GTK4 camera application using raw v4l2 and media-requests
14 days ago
a month ago

The development and maintainership of Megapixels has been moved to gitlab.com/postmarketos/megapixels


A GTK4 camera application that knows how to deal with the media request api. It uses opengl to debayer the raw sensor data for the preview.

chat: #megapixels:postmarketos.org on matrix


$ meson build
$ cd build
$ ninja
$ sudo ninja install


Megapixels checks multiple locations for it's configuration file and uses the first one it finds. As first step it will get the first compatible name in the device tree, in the case of a PinePhone this might be "pine64,pinephone-1.2". Then that dtname will be used as the filename in the search path in this order:

  • $XDG_CONFIG_DIR/megapixels/config/$dtname.ini
  • ~/.config/megapixels/config/$dtname.ini
  • /etc/megapixels/config/$dtname.ini
  • /usr/share/megapixels/config/$dtname.ini

The files in /usr/share/megapixels should be the config files distributed in this repository. The other locations allow the user or distribution to override config.

#Config file format

Configuration files are INI format files.


This provides global info, currently only the make and model keys exist, which is metadata added to the generated pictures.

#All other sections

These are the sections describing the sensors.

  • driver=ov5640 the name of the media node that provides the sensor and it's /dev/v4l-subdev* node.
  • media-driver=sun6i-csi the name of the media node that has this camera in it.
  • rotate=90 the rotation angle to make the sensor match the screen
  • mirrored=true whether the output is mirrored, useful for front-facing cameras
  • colormatrix= the DNG colormatrix1 attribute as 9 comma seperated floats
  • forwardmatrix= the DNG forwardmatrix1 attribute as 9 comma seperated floats
  • blacklevel=10 The DNG blacklevel attribute for this camera
  • whitelevel=255 The DNG whitelevel attribute for this camera
  • focallength=3.33 The focal length of the camera, for EXIF
  • cropfactor=10.81 The cropfactor for the sensor in the camera, for EXIF
  • fnumber=3.0 The aperture size of the sensor, for EXIF

These sections have two possibly prefixes: capture- and preview-. Both sets are required. Capture is used when a picture is taken, whereas preview is used when previewing.

  • width=640 and height=480 the resolution to use for the sensor
  • rate=15 the refresh rate in fps to use for the sensor
  • fmt=BGGR8 sets the pixel and bus formats used when capturing from the sensor, only BGGR8 is fully supported

#Post processing

Megapixels only captures raw frames and stores .dng files. It captures a 5 frame burst and saves it to a temporary location. Then the postprocessing script is run which will generate the final .jpg file and writes it into the pictures directory. Megapixels looks for the post processing script in the following locations:

  • ./postprocess.sh
  • $XDG_CONFIG_DIR/megapixels/postprocess.sh
  • ~/.config/megapixels/postprocess.sh
  • /etc/megapixels/postprocess.sh
  • /usr/share/megapixels/postprocess.sh

The bundled postprocess.sh script will copy the first frame of the burst into the picture directory as an DNG file and if dcraw and imagemagick are installed it will generate a JPG and also write that to the picture directory. It supports either the full dcraw or dcraw_emu from libraw.

It is possible to write your own post processing pipeline my providing your own postprocess.sh script at one of the above locations. The first argument to the script is the directory containing the temporary burst files and the second argument is the final path for the image without an extension. For more details see postprocess.sh in this repository.


