V4L2命令之 VIDIOC_ENUM_FMT VIDIOC_ENUM_FRAMESIZES VIDIOC_ENUM_FRAMEINTERVALS
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VIDIOC_ENUM_FMT
Name
VIDIOC_ENUM_FMT — Enumerate image formats // 列举设备所支持的格式
Synopsis
int ioctl( |
int fd, |
| int request, | |
struct v4l2_fmtdesc*argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
VIDIOC_ENUM_FMT
-
argp -
Description
To enumerate image formats applications initialize thetype and index field of struct v4l2_fmtdesc and call theVIDIOC_ENUM_FMT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code. All formats are enumerable by beginning at index zero and incrementing by one until EINVAL is returned.
Note that after switching input or output the list of enumerated image formats may be different.
Table A.24. struct v4l2_fmtdesc
| __u32 | index |
Number of the format in the enumeration, set by the application. This is in no way related to the pixelformat field. |
| __u32 | type |
Type of the data stream, set by the application.Only these types are valid here:V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE and V4L2_BUF_TYPE_VIDEO_OVERLAY. See Table 3.3, “enum v4l2_buf_type”. |
| __u32 | flags |
See Table A.25, “Image Format Description Flags” |
| __u8 | description[32] |
Description of the format, a NUL-terminated ASCII string. This information is intended for the user, for example: "YUV4:2:2". |
| __u32 | pixelformat |
The image format identifier. This is a four character code as computed by the v4l2_fourcc() macro: |
#define v4l2_fourcc(a,b,c,d) (((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24))
Several image formats are already defined by this specification in Chapter 2, Image Formats. Note these codes are not the same as those used in the Windows world. |
||
| __u32 | reserved[4] |
Reserved for future extensions. Drivers must setthe array to zero. |
Table A.25. Image Format Description Flags
V4L2_FMT_FLAG_COMPRESSED |
0x0001 | This is a compressed format. |
V4L2_FMT_FLAG_EMULATED |
0x0002 | This format is not native to the device but emulated through software (usually libv4l2), where possible try to use a native format instead for better performance. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.
- EINVAL
-
The struct v4l2_fmtdesc
typeis not supported or theindexis out ofbounds.
例子：
int enum_frame_formats(int dev) // dev是利用open打开的设备文件描述符
{
int ret;
struct v4l2_fmtdesc fmt;
memset(&fmt, 0, sizeof(fmt));
fmt.index = 0;
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
while ((ret = ioctl(dev, VIDIOC_ENUM_FMT, &fmt)) == 0) {
fmt.index++;
printf("{ pixelformat = '%c%c%c%c', description = '%s' }\n",
fmt.pixelformat & 0xFF, (fmt.pixelformat >> 8) & 0xFF,
(fmt.pixelformat >> 16) & 0xFF, (fmt.pixelformat >> 24) & 0xFF,
fmt.description);
ret = enum_frame_sizes(dev, fmt.pixelformat); // 列举该格式下的帧大小
if (ret != 0)
printf(" Unable to enumerate frame sizes.\n");
}
if (errno != EINVAL) {
printf("ERROR enumerating frame formats: %d\n", errno);
return errno;
}
return 0;
}
VIDIOC_ENUM_FRAMESIZES
Name
VIDIOC_ENUM_FRAMESIZES — Enumerate frame sizes
Synopsis
int ioctl( |
int fd, |
| int request, | |
struct v4l2_frmsizeenum *argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
VIDIOC_ENUM_FRAMESIZES
-
argp -
Pointer to a struct v4l2_frmsizeenum that contains an index and pixel format and receives a frame width and height.
Description
This ioctl allows applications to enumerate all frame sizes(i. e. width and height in pixels) that the device supports for the given pixel format.
The supported pixel formats can be obtained by using theVIDIOC_ENUM_FMT function.
The return value and the content of thev4l2_frmsizeenum.type field depend on the type of frame sizes the device supports. Here are the semantics of thefunction for the different cases:
// 一般在调用这个命令后，通过v4l2_frmsizeenum 的 type 域来决定后续的操作，
-
Discrete: The function returns success if the given index value (zero-based) is valid. The application should increase the index by one for each call until
EINVALis returned. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_DISCRETEby the driver. Of the union only thediscretemember isvalid. -
Step-wise: The function returns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_STEPWISEby the driver. Of the union only thestepwisemember isvalid. -
Continuous: This is a special case of the step-wise type above. The function returns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmsizeenum.typefield is set toV4L2_FRMSIZE_TYPE_CONTINUOUSby the driver. Of the union only thestepwisemember is valid and thestep_widthandstep_heightvalues are set to 1.
When the application calls the function with index zero, it must check the type field to determine the type of frame size enumeration the device supports. Only for the V4L2_FRMSIZE_TYPE_DISCRETE type does it make sense to increase the index value to receive more frame sizes.
Note that the order in which the frame sizes are returned has no special meaning. In particular does it not say anything about potential default format sizes.
