Soon after the V4L API was added to the kernel it was criticised as too inflexible. In August 1998 Bill Dirks proposed a number of improvements and began to work on documentation, example drivers and applications. With the help of other volunteers this eventually became the V4L2 API, not just an extension but a replacement for the V4L API. However it took another four years and two stable kernel releases until the new API was finally accepted for inclusion into the kernel in its present form.
1998-08-20: First version.
1998-08-27: The select() function was introduced.
1998-09-10: New video standard interface.
1998-09-18: The VIDIOC_NONCAP ioctl was replaced by the otherwise meaningless O_TRUNC open() flag, and the aliases O_NONCAP and O_NOIO were defined. Applications can set this flag if they intend to access controls only, as opposed to capture applications which need exclusive access. The VIDEO_STD_XXX identifiers are now ordinals instead of flags, and the video_std_construct() helper function takes id and transmission arguments.
1998-09-28: Revamped video standard. Made video controls individually enumerable.
1998-10-02: The id field was removed from struct struct video_standard and the color subcarrier fields were renamed. The ioctl VIDIOC_QUERYSTD ioctl was renamed to ioctl VIDIOC_ENUMSTD, VIDIOC_G_INPUT to ioctl VIDIOC_ENUMINPUT. A first draft of the Codec API was released.
1998-11-08: Many minor changes. Most symbols have been renamed. Some material changes to struct v4l2_capability.
1998-11-12: The read/write directon of some ioctls was misdefined.
1998-11-14: V4L2_PIX_FMT_RGB24 changed to V4L2_PIX_FMT_BGR24, and V4L2_PIX_FMT_RGB32 changed to V4L2_PIX_FMT_BGR32. Audio controls are now accessible with the VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls under names starting with V4L2_CID_AUDIO. The V4L2_MAJOR define was removed from videodev.h since it was only used once in the videodev kernel module. The YUV422 and YUV411 planar image formats were added.
1998-11-28: A few ioctl symbols changed. Interfaces for codecs and video output devices were added.
1999-01-14: A raw VBI capture interface was added.
1999-01-19: The VIDIOC_NEXTBUF ioctl was removed.
1999-01-27: There is now one QBUF ioctl, VIDIOC_QWBUF and VIDIOC_QRBUF are gone. VIDIOC_QBUF takes a v4l2_buffer as a parameter. Added digital zoom (cropping) controls.
Added a v4l to V4L2 ioctl compatibility layer to videodev.c. Driver writers, this changes how you implement your ioctl handler. See the Driver Writer’s Guide. Added some more control id codes.
1999-03-18: Fill in the category and catname fields of v4l2_queryctrl objects before passing them to the driver. Required a minor change to the VIDIOC_QUERYCTRL handlers in the sample drivers.
1999-03-31: Better compatibility for v4l memory capture ioctls. Requires changes to drivers to fully support new compatibility features, see Driver Writer’s Guide and v4l2cap.c. Added new control IDs: V4L2_CID_HFLIP, _VFLIP. Changed V4L2_PIX_FMT_YUV422P to _YUV422P, and _YUV411P to _YUV411P.
1999-04-04: Added a few more control IDs.
1999-04-07: Added the button control type.
1999-05-02: Fixed a typo in videodev.h, and added the V4L2_CTRL_FLAG_GRAYED (later V4L2_CTRL_FLAG_GRABBED) flag.
1999-05-20: Definition of VIDIOC_G_CTRL was wrong causing a malfunction of this ioctl.
1999-06-05: Changed the value of V4L2_CID_WHITENESS.
Version 0.20 introduced a number of changes which were not backward compatible with 0.19 and earlier versions. Purpose of these changes was to simplify the API, while making it more extensible and following common Linux driver API conventions.
Some typos in V4L2_FMT_FLAG symbols were fixed. struct v4l2_clip was changed for compatibility with v4l. (1999-08-30)
V4L2_TUNER_SUB_LANG1 was added. (1999-09-05)
All ioctl() commands that used an integer argument now take a pointer to an integer. Where it makes sense, ioctls will return the actual new value in the integer pointed to by the argument, a common convention in the V4L2 API. The affected ioctls are: VIDIOC_PREVIEW, VIDIOC_STREAMON, VIDIOC_STREAMOFF, VIDIOC_S_FREQ, VIDIOC_S_INPUT, VIDIOC_S_OUTPUT, VIDIOC_S_EFFECT. For example
err = ioctl (fd, VIDIOC_XXX, V4L2_XXX);
becomes
int a = V4L2_XXX; err = ioctl(fd, VIDIOC_XXX, &a);
All the different get- and set-format commands were swept into one VIDIOC_G_FMT and VIDIOC_S_FMT ioctl taking a union and a type field selecting the union member as parameter. Purpose is to simplify the API by eliminating several ioctls and to allow new and driver private data streams without adding new ioctls.