See the mailing list and issue tracker on https://sr.ht/~martijnbraam/Megapixels/

To send patches, follow this procedure:

  1. Change the default subject prefix from "PATCH" to "PATCH Megapixels" by running this command (only needed once).
    $ git config --local format.subjectPrefix "PATCH Megapixels"
  2. Rebase your commits on top of the latest master.
  3. Send them to the mailing list:
    $ git send-email --to="~martijnbraam/public-inbox@lists.sr.ht" origin/master

#Source code organization

  • ini.c contains a INI file format parser.
  • camera_config.c describes how cameras are configured. Contains no state.
  • main.c contains the entry point and UI portion of the application.
  • quickpreview.c implements fast preview functionality, including debayering, color correction, rotation, etc.
  • io_pipeline.c implements all IO interaction with V4L2 devices in a separate thread to prevent blocking.
  • process_pipeline.c implements all process done on captured images, including launching post-processing
  • pipeline.c Generic threaded message passing implementation based on glib, used to implement the pipelines.
  • camera.c V4L2 abstraction layer to make working with cameras easier
  • device.c V4L2 abstraction layer for devices

The primary image pipeline consists of the main application, the IO pipeline and the process pipeline. The main application sends commands to the IO pipeline, which in turn talks to the process pipeline, which then talks to the main application. This way neither IO nor processing blocks the main application and races are generally avoided.

Tests are located in tests/.


All tools are contained in tools/

  • list_devices lists all V4L2 devices and their hardware layout
  • camera_test lists controls and video modes of a specific camera and tests capturing data from it

#Linux video subsystem

Most of the logic is contained inside main.c, but before we look at it, it is convenient to have some basic notions about the Linux video subsystem that Megapixels directly uses (instead of, for example, using a higher level framework such as "gstreamer", as other camera apps do).

Typically, for "simple" video capture devices (such as some old webcams on a PC), the Linux kernel creates an entry on /dev/ called /dev/videoX (where X can be 0, 1, ...). The user can then open() that file descriptor, use standard ioctl()s on it to start/stop/configure the hardware and finally read() from it to obtain individual video frames.

In the PinePhone we have two cameras ("front" and "rear") but, surprinsingly, the Linux kernel does not expose two video devices but just a single one named /dev/video1.

This is because, on the PinePhone, there is one single "capture device" and two "image sensors" (one for each camera) attached to it:

    .-----------.         .--------------.
    |           |---------| front sensor ))))))
    |  Sensors  |         '--------------'
    | interface |         .--------------.
    |           |---------| rear sensor  ))))))
    '-----------'         '--------------'

The only video device exposed (/dev/video1) represents the "sensors interface" block, which can be configured at runtime to capture data from one sensor or the other.

But there is more: in order to configure the properties of each sensor (example: capture frame rate, auto exposure, ...), instead of issuing ioctl() calls on /dev/video1, the Linux kernel (for this particular case) exposes two extra devices (/dev/v4l-subdev0 for one sensor and /dev/v4l-subdev1 for the other one)

How does the user know that /dev/v4l-subdev0, /dev/v4l-subdev1 and /dev/video1 are related? Thanks to the "media subsystem": for "complex" cases such as this one, the Linux kernel exposes an extra device (/dev/mediaX, where X can be 0, 1, ...) that can be used to...

  • Obtain the list of related devices to that "media interface".
  • Link/unlink the different "blocks" at runtime.

Pheeew.... let's recap what we have to far:

  • /dev/mediaW represents the "whole camera hardware"
  • /dev/videoX is the "sensors interface" from where we will read() frames.
  • /dev/vl4-subdevY and /dev/vl4-subdevZ can be used to configure the sensors.

Notice how I used W, X, Y and Z instead of numbers. In the current kernel W==1, X==0, Y==0 and Z==1, but that might change in the future. That's why main() needs to figure them out by following this procedure:

  1. List all /dev/mediaX devices present (ex: /dev/media0, /dev/media1, ...)
  2. Query each of them with ioctl(MEDIA_IOC_DEVICE_INFO) until we find the entry managed by a driver named "sun6i-csi" (as that is the name of the driver of the sensor interface for the Allwinner SoC camera sensor that the PinePhone uses, which is provided on the *.ini file).
  3. Obtain a list of elements associated to that "media device" by calling ioctl(MEDIA_IOC_ENUM_ENTITIES).
  4. The entry called "ov5640" is the rear camera (as that is the name of the driver of the rear sensor, which is provided on the *.ini file). Save its device name (ex: /dev/v4l-subdev1) for later.
  5. The entry called "gc2145" is the front camera (as that is the name of the driver of the front sensor, which is provided on the *.ini file). Save its device name (ex: /dev/v4l-subdev0) for later.
  6. The entry called "sun6i-csi" is the sensors interface (same name as the driver in charge of the /dev/mediaX interface). Save its device name (ex: /dev/video1) for later.