Applications can assume that the enumeration data does not change without any inter action from the application itself. This means that the enumeration data is consistent if the application does not perform any other ioctl calls while it runs the frame size enumeration.
Structs
In the structs below, IN denotes avalue that has to be filled in by the application,OUT denotes values that the driver fills in. The application should zero out all members except for theIN fields.
Table A.26. struct v4l2_frmsize_discrete
| __u32 | width |
Width of the frame [pixel]. |
| __u32 | height |
Height of the frame [pixel]. |
Table A.27. struct v4l2_frmsize_stepwise
| __u32 | min_width |
Minimum frame width [pixel]. |
| __u32 | max_width |
Maximum frame width [pixel]. |
| __u32 | step_width |
Frame width step size [pixel]. |
| __u32 | min_height |
Minimum frame height [pixel]. |
| __u32 | max_height |
Maximum frame height [pixel]. |
| __u32 | step_height |
Frame height step size [pixel]. |
Table A.28. struct v4l2_frmsizeenum
| __u32 | index |
IN: Index of the given frame size in the enumeration. | |
| __u32 | pixel_format |
IN: Pixel format for which the frame sizes are enumerated. | |
| __u32 | type |
OUT: Frame size type the device supports. | |
| union | OUT: Frame size with the given index. | ||
| struct v4l2_frmsize_discrete | discrete |
||
| struct v4l2_frmsize_stepwise | stepwise |
||
| __u32 | reserved[2] |
Reserved space for future use. |
Enums
Table A.29. enum v4l2_frmsizetypes
V4L2_FRMSIZE_TYPE_DISCRETE |
1 | Discrete frame size. |
V4L2_FRMSIZE_TYPE_CONTINUOUS |
2 | Continuous frame size. |
V4L2_FRMSIZE_TYPE_STEPWISE |
3 | Step-wise defined frame size. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.
例子：
int enum_frame_sizes(int dev, __u32 pixfmt)
{
int ret;
struct v4l2_frmsizeenum fsize;
memset(&fsize, 0, sizeof(fsize));
fsize.index = 0;
fsize.pixel_format = pixfmt;
while ((ret = ioctl(dev, VIDIOC_ENUM_FRAMESIZES, &fsize)) == 0) {
if (fsize.type == V4L2_FRMSIZE_TYPE_DISCRETE) {
printf("{ discrete: width = %u, height = %u }\n",
fsize.discrete.width, fsize.discrete.height);
ret = enum_frame_intervals(dev, pixfmt,
fsize.discrete.width, fsize.discrete.height); //查找设备支持的 帧的间隔时间
if (ret != 0)
printf(" Unable to enumerate frame sizes.\n");
} else if (fsize.type == V4L2_FRMSIZE_TYPE_CONTINUOUS) {
printf("{ continuous: min { width = %u, height = %u } .. "
"max { width = %u, height = %u } }\n",
fsize.stepwise.min_width, fsize.stepwise.min_height,
fsize.stepwise.max_width, fsize.stepwise.max_height);
printf(" Refusing to enumerate frame intervals.\n");
break;
} else if (fsize.type == V4L2_FRMSIZE_TYPE_STEPWISE) {
printf("{ stepwise: min { width = %u, height = %u } .. "
"max { width = %u, height = %u } / "
"stepsize { width = %u, height = %u } }\n",
fsize.stepwise.min_width, fsize.stepwise.min_height,
fsize.stepwise.max_width, fsize.stepwise.max_height,
fsize.stepwise.step_width, fsize.stepwise.step_height);
printf(" Refusing to enumerate frame intervals.\n");
break;
}
fsize.index++;
}
if (ret != 0 && errno != EINVAL) {
printf("ERROR enumerating frame sizes: %d\n", errno);
return errno;
}
return 0;
}
VIDIOC_ENUM_FRAMEINTERVALS
Name
VIDIOC_ENUM_FRAMEINTERVALS — Enumerate frame intervals
Synopsis
int ioctl( |
int fd, |
| int request, | |
struct v4l2_frmivalenum *argp); |
Arguments
-
fd -
File descriptor returned by
open(). -
request -
VIDIOC_ENUM_FRAMEINTERVALS
-
argp -
Pointer to a struct v4l2_frmivalenum structure that contains a pixel format and size and receives a frame interval.
Description
This ioctl allows applications to enumerate all frame intervals that the device supports for the given pixel format and frame size.
The supported pixel formats and frame sizes can be obtained by using the VIDIOC_ENUM_FMT and VIDIOC_ENUM_FRAMESIZES functions.