This change obsoletes the following ioctls: VIDIOC_S_INFMT, VIDIOC_G_INFMT, VIDIOC_S_OUTFMT, VIDIOC_G_OUTFMT, VIDIOC_S_VBIFMT and VIDIOC_G_VBIFMT. The image format structure struct v4l2_format was renamed to struct v4l2_pix_format, while struct v4l2_format is now the envelopping structure for all format negotiations.
Similar to the changes above, the VIDIOC_G_PARM and VIDIOC_S_PARM ioctls were merged with VIDIOC_G_OUTPARM and VIDIOC_S_OUTPARM. A type field in the new struct v4l2_streamparm selects the respective union member.
This change obsoletes the VIDIOC_G_OUTPARM and VIDIOC_S_OUTPARM ioctls.
Control enumeration was simplified, and two new control flags were introduced and one dropped. The catname field was replaced by a group field.
Drivers can now flag unsupported and temporarily unavailable controls with V4L2_CTRL_FLAG_DISABLED and V4L2_CTRL_FLAG_GRABBED respectively. The group name indicates a possibly narrower classification than the category. In other words, there may be multiple groups within a category. Controls within a group would typically be drawn within a group box. Controls in different categories might have a greater separation, or may even appear in separate windows.
The struct v4l2_buffer timestamp was changed to a 64 bit integer, containing the sampling or output time of the frame in nanoseconds. Additionally timestamps will be in absolute system time, not starting from zero at the beginning of a stream. The data type name for timestamps is stamp_t, defined as a signed 64-bit integer. Output devices should not send a buffer out until the time in the timestamp field has arrived. I would like to follow SGI’s lead, and adopt a multimedia timestamping system like their UST (Unadjusted System Time). See http://web.archive.org/web/*/http://reality.sgi.com /cpirazzi_engr/lg/time/intro.html. UST uses timestamps that are 64-bit signed integers (not struct timeval’s) and given in nanosecond units. The UST clock starts at zero when the system is booted and runs continuously and uniformly. It takes a little over 292 years for UST to overflow. There is no way to set the UST clock. The regular Linux time-of-day clock can be changed periodically, which would cause errors if it were being used for timestamping a multimedia stream. A real UST style clock will require some support in the kernel that is not there yet. But in anticipation, I will change the timestamp field to a 64-bit integer, and I will change the v4l2_masterclock_gettime() function (used only by drivers) to return a 64-bit integer.
A sequence field was added to struct v4l2_buffer. The sequence field counts captured frames, it is ignored by output devices. When a capture driver drops a frame, the sequence number of that frame is skipped.
1999-12-23: In struct v4l2_vbi_format the reserved1 field became offset. Previously drivers were required to clear the reserved1 field.
2000-01-13: The V4L2_FMT_FLAG_NOT_INTERLACED flag was added.
2000-07-31: The linux/poll.h header is now included by videodev.h for compatibility with the original videodev.h file.
2000-11-20: V4L2_TYPE_VBI_OUTPUT and V4L2_PIX_FMT_Y41P were added.
2000-11-25: V4L2_TYPE_VBI_INPUT was added.
2000-12-04: A couple typos in symbol names were fixed.
2001-01-18: To avoid namespace conflicts the fourcc macro defined in the videodev.h header file was renamed to v4l2_fourcc.
2001-01-25: A possible driver-level compatibility problem between the videodev.h file in Linux 2.4.0 and the videodev.h file included in the videodevX patch was fixed. Users of an earlier version of videodevX on Linux 2.4.0 should recompile their V4L and V4L2 drivers.
2001-01-26: A possible kernel-level incompatibility between the videodev.h file in the videodevX patch and the videodev.h file in Linux 2.2.x with devfs patches applied was fixed.
2001-03-02: Certain V4L ioctls which pass data in both direction although they are defined with read-only parameter, did not work correctly through the backward compatibility layer. [Solution?]
2001-04-13: Big endian 16-bit RGB formats were added.
2001-09-17: New YUV formats and the VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCY ioctls were added. (The old VIDIOC_G_FREQ and VIDIOC_S_FREQ ioctls did not take multiple tuners into account.)