By the way, regarding steps 1 and 2, you can manually inspect the list of "elements" that are related to a given /dev/mediaX entry from user space using the media-ctl tool. This is what the current kernel and hardware revision return:

$ media-tcl -d /dev/media1 -p

Media controller API version 5.7.19
Media device information
driver          sun6i-csi
model           Allwinner Video Capture Device
bus info        
hw revision     0x0
driver version  5.7.19
Device topology
- entity 1: sun6i-csi (1 pad, 2 links)
            type Node subtype V4L flags 0
            device node name /dev/video1
        pad0: Sink
                <- "gc2145 4-003c":0 []
                <- "ov5640 4-004c":0 [ENABLED]
- entity 5: gc2145 4-003c (1 pad, 1 link)
            type V4L2 subdev subtype Sensor flags 0
            device node name /dev/v4l-subdev0
        pad0: Source
                [fmt:YUYV8_2X8/1280x720@1/10 field:none colorspace:srgb]
                -> "sun6i-csi":0 []
- entity 7: ov5640 4-004c (1 pad, 1 link)
            type V4L2 subdev subtype Sensor flags 0
            device node name /dev/v4l-subdev1
        pad0: Source
                [fmt:YUYV8_2X8/1280x720@1/30 colorspace:srgb xfer:srgb ycbcr:601 quantization:full-range]
                -> "sun6i-csi":0 [ENABLED]

...which means what we already know: sun6i-csi is the sensors interface sink (on /dev/video1) where the two sensors (gc2145 on /dev/v4l-subdev0 and ov5640 on /dev/v4l-subdev1 are connected). By default (or, at least, in the example above) the sensors interface is connected to the rear camera (ov5640) as its link is the only one "ENABLED".

Anyway... once main() has figured out the values of W, X, Y and Z, this is how all these device entries are used to manage the camera hardware:

  • Use ioctl(MEDIA_IOC_SETUP_LINK) on the /dev/mediaW entry to "link" the sensors interface with either the rear sensor or the front sensor (this is how we choose from which camera we will be capturing frames)
  • Use ioctl(VIDIOC_SUBDEV_...) on /dev/v4l-subdev{Y,Z} to configure the sensors.
  • Use ioctl(VIDIOC_...) on /dev/videoX to configure the sensors interface.
  • Use read() on /dev/videoX to capture frames.

The mechanism described on the last point (ie. use read() to capture frames), while possible, is not actually what main() does. Instead, a more complex mechanism (described here) is used, where a series of buffers are allocated, sent to /dev/videoX with ioctl(VIDIOC_QBUF) and then retrieved with ioctl(VIDIOC_DQBUF) once they have been filled with video frames (after having called ioctl(VIDIOC_STREAMON))... but it is basically the same as performing a read() (except that it has more flexibility).

#Source code walkthrough

As we have just seen on the previous section, in the current kernel version, and for the latest PinePhone revision (1.2a), the Linux kernel exposes 4 device entries to manage the camera hardware:

  • /dev/media1 to select the active camera ("front" or "rear")
  • /dev/vl4-subdev0 and /dev/vl4-subdev1 to configure the sensor of each camera (aperture, auto exposure, etc...)
  • /dev/video1 to capture frames (video stream and/or pictures)

However these device entries might change with future versions of the kernel and/or the hardware (for example, /dev/video3 instead of /dev/video1), and that's why function main() in main.c starts by trying to figure out the correct names.