The return value and the content of thev4l2_frmivalenum.type field depend on thetype of frame intervals the device supports. Here are the semantics ofthe function for the different cases:
-
Discrete: The function returns success if the given index value (zero-based) is valid. The application should increase the index by one for each call until
EINVALis returned. The `v4l2_frmivalenum.type`field is set to `V4L2_FRMIVAL_TYPE_DISCRETE` by the driver. Of theunion only the `discrete` member is valid. -
Step-wise: The function returns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmivalenum.typefield is set toV4L2_FRMIVAL_TYPE_STEPWISEby the driver. Of the union only thestepwisemember isvalid. -
Continuous: This is a special case of the step-wise type above. The function returns success if the given index value is zero and
EINVALfor any other index value. Thev4l2_frmivalenum.typefield is set toV4L2_FRMIVAL_TYPE_CONTINUOUSby the driver. Of the union only thestepwisemember is valid and thestepvalue is set to 1.
When the application calls the function with index zero, it must check the type field to determine the type of frame interval enumeration the device supports. Only for the V4L2_FRMIVAL_TYPE_DISCRETE type does it make sense to increase the index value to receive more frame intervals.
Note that the order in which the frame intervals are returned has no special meaning. In particular does it not say anything about potential default frame intervals.
Applications can assume that the enumeration data does not change without any inter action from the application itself. This means that the enumeration data is consistent if the application does not perform any other ioctl calls while it runs the frame interval enumeration.
Notes
-
Frame intervals and framerates: The V4L2 API uses frame intervals instead of framerates. Given the frame interval the frame rate can be computed asfollows:
frame_rate = 1 / frame_interval - frame intervals 用分数表示
Structs
In the structs below, IN denotes avalue that has to be filled in by the application,OUT denotes values that the driver fills in. The application should zero out all members except for theIN fields.
Table A.30. struct v4l2_frmival_stepwise
| struct v4l2_fract | min |
Minimum frame interval [s]. |
| struct v4l2_fract | max |
Maximum frame interval [s]. |
| struct v4l2_fract | step |
Frame interval step size [s]. |
Table A.31. struct v4l2_frmivalenum
| __u32 | index |
IN: Index of the given frame interval in theenumeration. | |
| __u32 | pixel_format |
IN: Pixel format for which the frame intervals areenumerated. | |
| __u32 | width |
IN: Frame width for which the frame intervals areenumerated. | |
| __u32 | height |
IN: Frame height for which the frame intervals areenumerated. | |
| __u32 | type |
OUT: Frame interval type the device supports. | |
| union | OUT: Frame interval with the given index. | ||
| struct v4l2_fract | discrete |
Frame interval [s]. | |
| struct v4l2_frmival_stepwise | stepwise |
||
| __u32 | reserved[2] |
Reserved space for future use. |
Enums
Table A.32. enum v4l2_frmivaltypes
V4L2_FRMIVAL_TYPE_DISCRETE |
1 | Discrete frame interval. |
V4L2_FRMIVAL_TYPE_CONTINUOUS |
2 | Continuous frame interval. |
V4L2_FRMIVAL_TYPE_STEPWISE |
3 | Step-wise defined frame interval. |
Return Value
On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.
例子：
int enum_frame_intervals(int dev, __u32 pixfmt, __u32 width, __u32 height)
{
int ret;
struct v4l2_frmivalenum fival;
memset(&fival, 0, sizeof(fival));
fival.index = 0;
fival.pixel_format = pixfmt;
fival.width = width;
fival.height = height;
printf("\tTime interval between frame: ");
while ((ret = ioctl(dev, VIDIOC_ENUM_FRAMEINTERVALS, &fival)) == 0) {
if (fival.type == V4L2_FRMIVAL_TYPE_DISCRETE) {
printf("%u/%u, ",
fival.discrete.numerator, fival.discrete.denominator); //输出分数
} else if (fival.type == V4L2_FRMIVAL_TYPE_CONTINUOUS) {
printf("{min { %u/%u } .. max { %u/%u } }, ",
fival.stepwise.min.numerator, fival.stepwise.min.numerator,
fival.stepwise.max.denominator, fival.stepwise.max.denominator);
break;
} else if (fival.type == V4L2_FRMIVAL_TYPE_STEPWISE) {
printf("{min { %u/%u } .. max { %u/%u } / "
"stepsize { %u/%u } }, ",
fival.stepwise.min.numerator, fival.stepwise.min.denominator,
fival.stepwise.max.numerator, fival.stepwise.max.denominator,
fival.stepwise.step.numerator, fival.stepwise.step.denominator);
break;
}
fival.index++;
}
printf("\n");
if (ret != 0 && errno != EINVAL) {
printf("ERROR enumerating frame intervals: %d\n", errno);
return errno;
}
return 0;
}
上面的全部例子程序运行结果如下：
{ pixelformat = 'YUYV', description = 'YUV 4:2:2 (YUYV)' }
{ discrete: width = 640, height = 480 }
Time interval between frame: 1/30,
{ discrete: width = 352, height = 288 }
Time interval between frame: 1/30,
{ discrete: width = 320, height = 240 }
Time interval between frame: 1/30,
{ discrete: width = 176, height = 144 }
Time interval between frame: 1/30,
{ discrete: width = 160, height = 120 }
Time interval between frame: 1/30,