2000-09-18: V4L2_BUF_TYPE_VBI was added. This may break compatibility as the VIDIOC_G_FMT and VIDIOC_S_FMT ioctls may fail now if the struct struct v4l2_fmt type field does not contain V4L2_BUF_TYPE_VBI. In the documentation of the struct v4l2_vbi_format offset field the ambiguous phrase “rising edge” was changed to “leading edge”.
A number of changes were made to the raw VBI interface.
Added sliced VBI interface proposal.
Around October-November 2002, prior to an announced feature freeze of Linux 2.5, the API was revised, drawing from experience with V4L2 0.20. This unnamed version was finally merged into Linux 2.5.46.
As specified in Related Devices, drivers must make related device functions available under all minor device numbers.
The open() function requires access mode O_RDWR regardless of the device type. All V4L2 drivers exchanging data with applications must support the O_NONBLOCK flag. The O_NOIO flag, a V4L2 symbol which aliased the meaningless O_TRUNC to indicate accesses without data exchange (panel applications) was dropped. Drivers must stay in “panel mode” until the application attempts to initiate a data exchange, see Opening and Closing Devices.
The struct v4l2_capability changed dramatically. Note that also the size of the structure changed, which is encoded in the ioctl request code, thus older V4L2 devices will respond with an EINVAL error code to the new ioctl VIDIOC_QUERYCAP ioctl.
There are new fields to identify the driver, a new RDS device function V4L2_CAP_RDS_CAPTURE, the V4L2_CAP_AUDIO flag indicates if the device has any audio connectors, another I/O capability V4L2_CAP_ASYNCIO can be flagged. In response to these changes the type field became a bit set and was merged into the flags field. V4L2_FLAG_TUNER was renamed to V4L2_CAP_TUNER, V4L2_CAP_VIDEO_OVERLAY replaced V4L2_FLAG_PREVIEW and V4L2_CAP_VBI_CAPTURE and V4L2_CAP_VBI_OUTPUT replaced V4L2_FLAG_DATA_SERVICE. V4L2_FLAG_READ and V4L2_FLAG_WRITE were merged into V4L2_CAP_READWRITE.
The redundant fields inputs, outputs and audios were removed. These properties can be determined as described in Video Inputs and Outputs and Audio Inputs and Outputs.
The somewhat volatile and therefore barely useful fields maxwidth, maxheight, minwidth, minheight, maxframerate were removed. This information is available as described in Data Formats and Video Standards.
V4L2_FLAG_SELECT was removed. We believe the select() function is important enough to require support of it in all V4L2 drivers exchanging data with applications. The redundant V4L2_FLAG_MONOCHROME flag was removed, this information is available as described in Data Formats.
In struct v4l2_input the assoc_audio field and the capability field and its only flag V4L2_INPUT_CAP_AUDIO was replaced by the new audioset field. Instead of linking one video input to one audio input this field reports all audio inputs this video input combines with.
New fields are tuner (reversing the former link from tuners to video inputs), std and status.
Accordingly struct v4l2_output lost its capability and assoc_audio fields. audioset, modulator and std where added instead.
The struct v4l2_audio field audio was renamed to index, for consistency with other structures. A new capability flag V4L2_AUDCAP_STEREO was added to indicated if the audio input in question supports stereo sound. V4L2_AUDCAP_EFFECTS and the corresponding V4L2_AUDMODE flags where removed. This can be easily implemented using controls. (However the same applies to AVL which is still there.)
Again for consistency the struct v4l2_audioout field audio was renamed to index.
The struct v4l2_tuner input field was replaced by an index field, permitting devices with multiple tuners. The link between video inputs and tuners is now reversed, inputs point to their tuner. The std substructure became a simple set (more about this below) and moved into struct v4l2_input. A type field was added.
Accordingly in struct v4l2_modulator the output was replaced by an index field.
In struct v4l2_frequency the port field was replaced by a tuner field containing the respective tuner or modulator index number. A tuner type field was added and the reserved field became larger for future extensions (satellite tuners in particular).
The idea of completely transparent video standards was dropped. Experience showed that applications must be able to work with video standards beyond presenting the user a menu. Instead of enumerating supported standards with an ioctl applications can now refer to standards by v4l2_std_id and symbols defined in the videodev2.h header file. For details see Video Standards. The VIDIOC_G_STD and VIDIOC_S_STD now take a pointer to this type as argument. ioctl VIDIOC_QUERYSTD was added to autodetect the received standard, if the hardware has this capability. In struct v4l2_standard an index field was added for ioctl VIDIOC_ENUMSTD. A v4l2_std_id field named id was added as machine readable identifier, also replacing the transmission field. The misleading framerate field was renamed to frameperiod. The now obsolete colorstandard information, originally needed to distguish between variations of standards, were removed.