It does so by checking the hardware revision in /proc/device-tree/compatible and then opening the corresponding .ini file from the config folder (ex: pine64,pinephone-1.2.ini for the latest PinePhone revision as of today, pine64,pinetab.ini for the PineTab, etc...).

The .ini file contains the name of the driver that manages the /dev/mediaX interface (csi entry on the device section) and, from there, main() can figure out the rest of the device names as already explained on the previous section.

    config/*.ini ---------------.
        |                       |
        |                       V
        |          .~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        |          :                           :
        |          :  .----> /dev/video1       :
        V          :  |                        :
    /dev/media1 ------+----> /dev/v4l-subdev0  :
                   :  |                        :
                   :  '----> /dev/v4l-subdev1  :
                   :                           :

Anyway... in addition to figuring out these entry names, main() also prepares the GTK widgets layout and installs a series of callbacks. Among them we find these two:

  1. One on the "switch camera button" (on_camera_switch_clicked()) which uses /dev/media1 to switch between the front and rear cameras. Every time this happens, the sensors and the sensors interface are reconfigured according to the parameters provided on the .ini file using /dev/video1, /dev/v4l-subdev0 and /v4l-subdev1.
    |--> stop_capturing()
    |    `--> ioctl('/dev/video1', ...)           # Stop processing frames
    |--> setup_front() or setup_rear()
    |    |--> ioctl('/dev/media1', ...)
    |    `--> init_sensor()
    |         `--> ioctl('/dev/v4l-subdev{0,1}')  # Reconfigure sensor
    |--> init_device()
    |    `--> ioctl('/dev/video1')                # Reconfigure sensors interface
    `--> start_capturing()
         `--> ioctl('/dev/video1')                # Resume capturing frames
  2. Another one on the "take a photo button" (on_shutter_clicked()) which will use /dev/v4l-subdev{0,1} to disable hardware "auto gain" and "auto exposure" and initiate the "single frame capture process" (described later).

Finally, before calling GTK's main loop, main() installs another function (get_frame()) on the "nothing else todo" GTK slot. It will thus be called continuosly as long as there are no other GTK events queued (ie. almost always).

This get_frame() function is where the magic happens: it will call read_frame() to read() from the /dev/video1 device an image frame and then call process_image() to process it.

NOTE: As explained at the end of the Linux video subsystem section, it is a bit more complex than that (that's why you will find a ioctl() instead of a read() inside read_frame()), but for all purposes, you can ignore this fact.

So... let's recap: as long as the user does not click on any application button, the process_image() function is being called all the time with a pointer to the latest captured frame. What does it do with it?

The captured frame buffer contains "RAW data", whose format depends on the value specified on the .ini file for each sensor. Right now we are using BGGR8 for both of them, so the function that takes this buffer to process it is always the same (quick_debayer_bggr8()). The result is a buffer of "standard pixels" that can be drawn to screen using GTK/cairo functions.

When the user clicks on the "take a photo button", however, a special global variable (capture) is set so that the next N times (currently N==10), the process_image() will do something different:

  1. It will first retrieve the latest "auto gain" and "auto exposure" values (remember they were disabled when the user clicked on the "take a photo button").
  2. It will save the latest captured buffer (in "RAW data" format, ie. BGGR8) to a .dng file using the "TIFF" library, which makes it possible to attach all the needed metadata (which Megapixels extracts from the hardware itself and/or the values on the .ini file).
  3. In addition, only the very last time (from the N times):
    • The captured buffer is run through quick_debayer_bggr8() and the result printed to the UI.
    • The postprocess.sh script (see the Post processing section) is called with two arguments: the path to the /tmp folder where the N .dng images have been saved and the path and filename where the resulting post-processed (typically JPEG) image should be saved to (as a result of running postprocess.sh)
    • "Auto exposure" and "auto gain" are re-enabled.

In other words: every time the user clicks on the "take a photo button", N RAW images are saved and postprocess.sh called, which is expected to take those N images and generate a final JPEG.