Versione delle 11:51, 16 mag 2021

La risorsa libv4l è una raccolta di librerie per la gestione dei dispositivi video4linux2 evitando che si debba scrivere del codice separato, nella stessa classe, per i diversi dispositivi. libv4l è composta da tre librerie differenti: libv4lconvert, libv4l1 e libv4l2.

La libreria libv4l2, che qui ci interessa, mette a disposizione l'API v4l2 per i dispositivi v4l2.

Per poter fruire delle risorse fornite dalla libreria v4l2 è necessario richiamare nell'applicazione Gambas la seguente libreria dinamica condivisa: "libv4l2.so.0.0.0"

Mostriamo di seguito un possibile codice per la cattura di un'immagine attraverso una webcam.

L'esempio funziona solo su sistemi a 64bit.

L'immagine sarà salvata in un file formato ppm:

Public Struct BUFFER
  start As Pointer
  length As Integer
End Struct

Library "libv4l2:0.0.0"

Public Struct v4l2_requestbuffers
  count As Integer
  type As Integer
  memory_ As Integer
  reserved[2] As Integer
End Struct

Private Enum V4L2_BUF_TYPE_VIDEO_CAPTURE = 1, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY,
             V4L2_BUF_TYPE_VBI_CAPTURE, V4L2_BUF_TYPE_VBI_OUTPUT, V4L2_BUF_TYPE_SLICED_VBI_CAPTURE,
             V4L2_BUF_TYPE_SLICED_VBI_OUTPUT
Private Enum V4L2_MEMORY_MMAP = 1, V4L2_MEMORY_USERPTR, V4L2_MEMORY_OVERLAY, V4L2_MEMORY_DMABUF
Private Const V4L2_PIX_FMT_RGB24 As Integer = 859981650
Private Const V4L2_FIELD_INTERLACED As Integer = 4
Private Const VIDIOC_S_FMT As Long = 3234878981
Private Const VIDIOC_REQBUFS As Long = 3222558216
Private Const VIDIOC_QUERYBUF As Long = 3227014665
Private Const VIDIOC_QBUF As Long = 3227014671
Private Const VIDIOC_STREAMON As Long = 1074026002
Private Const VIDIOC_DQBUF As Long = 3227014673

' int v4l2_open(const char *file, int oflag, ...)
' Open a V4L2 device.
Private Extern v4l2_open(fl As String, oflag As Integer, alterum As Integer) As Integer

' int v4l2_ioctl(int fd, unsigned long int request, ...)
' Program a V4L2 device.
Private Extern v4l2_ioctl_pointer(fd As Integer, request As Long, arg As Pointer) As Integer Exec "v4l2_ioctl"

' int v4l2_ioctl(int fd, unsigned long int request, ...)
' Program a V4L2 device.
Private Extern v4l2_ioctl_request(fd As Integer, request As Long, arg As V4l2_requestbuffers) As Integer Exec "v4l2_ioctl"

' void *v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset)
' Map device memory into application address space.
Private Extern v4l2_mmap(startP As Pointer, lengthI As Integer, prot As Integer, flags As Integer, fdI As Integer, offsetL As Long) As Pointer

' int v4l2_munmap(void *start, size_t length)
' Unmap device memory.
Private Extern v4l2_munmap(startP As Pointer, lengthI As Integer) As Integer

' int v4l2_close(int fd)
' Close a V4L2 device.
Private Extern v4l2_close(fd As Integer)


Library "libc:6"

Public Struct timeval
  tv_sec As Long
  tv_usec As Long
End Struct

Private Enum O_RDONLY = 0, O_WRONLY, O_RDWR, O_NONBLOCK = &2000
Private Enum PROT_NONE = 0, PROT_READ, PROT_WRITE
Private Const MAP_SHARED As Integer = 1
 
' int select (int __nfds, fd_set *__restrict __readfds, fd_set *__restrict __writefds, fd_set *__restrict __exceptfds, struct timeval *__restrict __timeout)
' Blocks the calling process until there is activity on any of the specified sets of file descriptors, or until the timeout period has expired.
Private Extern select_C(nfds As Integer, readfds As Pointer, writefds As Pointer, exceptfds As Pointer, timeout As Timeval) As Integer Exec "select"
 

Public Sub Main()
 
 Dim dispositivo, file_ppm As String
 Dim fd, r, n_buff, i, offset As Integer
 Dim width, height, index, byte_usati, pix As Integer
 Dim reqbuf As New V4l2_requestbuffers
 Dim p As Pointer
 Dim st As Stream
 Dim buffers As New BUFFER[2]
 Dim tv As New Timeval
 Dim fex As File
 
' Impostare il dispositivo video adeguato:
 dispositivo = "/dev/video0"
 file_ppm = "/tmp/immago.ppm"   ' Il percorso ove sarà salvato il file immagine ppm creato
 
 Print "\e[31mAttendere......\e[0m"
 fd = v4l2_open(dispositivo, O_RDWR Or O_NONBLOCK, 0)
 If fd < 0 Then Error.Raise("Impossibile aprire il file-device !")
     