Struct v4l2_enumstd ceased to be. ioctl VIDIOC_ENUMSTD now takes a pointer to a struct v4l2_standard directly. The information which standards are supported by a particular video input or output moved into struct v4l2_input and struct v4l2_output fields named std, respectively.
The struct v4l2_queryctrl fields category and group did not catch on and/or were not implemented as expected and therefore removed.
The VIDIOC_TRY_FMT ioctl was added to negotiate data formats as with VIDIOC_S_FMT, but without the overhead of programming the hardware and regardless of I/O in progress.
In struct v4l2_format the fmt union was extended to contain struct v4l2_window. All image format negotiations are now possible with VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT; ioctl. The VIDIOC_G_WIN and VIDIOC_S_WIN ioctls to prepare for a video overlay were removed. The type field changed to type enum v4l2_buf_type and the buffer type names changed as follows.
| Old defines | enum v4l2_buf_type |
|---|---|
| V4L2_BUF_TYPE_CAPTURE | V4L2_BUF_TYPE_VIDEO_CAPTURE |
| V4L2_BUF_TYPE_CODECIN | Omitted for now |
| V4L2_BUF_TYPE_CODECOUT | Omitted for now |
| V4L2_BUF_TYPE_EFFECTSIN | Omitted for now |
| V4L2_BUF_TYPE_EFFECTSIN2 | Omitted for now |
| V4L2_BUF_TYPE_EFFECTSOUT | Omitted for now |
| V4L2_BUF_TYPE_VIDEOOUT | 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_PRIVATE_BASE | V4L2_BUF_TYPE_PRIVATE (but this is deprecated) |
In struct v4l2_fmtdesc a enum v4l2_buf_type field named type was added as in struct v4l2_format. The VIDIOC_ENUM_FBUFFMT ioctl is no longer needed and was removed. These calls can be replaced by ioctl VIDIOC_ENUM_FMT with type V4L2_BUF_TYPE_VIDEO_OVERLAY.
In struct v4l2_pix_format the depth field was removed, assuming applications which recognize the format by its four-character-code already know the color depth, and others do not care about it. The same rationale lead to the removal of the V4L2_FMT_FLAG_COMPRESSED flag. The V4L2_FMT_FLAG_SWCONVECOMPRESSED flag was removed because drivers are not supposed to convert images in kernel space. A user library of conversion functions should be provided instead. The V4L2_FMT_FLAG_BYTESPERLINE flag was redundant. Applications can set the bytesperline field to zero to get a reasonable default. Since the remaining flags were replaced as well, the flags field itself was removed.
The interlace flags were replaced by a enum v4l2_field value in a newly added field field.
| Old flag | enum v4l2_field |
|---|---|
| V4L2_FMT_FLAG_NOT_INTERLACED | ? |
| V4L2_FMT_FLAG_INTERLACED = V4L2_FMT_FLAG_COMBINED | V4L2_FIELD_INTERLACED |
| V4L2_FMT_FLAG_TOPFIELD = V4L2_FMT_FLAG_ODDFIELD | V4L2_FIELD_TOP |
| V4L2_FMT_FLAG_BOTFIELD = V4L2_FMT_FLAG_EVENFIELD | V4L2_FIELD_BOTTOM |
| - | V4L2_FIELD_SEQ_TB |
| - | V4L2_FIELD_SEQ_BT |
| - | V4L2_FIELD_ALTERNATE |
The color space flags were replaced by a enum v4l2_colorspace value in a newly added colorspace field, where one of V4L2_COLORSPACE_SMPTE170M, V4L2_COLORSPACE_BT878, V4L2_COLORSPACE_470_SYSTEM_M or V4L2_COLORSPACE_470_SYSTEM_BG replaces V4L2_FMT_CS_601YUV.
In struct v4l2_requestbuffers the type field was properly defined as enum v4l2_buf_type. Buffer types changed as mentioned above. A new memory field of type enum v4l2_memory was added to distinguish between I/O methods using buffers allocated by the driver or the application. See Input/Output for details.
