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• Mapping v4l2 pixel format codes to ffmpeg pixel format names

  12 décembre 2019, par DekiChan

  I’m building an app that is capturing frames from any v4l2 supported webcam. I’m controlling the camera with v4l2 (golang package https://github.com/blackjack/webcam) and one of the things I need is pixel format. However, what I get is uint32 representing the format in v4l2.

  I do the streaming by wrapping ffmpeg command and reading from its stdout :

  args := []string{
  
      "-f", "v4l2",
  
      "-c:v", "rawvideo",
  
      "-pix_fmt", pixFmt,
  
      "-s", size,
  
      "-i", input,
  
      "-f", "image2pipe",
  
      "-c:v", "mjpeg",
  
      "-qscale:v", "5",
  
      "-", // pipes to stdout
  
  }
  
  
  
  cmd := exec.Command("ffmpeg", args...)

  Now the problem is that -pix_fmt option requires format name in string that is recognized by ffmpeg (for example 8-bit Bayer RGGB8 corresponds to bayer_rggb8, while this format in v4l2 has code 1111967570).

  I found the mapping code for it in ffmpeg’s libavdevice : https://github.com/FFmpeg/FFmpeg/blob/master/libavdevice/v4l2-common.h includes the functions I need, but somehow I’m not able to include <libavdevice></libavdevice>v4l2-common.h> in CGo (no problems with <libavdevice></libavdevice>avdevice.h>) :

  package conv
  
  
  
  /*
  
      #cgo pkg-config: libavdevice
  
      #include <libavdevice></libavdevice>avdevice.h>
  
      #include <libavdevice></libavdevice>v4l2-common.h>
  
  */
  
  import "C"
  
  
  
  type (
  
  )
  
  
  
  // PixFmtToFF takes v4l2 pixel format code and converts it to a string
  
  // recognized by ffmpeg command
  
  // example: 1111967570 to "bayer_rggb8"
  
  func PixFmtToFF(v4l2code uint32) (string, error) {
  
      // should work something like this, couldn't test
  
      codecID := C.ff_fmt_v4l2codec(v4l2code)
  
      pixFmt := C.ff_fmt_v4l2ff(vfd.pixelformat, codec_id)
  
      return C.av_get_pix_fmt_name(pixFmt)
  
  }

  Build output returns :

  go build \
  
          -mod=vendor \
  
          -o ./bin/camera-api \
  
          ./cmd/camera-api
  
  # kca/internal/camera/conv
  
  internal/camera/conv/convert.go:6:11: fatal error: libavdevice/v4l2-common.h: No such file or directory
  
    #include <libavdevice></libavdevice>v4l2-common.h>
  
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  compilation terminated.
  
  make: *** [Makefile:13: build/camera-api] Error 2
  
  make: *** [builder/camera-api] Error 2

  Output of pkg-config --libs libavdevice is -L/usr/local/lib -lavdevice.

  So, I have two questions :

  1) How could I include v4l2-common.h so I can use the mapping functions ?

  2) Is there a better way ? I need ffmpeg wrapping so we can support wide range of formats for captured frames.

• Catalyst Open Source Academy

  16 janvier 2015, par Matthieu Aubry — Community

  The Open Source Academy is an initiative designed to provide training and work experience for young New Zealand technologists. Catalyst organises the Academy to show young technologists how to participate in open source communities and to fully explore their passion for IT through freely available open source tools.

  It has been running annually since 2011. We are proud that Piwik project could participate in the Academy again this year !

  What students got done

  It’s amazing what a few young students can get done in four days of participating in an open source project like Piwik ! They were able to quickly get started with Piwik, and continued to make useful contributions to the Piwik analytics platform.

  New Darkness theme

  Liam has created a new dark theme for Piwik called Darkness.

  

  To create the theme, Liam had to improve Piwik core stylesheets and created this pull request : Reuse the LESS variable for white color across all stylesheets.

  Accessibility improvements

  We were lucky to spend time with Julius, a Catalyst employee who is blind. He showed us in great detail how difficult and time consuming it can be for a blind user to use Piwik. For example we noticed how complicated it was for Julius to navigate the menus, to get to the main content, and to use the calendar and the Website selector. During this presentation we also noticed that Piwik was not yet usable with the keyboard.

  As a result of this session with Julius we got to work with the students to improve accessibility in Piwik.

  

  (photo source)

  List of accessibility improvements

  All these pull requests were created by the students and have been successfully merged into Piwik :

  • see the focus of menu items when tabbing through the page
  • Tweaks to improve accessibility (section around graph, better tooltip, set language in html)
  • Added h3 tags to widget headers
  • when main Menu is focused, display submenu
  • Add “Skip to content” feature when user presses tab key
  • Improved accessibility by putting offscreen text in the widget title

  To learn more about accessibility in Piwik check out this issue on our tracker.

  Summary

  Working with young students was fun and interesting. We were excited to see how much they got done in such a short time !

  At Piwik and Piwik PRO we are committed to building the best open analytics platform, and we will continue to support students who want to take part in the Piwik adventure.

  Be well,

• Catalyst Open Source Academy

  16 janvier 2015, par Matthieu Aubry — Community

  The Open Source Academy is an initiative designed to provide training and work experience for young New Zealand technologists. Catalyst organises the Academy to show young technologists how to participate in open source communities and to fully explore their passion for IT through freely available open source tools.

  It has been running annually since 2011. We are proud that Piwik project could participate in the Academy again this year !

  What students got done

  It’s amazing what a few young students can get done in four days of participating in an open source project like Piwik ! They were able to quickly get started with Piwik, and continued to make useful contributions to the Piwik analytics platform.

  New Darkness theme

  Liam has created a new dark theme for Piwik called Darkness.

  

  To create the theme, Liam had to improve Piwik core stylesheets and created this pull request : Reuse the LESS variable for white color across all stylesheets.

  Accessibility improvements

  We were lucky to spend time with Julius, a Catalyst employee who is blind. He showed us in great detail how difficult and time consuming it can be for a blind user to use Piwik. For example we noticed how complicated it was for Julius to navigate the menus, to get to the main content, and to use the calendar and the Website selector. During this presentation we also noticed that Piwik was not yet usable with the keyboard.

  As a result of this session with Julius we got to work with the students to improve accessibility in Piwik.

  

  (photo source)

  List of accessibility improvements

  All these pull requests were created by the students and have been successfully merged into Piwik :

  • see the focus of menu items when tabbing through the page
  • Tweaks to improve accessibility (section around graph, better tooltip, set language in html)
  • Added h3 tags to widget headers
  • when main Menu is focused, display submenu
  • Add “Skip to content” feature when user presses tab key
  • Improved accessibility by putting offscreen text in the widget title

  To learn more about accessibility in Piwik check out this issue on our tracker.

  Summary

  Working with young students was fun and interesting. We were excited to see how much they got done in such a short time !

  At Piwik and Piwik PRO we are committed to building the best open analytics platform, and we will continue to support students who want to take part in the Piwik adventure.

  Be well,