Linux视频编程必备：V4L2 API详解与实例

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“V4L2_API_Specification” 《Video for Linux Two 2.6.32版》是由Bill Dirks编写的文档，详细阐述了Linux系统中的V4L2（Video for Linux Two）API的使用方法，包含了丰富的示例程序，对进行Linux图像编程的开发者来说是不可或缺的参考资料。该文档对于深入了解图形图像处理技术具有很高的价值。 V4L2是Linux内核中的一个接口，用于支持视频捕获、视频输出、视频编码解码以及数字电视接收等多媒体功能。V4L2 API提供了与硬件设备交互的一系列函数调用，使得开发者能够方便地实现视频流的捕获、处理和显示。这个API在不断演进中，以适应更多新型硬件的需求和多媒体应用的复杂性。在文档中，作者们如Hans Verkuil、Martin Rubli、Andy Walls和Mauro Carvalho Chehab等对多个关键特性进行了设计和文档化。例如： 1. VIDIOC_LOG_STATUS ioctl：由Hans Verkuil设计和文档化的这个ioctl调用，允许应用程序获取驱动程序的状态信息，这对于调试和理解设备行为非常有用。 2. 扩展控制ioctls：这些ioctls扩展了V4L2的功能，使得开发者可以更加灵活地控制视频设备的参数，如亮度、对比度、饱和度等。 3. Sliced VBI API：Hans Verkuil也负责了sliced垂直消隐间隔（Vertical Blanking Interval, VBI）数据的API设计，这在处理电视信号中嵌入的辅助信息，如Teletext或 Closed Captioning时至关重要。 4. VIDIOC_ENUM_FRAMESIZES和VIDIOC_ENUM_FRAMEINTERVALS ioctls：由Martin Rubli设计和文档化，这些ioctls用于枚举设备支持的帧尺寸和帧率，帮助开发者选择最佳的视频设置。 5. 字幕和元数据支持：Andy Walls贡献了关于V4L2_MPEG_STREAM_VBI_FMT_IVTV MPEG流中嵌入的sliced VBI数据格式的文档，这对于处理含有字幕和其他元数据的视频流非常有帮助。 6. libv4l库和示例代码：Mauro Carvalho Chehab不仅文档化了libv4l库，还设计并添加了v4l2grab示例程序，帮助用户理解和使用V4L2 API，并提供了远程控制器章节，增强了开发者对硬件控制的理解。此外，V4L2 API规范遵循GNU Free Documentation License，允许自由复制、分发和修改，以促进知识共享和开源社区的发展。这个文档不仅是Linux图像编程的基石，也是驱动开发、媒体处理和嵌入式系统设计者的重要参考资料。

Introduction

Video For Linux Two is the second version of the Video For Linux API, a kernel interface for analog radio and video capture and

output drivers.

Early drivers used ad-hoc interfaces. These were replaced in Linux 2.2 by Alan Cox' V4L API, based on the interface of the bttv

driver. In 1999 Bill Dirks started the development of V4L2 to fix some shortcomings of V4L and to support a wider range of devices.

The API was revised again in 2002 prior to its inclusion in Linux 2.5/2.6, and work continues on improvements and additions while

maintaining compatibility with existing drivers and applications. In 2006/2007 efforts began on FreeBSD drivers with a V4L2

interface.

This book documents the V4L2 API. Intended audience are driver and application writers.

If you have questions or ideas regarding the API, please write to the linux-media mailing list: https://linuxtv.org/lists.php.

The latest version of this document and the DocBook SGML sources are part of the https://linuxtv.org/repo/ repository. The online

version is available here: https://linuxtv.org/downloads/video4linux/API/V4L2_API.

Chapter 1. Common API Elements

Programming a V4L2 device consists of these steps:

 Opening the device

 Changing device properties, selecting a video and audio input, video standard, picture brightness a. o.

 Negotiating a data format

 Negotiating an input/output method

 The actual input/output loop

 Closing the device

When an application attempts to open a device special file with major number 81 and minor number 0, 1, or 64, load

"mydriver" (and the "videodev" module it depends upon).

device with minor number 64 and 65 (base 64).

When no minor number is given as module option the driver supplies a default. Chapter 4, Interfaces recommends the base minor

numbers to be used for the various device types. Obviously minor numbers must be unique. When the number is already in use

the offending device will not be registered.

By convention system administrators create various character device special files with these major and minor numbers in

the /dev directory. The names recomended for the different V4L2 device types are listed in Chapter 4, Interfaces.

The creation of character special files (with mknod) is a privileged operation and devices cannot be opened by major and minor

number. That means applications cannot reliable scan for loaded or installed drivers. The user must enter a device name, or the

application can try the conventional device names.

Under the device filesystem (devfs) the minor number options are ignored. V4L2 drivers (or by proxy the "videodev" module)

automatically create the required device files in the /dev/v4ldirectory using the conventional device names above.

1.1.2. Related Devices

Devices can support several related functions. For example video capturing, video overlay and VBI capturing are related because

these functions share, amongst other, the same video input and tuner frequency. V4L and earlier versions of V4L2 used the same

device name and minor number for video capturing and overlay, but different ones for VBI. Experience showed this approach has

several problems

[3]

, and to make things worse the V4L videodev module used to prohibit multiple opens of a device.

As a remedy the present version of the V4L2 API relaxed the concept of device types with specific names and minor numbers. For

compatibility with old applications drivers must still register different minor numbers to assign a default function to the device. But if

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君乙杨

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