Catturare un'immagine con una webcam mediante le funzioni esterne del API di libv4l2
Da Gambas-it.org - Wikipedia.
Versione del 3 lug 2015 alle 02:03 di Vuott (Discussione | contributi)
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'immagine sarà salvata in un file formato ppm:
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 Public Struct BUFFER start As Pointer length As Integer End Struct Public Struct timeval tv_sec As Long tv_usec As Long 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" ' 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" ' void exit(int status) ' Terminates the calling process immediately. Private Extern exit_C(status As Integer) Exec "exit" Public Sub Main() Dim dispositivo As String Dim 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 dispositivo = "/dev/video0" file_ppm = "/tmp/immago.ppm" ' Il percorso ove sarà salvato il file immagine ppm creato 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 Do r = v4l2_ioctl_pointer(fd, VIDIOC_S_FMT, p) Loop 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 Do r = v4l2_ioctl_request(fd, VIDIOC_REQBUFS, reqbuf) Loop 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 Do r = v4l2_ioctl_pointer(fd, VIDIOC_QUERYBUF, p) Loop 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 Do r = v4l2_ioctl_pointer(fd, VIDIOC_QBUF, p) Loop Until r > -1 If r = -1 Then Error.Raise("Errore !") Next st.Close Free(p) i = V4L2_BUF_TYPE_VIDEO_CAPTURE Do r = v4l2_ioctl_pointer(fd, VIDIOC_STREAMON, VarPtr(i)) Loop Until r > -1 If r = -1 Then Error.Raise("Errore !") Do tv.tv_sec = 1 r = select_C(fd + 1, Null, Null, Null, tv) Loop 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 Do r = v4l2_ioctl_pointer(fd, VIDIOC_DQBUF, p) Loop 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 p = buffers[index].start Seek #fex, 15 For i = 0 To byte_usati - 1 Write #fex, Byte@(p) As Byte p = p + 1 Next fex.Close st.Close ' In chiusura: i = V4L2_BUF_TYPE_VIDEO_CAPTURE Do r = v4l2_ioctl_pointer(fd, VIDIOC_STREAMON, VarPtr(i)) Loop 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) exit_C(0) End