 p = Alloc(208)
 st = Memory p For Read Write
 Write #st, V4L2_BUF_TYPE_VIDEO_CAPTURE As Integer
 Seek #st, 8
 Write #st, 640 As Integer
 Write #st, 480 As Integer
 Write #st, V4L2_PIX_FMT_RGB24 As Integer
 Write #st, V4L2_FIELD_INTERLACED As Integer

 Repeat
   r = v4l2_ioctl_pointer(fd, VIDIOC_S_FMT, p)
 Until r > -1
 If r = -1 Then Error.Raise("Errore !")
 Seek #st, 8
 Read #st, width
 Read #st, height
 Read #st, pix
 If pix <> V4L2_PIX_FMT_RGB24 Then Error.Raise("Libv4l non accetta il formato RGB24.\nImpossibile procedere !")
 If (width <> 640) Or (height <> 480) Then Print "Attenzione: il dispositivo imposterà l'immagine alle dimensioni";; width; "x"; height
 st.Close
 Free(p)
 
 With reqbuf
   .count = 2
   .type = V4L2_BUF_TYPE_VIDEO_CAPTURE
   .memory_ = V4L2_MEMORY_MMAP
 End With
 
 Repeat
   r = v4l2_ioctl_request(fd, VIDIOC_REQBUFS, reqbuf)
 Until r > -1
 If r = -1 Then Error.Raise("Errore !")
 
 p = Alloc(88)
 st = Memory p For Read Write
 For n_buff = 0 To reqbuf.count - 1
   Seek #st, 0
   Write #st, n_buff As Integer
   Write #st, V4L2_BUF_TYPE_VIDEO_CAPTURE As Integer
   Seek #st, 60
   Write #st, V4L2_MEMORY_MMAP As Integer
   Repeat
     r = v4l2_ioctl_pointer(fd, VIDIOC_QUERYBUF, p)
   Until r > -1
   If r = -1 Then Error.Raise("Errore !")
   Seek #st, 72
   With buffers[n_buff] = New BUFFER
     Read #st, i
     .length = i
   End With
   Seek #st, 64
   Read #st, offset
   buffers[n_buff].start = v4l2_mmap(Null, i, PROT_READ Or PROT_WRITE, MAP_SHARED, fd, offset)
   If IsNull(buffers[n_buff].start) Then Error.Raise("Errore: puntatore nullo !")
 Next
 st.Close
 Free(p)
 
 p = Alloc(88)
 st = Memory p For Write
 For i = 0 To n_buff - 1
   Seek #st, 0
   Write #st, i As Integer
   Write #st, V4L2_BUF_TYPE_VIDEO_CAPTURE As Integer
   Seek #st, 60
   Write #st, V4L2_MEMORY_MMAP As Integer
   Repeat
     r = v4l2_ioctl_pointer(fd, VIDIOC_QBUF, p)
   Until r > -1
   If r = -1 Then Error.Raise("Errore !")
 Next
 st.Close
 Free(p)
 
 i = V4L2_BUF_TYPE_VIDEO_CAPTURE
 Repeat
   r = v4l2_ioctl_pointer(fd, VIDIOC_STREAMON, VarPtr(i))
 Until r > -1
 If r = -1 Then Error.Raise("Errore !")
      
 Repeat
   tv.tv_sec = 1
   r = select_C(fd + 1, Null, Null, Null, tv)
 Until r > -1
 If r = -1 Then Error.Raise("Errore !")
 
 p = Alloc(88)
 st = Memory p For Read Write
 Seek #st, 4
 Write #st, V4L2_BUF_TYPE_VIDEO_CAPTURE As Integer
 Seek #st, 60
 Write #st, V4L2_MEMORY_MMAP As Integer
 Repeat
   r = v4l2_ioctl_pointer(fd, VIDIOC_DQBUF, p)
 Until r > -1
 If r = -1 Then Error.Raise("Errore !")
 Seek #st, 8
 Read #st, byte_usati
  

' Scrive il file immagine .ppm:
 File.Save(file_ppm, "P6\x0A" & CStr(width) & Chr(32) & CStr(height) & " 255\x0A")
  
 fex = open file_ppm For Write
 If IsNull(fex) Then Error.Raise("Impossibile aprire in scrittura il file immagine !")

 Seek #st, 0
 Read #st, index
 st.Close
 Free(p)
 
 p = buffers[index].start
 Seek #fex, 15
 For i = 1 To byte_usati
   Write #fex, Byte@(p) As Byte
   p = p + 1
 Next
 fex.Close
       

' In chiusura:
 i = V4L2_BUF_TYPE_VIDEO_CAPTURE
 Repeat
   r = v4l2_ioctl_pointer(fd, VIDIOC_STREAMON, VarPtr(i))
 Until r > -1
 If r = -1 Then Error.Raise("Errore !")

 For i = 0 To n_buff - 1
   v4l2_munmap(buffers[i].start, buffers[i].length)
 Next
 v4l2_close(fd)
 
End

Riferimenti

• http://linuxtv.org/downloads/v4l-dvb-apis/index.html
• http://www.exploits.org/v4l/