In struct v4l2_buffer the type field was properly defined as enum v4l2_buf_type. Buffer types changed as mentioned above. A field field of type enum v4l2_field was added to indicate if a buffer contains a top or bottom field. The old field flags were removed. Since no unadjusted system time clock was added to the kernel as planned, the timestamp field changed back from type stamp_t, an unsigned 64 bit integer expressing the sample time in nanoseconds, to struct timeval. With the addition of a second memory mapping method the offset field moved into union m, and a new memory field of type enum v4l2_memory was added to distinguish between I/O methods. See Input/Output for details.
The V4L2_BUF_REQ_CONTIG flag was used by the V4L compatibility layer, after changes to this code it was no longer needed. The V4L2_BUF_ATTR_DEVICEMEM flag would indicate if the buffer was indeed allocated in device memory rather than DMA-able system memory. It was barely useful and so was removed.
In struct v4l2_framebuffer the base[3] array anticipating double- and triple-buffering in off-screen video memory, however without defining a synchronization mechanism, was replaced by a single pointer. The V4L2_FBUF_CAP_SCALEUP and V4L2_FBUF_CAP_SCALEDOWN flags were removed. Applications can determine this capability more accurately using the new cropping and scaling interface. The V4L2_FBUF_CAP_CLIPPING flag was replaced by V4L2_FBUF_CAP_LIST_CLIPPING and V4L2_FBUF_CAP_BITMAP_CLIPPING.
In struct v4l2_clip the x, y, width and height field moved into a c substructure of type struct v4l2_rect. The x and y fields were renamed to left and top, i. e. offsets to a context dependent origin.
In struct v4l2_window the x, y, width and height field moved into a w substructure as above. A field field of type v4l2_field was added to distinguish between field and frame (interlaced) overlay.
The digital zoom interface, including struct struct v4l2_zoomcap, struct struct v4l2_zoom, V4L2_ZOOM_NONCAP and V4L2_ZOOM_WHILESTREAMING was replaced by a new cropping and scaling interface. The previously unused struct struct v4l2_cropcap and struct v4l2_crop where redefined for this purpose. See Image Cropping, Insertion and Scaling for details.
In struct v4l2_vbi_format the SAMPLE_FORMAT field now contains a four-character-code as used to identify video image formats and V4L2_PIX_FMT_GREY replaces the V4L2_VBI_SF_UBYTE define. The reserved field was extended.
In struct v4l2_captureparm the type of the timeperframe field changed from unsigned long to struct v4l2_fract. This allows the accurate expression of multiples of the NTSC-M frame rate 30000 / 1001. A new field readbuffers was added to control the driver behaviour in read I/O mode.
Similar changes were made to struct v4l2_outputparm.
The struct v4l2_performance and VIDIOC_G_PERF ioctl were dropped. Except when using the read/write I/O method, which is limited anyway, this information is already available to applications.
The example transformation from RGB to YCbCr color space in the old V4L2 documentation was inaccurate, this has been corrected in Image Formats.
A new capability flag V4L2_CAP_RADIO was added for radio devices. Prior to this change radio devices would identify solely by having exactly one tuner whose type field reads V4L2_TUNER_RADIO.
An optional driver access priority mechanism was added, see Application Priority for details.
The audio input and output interface was found to be incomplete.
Previously the VIDIOC_G_AUDIO ioctl would enumerate the available audio inputs. An ioctl to determine the current audio input, if more than one combines with the current video input, did not exist. So VIDIOC_G_AUDIO was renamed to VIDIOC_G_AUDIO_OLD, this ioctl was removed on Kernel 2.6.39. The ioctl VIDIOC_ENUMAUDIO ioctl was added to enumerate audio inputs, while VIDIOC_G_AUDIO now reports the current audio input.
The same changes were made to VIDIOC_G_AUDOUT and VIDIOC_ENUMAUDOUT.
Until further the “videodev” module will automatically translate between the old and new ioctls, but drivers and applications must be updated to successfully compile again.
The ioctl VIDIOC_OVERLAY ioctl was incorrectly defined with write-read parameter. It was changed to write-only, while the write-read version was renamed to VIDIOC_OVERLAY_OLD. The old ioctl was removed on Kernel 2.6.39. Until further the “videodev” kernel module will automatically translate to the new version, so drivers must be recompiled, but not applications.
Video Overlay Interface incorrectly stated that clipping rectangles define regions where the video can be seen. Correct is that clipping rectangles define regions where no video shall be displayed and so the graphics surface can be seen.
The VIDIOC_S_PARM and VIDIOC_S_CTRL ioctls were defined with write-only parameter, inconsistent with other ioctls modifying their argument. They were changed to write-read, while a _OLD suffix was added to the write-only versions. The old ioctls were removed on Kernel 2.6.39. Drivers and applications assuming a constant parameter need an update.
In RGB Formats the following pixel formats were incorrectly transferred from Bill Dirks’ V4L2 specification. Descriptions below refer to bytes in memory, in ascending address order.
| Symbol | In this document prior to revision 0.5 | Corrected |
|---|---|---|
| V4L2_PIX_FMT_RGB24 | B, G, R | R, G, B |
| V4L2_PIX_FMT_BGR24 | R, G, B | B, G, R |
| V4L2_PIX_FMT_RGB32 | B, G, R, X | R, G, B, X |
| V4L2_PIX_FMT_BGR32 | R, G, B, X | B, G, R, X |
The V4L2_PIX_FMT_BGR24 example was always correct.
In Image Properties the mapping of the V4L VIDEO_PALETTE_RGB24 and VIDEO_PALETTE_RGB32 formats to V4L2 pixel formats was accordingly corrected.
Unrelated to the fixes above, drivers may still interpret some V4L2 RGB pixel formats differently. These issues have yet to be addressed, for details see RGB Formats.
The capture example in Video Capture Example called the VIDIOC_S_CROP ioctl without checking if cropping is supported. In the video standard selection example in Video Standards the VIDIOC_S_STD call used the wrong argument type.
Two new field orders V4L2_FIELD_INTERLACED_TB and V4L2_FIELD_INTERLACED_BT were added. See v4l2_field for details.
Three new clipping/blending methods with a global or straight or inverted local alpha value were added to the video overlay interface. See the description of the VIDIOC_G_FBUF and VIDIOC_S_FBUF ioctls for details.
A new global_alpha field was added to v4l2_window, extending the structure. This may break compatibility with applications using a struct struct v4l2_window directly. However the VIDIOC_G/S/TRY_FMT ioctls, which take a pointer to a v4l2_format parent structure with padding bytes at the end, are not affected.
The format of the chromakey field in struct v4l2_window changed from “host order RGB32” to a pixel value in the same format as the framebuffer. This may break compatibility with existing applications. Drivers supporting the “host order RGB32” format are not known.
VIDIOC_QUERYCAP now returns a per-subsystem version instead of a per-driver one.
Standardize an error code for invalid ioctl.
Added V4L2_CTRL_TYPE_BITMASK.
The X Video Extension (abbreviated XVideo or just Xv) is an extension of the X Window system, implemented for example by the XFree86 project. Its scope is similar to V4L2, an API to video capture and output devices for X clients. Xv allows applications to display live video in a window, send window contents to a TV output, and capture or output still images in XPixmaps [1]. With their implementation XFree86 makes the extension available across many operating systems and architectures.
Because the driver is embedded into the X server Xv has a number of advantages over the V4L2 video overlay interface. The driver can easily determine the overlay target, i. e. visible graphics memory or off-screen buffers for a destructive overlay. It can program the RAMDAC for a non-destructive overlay, scaling or color-keying, or the clipping functions of the video capture hardware, always in sync with drawing operations or windows moving or changing their stacking order.
To combine the advantages of Xv and V4L a special Xv driver exists in XFree86 and XOrg, just programming any overlay capable Video4Linux device it finds. To enable it /etc/X11/XF86Config must contain these lines:
Section "Module"
Load "v4l"
EndSection
As of XFree86 4.2 this driver still supports only V4L ioctls, however it should work just fine with all V4L2 devices through the V4L2 backward-compatibility layer. Since V4L2 permits multiple opens it is possible (if supported by the V4L2 driver) to capture video while an X client requested video overlay. Restrictions of simultaneous capturing and overlay are discussed in Video Overlay Interface apply.
Only marginally related to V4L2, XFree86 extended Xv to support hardware YUV to RGB conversion and scaling for faster video playback, and added an interface to MPEG-2 decoding hardware. This API is useful to display images captured with V4L2 devices.
V4L2 does not support digital terrestrial, cable or satellite broadcast. A separate project aiming at digital receivers exists. You can find its homepage at https://linuxtv.org. The Linux DVB API has no connection to the V4L2 API except that drivers for hybrid hardware may support both.
[to do - OSS/ALSA]
The following V4L2 API elements are currently experimental and may change in the future.
The following V4L2 API elements were superseded by new interfaces and should not be implemented in new drivers.
| [1] | This is not implemented in XFree86. |