPool[dst_n++];
            if (p_dst == p_src)
                continue;
            memcpy(p_dst, p_src, sizeof(ImFontBaked));
            builder->BakedMap.SetVoidPtr(p_dst->BakedId, p_dst);
        }
        IM_ASSERT(dst_n + builder->BakedDiscardedCount == src_n);
        builder->BakedPool.Size -= builder->BakedDiscardedCount;
        builder->BakedDiscardedCount = 0;
    }

    // Update texture status
    for (int tex_n = 0; tex_n < atlas->TexList.Size; tex_n++)
    {
        ImTextureData* tex = atlas->TexList[tex_n];
        bool remove_from_list = false;
        if (tex->Status == ImTextureStatus_OK)
        {
            tex->Updates.resize(0);
            tex->UpdateRect.x = tex->UpdateRect.y = (unsigned short)~0;
            tex->UpdateRect.w = tex->UpdateRect.h = 0;
        }
        if (tex->Status == ImTextureStatus_WantCreate && atlas->RendererHasTextures)
            IM_ASSERT(tex->TexID == ImTextureID_Invalid && tex->BackendUserData == NULL && "Backend set texture's TexID/BackendUserData but did not update Status to OK.");