Function ImFontAtlasDebugWriteTexToDisk

imgui_tiny_app/imgui/imgui_draw.cpp:4072–4078

#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "../stb/stb_image_write.h"
static void ImFontAtlasDebugWriteTexToDisk(ImTextureData* tex, const char* description)
{
    ImGuiContext& g = *GImGui;
    char buf[128];
    ImFormatString(buf, IM_COUNTOF(buf), "[%05d] Texture #%03d - %s.png", g.FrameCount, tex->UniqueID, description);
    stbi_write_png(buf, tex->Width, tex->Height, tex->BytesPerPixel, tex->Pixels, tex->GetPitch()); // tex->BytesPerPixel is technically not component, but ok for the formats we support.
}
#endif

void ImFontAtlasTextureRepack(ImFontAtlas* atlas, int w, int h)