Catturare un'immagine con una webcam mediante le funzioni esterne del API di libv4l2
Da Gambas-it.org - Wikipedia.
Versione del 20 lug 2024 alle 09:05 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 libreria 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:
Public Struct Buffer
start As Pointer
length As Integer
End Struct
Library "libc:6"
Public Struct timeval
tv_sec As Long
tv_usec As Long
End Struct
Public Struct v4l2_requestbuffers
count As Integer
type As Integer
memory_ As Integer
capabilities As Integer
flags As Byte
reserved[3] As Byte
End Struct
Public Struct v4l2_timecode
type As Integer
flags As Integer
frames As Byte
seconds As Byte
minutes As Byte
hours As Byte
userbits[4] As Byte
End Struct
Public Struct v4l2_buffer
index As Integer
type As Integer
bytesused As Integer
flags As Integer
field As Integer
timestamp As Struct Timeval
timecode As Struct V4l2_timecode
sequence As Integer
memory_ As Integer
m As Long
length As Integer
reserved2 As Integer
request_fd As Integer
End Struct
Private Enum O_RDONLY = 0, O_WRONLY, O_RDWR, O_NONBLOCK = &2000
Private Enum PROT_NONE = 0, PROT_READ, PROT_WRITE
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, V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY, V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE,
V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE, V4L2_BUF_TYPE_SDR_CAPTURE, V4L2_BUF_TYPE_SDR_OUTPUT,
V4L2_BUF_TYPE_META_CAPTURE, V4L2_BUF_TYPE_META_OUTPUT
Private Enum V4L2_FIELD_ANY, V4L2_FIELD_NONE, V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM, V4L2_FIELD_INTERLACED,
V4L2_FIELD_SEQ_TB, V4L2_FIELD_SEQ_BT, V4L2_FIELD_ALTERNATE, V4L2_FIELD_INTERLACED_TB,
V4L2_FIELD_INTERLACED_BT
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 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_STREAMOFF As Long = 1074026003
Private Const VIDIOC_DQBUF As Long = 3227014673
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"
Library "libv4l2:0.0.0"
' 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_buffer(fd As Integer, request As Long, arg As V4l2_buffer) 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(start As Pointer, lengthI As Integer, prot As Integer, flags As Integer, fd As Integer, offset 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)
Public Sub Main()
Dim buf As New V4l2_buffer
Dim req As New V4l2_requestbuffers
Dim buffers As Buffer[]
Dim tv As New Timeval
Dim fout As File
Dim fmt, p As Pointer
Dim st As Stream
Dim fd, r, width, height, pix, type As Integer
Dim ppm As String
Write "\e[31m\e[5mAcquisizione immagini...\e[0m"
Flush
fd = v4l2_open("/dev/video0", O_RDWR Or O_NONBLOCK, 0)
fmt = Alloc(SizeOf(gb.Byte), 208)
st = Memory fmt 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, fmt)
If r = -1 Then Error.Raise("Errore !")
Until r > -1
Seek #st, 8
Read #st, width
Read #st, height
Read #st, pix
If (width <> 640) Or (height <> 480) Then Print "Attenzione: il dispositivo imposterà l'immagine alle dimensioni";; width; "x"; height
If pix <> V4L2_PIX_FMT_RGB24 Then Error.Raise("Libv4l non accetta il formato RGB24.\nImpossibile procedere !")
st.Close
Free(fmt)
With req
.count = 1
.type = V4L2_BUF_TYPE_VIDEO_CAPTURE
.memory_ = V4L2_MEMORY_MMAP
End With
Repeat
r = v4l2_ioctl_request(fd, VIDIOC_REQBUFS, req)
If r = -1 Then Error.Raise("Errore !")
Until r > -1
buffers = New Buffer[req.count]
For n_buffers As Integer = 0 To req.count - 1
With buf
.index = n_buffers
.type = V4L2_BUF_TYPE_VIDEO_CAPTURE
.memory_ = V4L2_MEMORY_MMAP
End With
Repeat
r = v4l2_ioctl_buffer(fd, VIDIOC_QUERYBUF, buf)
If r = -1 Then Error.Raise("Errore !")
Until r > -1
With buffers[n_buffers] = New Buffer
.length = buf.length
.start = v4l2_mmap(Null, buf.length, PROT_READ Or PROT_WRITE, MAP_SHARED, fd, buf.m)
End With
Next
For i As Integer = 0 To n_buffers - 1
With buf
.index = i
.type = V4L2_BUF_TYPE_VIDEO_CAPTURE
.memory_ = V4L2_MEMORY_MMAP
End With
Repeat
r = v4l2_ioctl_buffer(fd, VIDIOC_QBUF, buf)
If r = -1 Then Error.Raise("Errore !")
Until r > -1
Next
type = V4L2_BUF_TYPE_VIDEO_CAPTURE
Repeat
r = v4l2_ioctl_pointer(fd, VIDIOC_STREAMON, VarPtr(type))
If r = -1 Then Error.Raise("Errore !")
Until r > -1
Repeat
tv.tv_sec = 1
r = select_C(fd + 1, Null, Null, Null, tv)
If r = -1 Then Error.Raise("Errore !")
Until r > -1
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE
buf.memory_ = V4L2_MEMORY_MMAP
Repeat
r = v4l2_ioctl_buffer(fd, VIDIOC_DQBUF, buf)
If r = -1 Then Error.Raise("Errore !")
Until r > -1
ppm = "/tmp/V4L" &/ Format(Now, "dd-mm-yyyy_hh:nn:ss") & ".ppm"
' Scrive i dati d'intestazione del file immagine PPM:
File.Save(ppm, "P6\x0A" & CStr(width) & Chr(32) & CStr(height) & " 255\x0A")
fout = Open ppm For Write Append
p = buffers[buf.index].start
If p == 0 Then Error.Raise("Errore !")
' Aggiunge i dati immagine al file PPM:
Write #fout, p, buf.bytesused ' [nota 1]
fout.Close
' Va in chiusura:
i = V4L2_BUF_TYPE_VIDEO_CAPTURE
Repeat
r = v4l2_ioctl_pointer(fd, VIDIOC_STREAMOFF, VarPtr(i))
If r = -1 Then Error.Raise("Errore !")
Until r > -1
For i = 0 To n_buffers - 1
v4l2_munmap(buffers[i].start, buffers[i].length)
Next
v4l2_close(fd)
Write "\rAcquisite n. " & t & " immagini !"
End
Note
[1] Per questo uso del Puntatore con WRITE vedere: https://www.gambas-it.org/wiki/index.php/Write#Scrivere_in_un_file_i_dati_contenuti_in_un.27area_di_memoria_puntata_da_un_Puntatore
