ThinkPad Infrared Camera

One of the options available when configuring the my ThinkPad was an Infrared camera. The main selling point being “Windows Hello” facial recognition based login. While I wasn’t planning on keeping Windows on the system, I was curious to see what I could do with it under Linux. Hopefully this is of use to anyone else trying to get it to work.

The camera is manufactured by Chicony Electronics (probably a CKFGE03 or similar), and shows up as two USB devices:

04f2:b5ce Integrated Camera
04f2:b5cf Integrated IR Camera

Both devices are bound by the uvcvideo driver, showing up as separate video4linux devices. Interestingly, the IR camera seems to be assigned /dev/video0, so generally gets picked by apps in preference to the colour camera. Unfortunately, the image it produces comes up garbled:

So it wasn’t going to be quite so easy to get things working. Looking at the advertised capture modes, the camera supports Motion-JPEG and YUYV raw mode. So I tried capturing a few JPEG frames with the following GStreamer pipeline:

gst-launch-1.0 v4l2src device=/dev/video0 num-buffers=10 ! image/jpeg ! multifilesink location="frame-%02d.jpg"

Unlike in raw mode, the red illumination LEDs started flashing when in JPEG mode, which resulted in frames having alternating exposures. Here’s one of the better exposures:

What is interesting is that the JPEG frames have a different aspect ratio to the raw version: a more normal 640×480 rather than 400×480. So to start, I captured a few raw frames:

gst-launch-1.0 v4l2src device=/dev/video0 num-buffers=10 ! "video/x-raw,format=(string)YUY2" ! multifilesink location="frame-%02d.raw"

The illumination LEDs stayed on constantly while recording in raw mode. The contents of the raw frames show something strange:

00000000  11 48 30 c1 04 13 44 20  81 04 13 4c 20 41 04 13  |.H0...D ...L A..|
00000010  40 10 41 04 11 40 10 81  04 11 44 00 81 04 12 40  |@.A..@....D....@|
00000020  00 c1 04 11 50 10 81 04  12 4c 10 81 03 11 44 00  |....P....L....D.|
00000030  41 04 10 48 30 01 04 11  40 10 01 04 11 40 10 81  |A..H0...@....@..|

The advertised YUYV format encodes two pixels in four bytes, so you would expect any repeating patterns to occur at a period of four bytes. But the data in these frames seems to repeat at a period of five bytes.

Looking closer it is actually repeating at a period of 10 bits, or four packed values for every five bytes. Furthermore, the 800 byte rows work out to 640 pixels when interpreted as packed 10 bit values (rather than the advertised 400 pixels), which matches the dimensions of the JPEG mode.

The following Python code can unpack the 10-bit pixel values:

def unpack(data):
    result = []
    for i in range(0, len(data), 5):
        block = (data[i] |
                 data[i+1] << 8 |
                 data[i+2] << 16 |
                 data[i+3] << 24 |
                 data[i+4] << 32)
        result.append((block >> 0) & 0x3ff)
        result.append((block >> 10) & 0x3ff)
        result.append((block >> 20) & 0x3ff)
        result.append((block >> 30) & 0x3ff)
    return result

After adjusting the brightness while converting to 8-bit greyscale, I get a usable image. Compare a fake YUYV frame with the decoded version:

I suppose this logic could be wrapped up in a GStreamer element to get usable infrared video capture.

I’m still not clear why the camera would lie about the pixel format it produces. My best guess is that they wanted to use the standard USB Video Class driver on Windows, and this let them get at the raw data to process in user space.

Experimenting with C++ Reflection

For a number of projects I’ve worked on at Canonical have involved using GObject based libraries from C++. To make using these APIs easier and safer, we’ve developed a few helper utilities. One of the more useful ones was a class (originally by Jussi Pakkenen) that presents an API similar to the C++11 smart pointers but specialised for GObject types we called gobj_ptr. This ensures that objects are unrefed when the smart pointer goes out of scope, and increments the reference count when copying the pointer.

However, even with the use of this class I still end up with a bunch of type cast macros littering my code, so I was wondering if there was anything I could do about that. With the current C++ language the answer seems to be “no”, since GObject’s convention of subclasses as structures whose first member is a value of the parent structure type is at a higher level but then I found a paper describing an extension for C++ Static Reflection. This looked like it could help, but seems to have missed the boat for C++17. However, there is a sample implementation for Clang written by Matus Chochlik, so I downloaded and compiled that to have a play.

At its heart, there is a new reflexpr() operation that takes a type as an argument, and returns a type-like “metaobject” that describes the type. For example:

using MO = reflexpr(sometype);

These metaobjects can then be manipulated using various helpers in the std::meta namespace. For example, std::meta::reflects_same_v<MO1,MO2> will tell you whether two metaobjects represent the same thing. There were a few other useful operations:

  • std::meta::Class<MO> will return true if the metaobject represents a class or struct (which are effectively interchangeable in C++).
  • std::meta::get_data_members_m<MO> will return a metaobject representing the members of a class/struct.
  • From a sequence metaobject, we can determine its length with std::meta::get_size_v<MO>, and retrieve the metaobject elements in the sequence with std::meta::get_element_m<MO>
  • We can get a metaobject representing the type for a data member metaobject with std::meta::get_type_m<MO>.

Put all this together, and we’ve got the building blocks to walk the GObject inheritance hierarchy at compile time. Now rather than spread the reflection magic throughout my code, I used it to declare a templated compile time constant:

template <typename Base, typename Derived>
constexpr bool is_base_of = ...

With this, an expression like is_base_of<GObject, GtkWidget> will evaluate true, while something like is_base_of<GtkContainer, GtkLabel> will evaluate false.

As a simple example, this constant could be used to implement an up-cast helper:

template <typename Target, typename Source>
inline Target* gobj_cast(Source* v)
    static_assert(is_base_of<Target, Source>,
                  "Could not verify that target type is a base of source type");
    return reinterpret_cast<Target*>(v);

If we can verify that this is a correct up-cast, this function will compile down to a simple type cast. Otherwise, compilation will fail on the static_assert, printing a relatively short and readable error message.

The same primitive could be used for other things, such as allowing you to construct a gobj_ptr<T> from an instance of a subclass, or copying one gobj_ptr to another one representing a parent class.

It’d be nice to implement something like dynamic_cast for down-casting, but I don’t think even static reflection will help us map from a struct type to the corresponding helper function that returns the GType.

If you want to experiment with this, the code used to implement all of the above can be found in the following repository: