基于UVC的USB摄像头的移植很简单，只需要在menuconfig中进行适当的配置即可，如下所示：
Device Drivers --->
<*> Multimedia support --->
[*] Video capture adapters--->
[*] V4L USBdevices --->
<*> USB Video Class (UVC)
[*] UVC
input events devicesupport
<*> GSPCA based webcams --->
[*] USB support --->
[*] USB announce
newdevices
<*>
USB Gadget Support--->
<*> USB
Webcam Gadget
经过上述配置以后的系统就能够正确识别出UVC的USB摄像头。把我的罗技C525摄像头插到开发板上，就会显示下列信息：
usb 1-1: new full-speed USB device number 2using s3c2410-ohci
usb 1-1: New USB device found,idVendor=046d, idProduct=0826
usb 1-1: New USB device strings: Mfr=0,Product=2, SerialNumber=1
usb 1-1: Product: HD Webcam C525
usb 1-1: SerialNumber: 19E19A50
uvcvideo: Found UVC 1.00 device HD WebcamC525 (046d:0826)
input: HD Webcam C525 as/devices/platform/s3c2410-ohci/usb1/1-1/1-1:1.2/input/input3
并且在/dev目录下，会自动创建video0设备节点，这就说明该摄像头可以正常使用了。
下面就介绍对摄像头的测试，在这里我们是利用摄像头采集视频信号并在LCD上实时显示。要完成上述工作，需要解决三个关键问题。一是V4L2编程；二是LCD的显示；三是彩色空间的转换。
V4L2是内核提供给应用程序访问音、视频驱动的统一接口。它的基本工作流程为：
打开设备文件--->查询设备属性--->设置视频格式--->请求分配内存，并映射到用户空间--->申请到的内存入队列，开始视频采集--->内存出队列，得到原始数据，这时就可以对原始数据进行适当处理--->内存重新入队列，以便循环--->停止采集--->关闭设备。
LCD的显示较简单，主要就是把framebuffer映射到用户空间，对该空间进行操作即可。
摄像头输出的主要格式是YUV422，该格式不能直接用于LCD的显示，所以需要彩色空间的转换，即把它转换为RGB格式。转换的公式其实很简单，但考虑到用于嵌入式系统，因此需要一些优化处理。
下面就给出一段完整的视频显示的程序，我的开发板的LCD分辨率为320×240，彩色空间为RGB32，摄像头输出设置为176×144。
#include <unistd.h>
#include <linux/fb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <getopt.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <malloc.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <asm/types.h>
#include <linux/videodev2.h>
#define CAMERA_DEVICE "/dev/video0"
#define FB_DEVICE "/dev/fb0"
#define VIDEO_WIDTH 176
#define VIDEO_HEIGHT 144
#define BUFFER_COUNT 4
const int rvarrxyp[]={
-180,-179,-177,-176,-175,-173,-172,-170,-169,-167,-166,-165,-163,-162,-160,-159,-158,-156,-155,-153,-152,-151,
-149,-148,-146,-145,-144,-142,-141,-139,-138,-137,-135,-134,-132,-131,-129,-128,-127,-125,-124,-122,-121,-120,
-118,-117,-115,-114,-113,-111,-110,-108,-107,-106,-104,-103,-101,-100,-99,-97,-96,-94,-93,-91,-90,-89,-87,-86,
-84,-83,-82,-80,-79,-77,-76,-75,-73,-72,-70,-69,-68,-66,-65,-63,-62,-61,-59,-58,-56,-55,-53,-52,-51,-49,-48,
-46,-45,-44,-42,-41,-39,-38,-37,-35,-34,-32,-31,-30,-28,-27,-25,-24,-23,-21,-20,-18,-17,-15,-14,-13,-11,-10,
-8,-7,-6,-4,-3,-1,0,1,3,4,6,7,8,10,11,13,14,15,17,18,20,21,23,24,25,27,28,30,31,32,34,35,37,38,39,41,42,44,45,
46,48,49,51,52,53,55,56,58,59,61,62,63,65,66,68,69,70,72,73,75,76,77,79,80,82,83,84,86,87,89,90,91,93,94,96,
97,99,100,101,103,104,106,107,108,110,111,113,114,115,117,118,120,121,122,124,125,127,128,129,131,132,134,135,
137,138,139,141,142,144,145,146,148,149,151,152,153,155,156,158,159,160,162,163,165,166,167,169,170,172,173,175,
176,177,179};
const int guarrxyp[]={
-44,-44,-44,-43,-43,-42,-42,-42,-41,-41,-41,-40,-40,-40,-39,-39,-39,-38,-38,-38,-37,-37,-37,-36,-36,-36,-35,-35,
-35,-34,-34,-34,-33,-33,-32,-32,-32,-31,-31,-31,-30,-30,-30,-29,-29,-29,-28,-28,-28,-27,-27,-27,-26,-26,-26,-25,
-25,-25,-24,-24,-23,-23,-23,-22,-22,-22,-21,-21,-21,-20,-20,-20,-19,-19,-19,-18,-18,-18,-17,-17,-17,-16,-16,-16,
-15,-15,-15,-14,-14,-13,-13,-13,-12,-12,-12,-11,-11,-11,-10,-10,-10,-9,-9,-9,-8,-8,-8,-7,-7,-7,-6,-6,-6,-5,-5,-4,
-4,-4,-3,-3,-3,-2,-2,-2,-1,-1,-1,0,0,0,1,1,1,2,2,2,3,3,3,4,4,4,5,5,6,6,6,7,7,7,8,8,8,9,9,9,10,10,10,11,11,11,12,12,
12,13,13,13,14,14,15,15,15,16,16,16,17,17,17,18,18,18,19,19,19,20,20,20,21,21,21,22,22,22,23,23,23,24,24,25,25,25,
26,26,26,27,27,27,28,28,28,29,29,29,30,30,30,31,31,31,32,32,32,33,33,34,34,34,35,35,35,36,36,36,37,37,37,38,38,38,
39,39,39,40,40,40,41,41,41,42,42,42,43,43,44,44};
const int gvarrxyp[]={
-92,-91,-90,-90,-89,-88,-87,-87,-86,-85,-85,-84,-83,-82,-82,-81,-80,-80,-79,-78,-77,-77,-76,-75,-75,-74,-73,-72,
-72,-71,-70,-70,-69,-68,-67,-67,-66,-65,-65,-64,-63,-62,-62,-61,-60,-60,-59,-58,-57,-57,-56,-55,-54,-54,-53,-52,-52,
-51,-50,-49,-49,-48,-47,-47,-46,-45,-44,-44,-43,-42,-42,-41,-40,-39,-39,-38,-37,-37,-36,-35,-34,-34,-33,-32,-32,-31,
-30,-29,-29,-28,-27,-27,-26,-25,-24,-24,-23,-22,-22,-21,-20,-19,-19,-18,-17,-16,-16,-15,-14,-14,-13,-12,-11,-11,-10,
-9,-9,-8,-7,-6,-6,-5,-4,-4,-3,-2,-1,-1,0,1,1,2,3,4,4,5,6,6,7,8,9,9,10,11,11,12,13,14,14,15,16,16,17,18,19,19,20,21,
22,22,23,24,24,25,26,27,27,28,29,29,30,31,32,32,33,34,34,35,36,37,37,38,39,39,40,41,42,42,43,44,44,45,46,47,47,48,49,
49,50,51,52,52,53,54,54,55,56,57,57,58,59,60,60,61,62,62,63,64,65,65,66,67,67,68,69,70,70,71,72,72,73,74,75,75,76,77,
77,78,79,80,80,81,82,82,83,84,85,85,86,87,87,88,89,90,90,91};
const int buarrxyp[]={
-228,-226,-224,-222,-221,-219,-217,-215,-213,-212,-210,-208,-206,-205,-203,-201,-199,-197,-196,-194,-192,-190,-189,
-187,-185,-183,-181,-180,-178,-176,-174,-173,-171,-169,-167,-165,-164,-162,-160,-158,-157,-155,-153,-151,-149,-148,
-146,-144,-142,-141,-139,-137,-135,-133,-132,-130,-128,-126,-125,-123,-121,-119,-117,-116,-114,-112,-110,-109,-107,
-105,-103,-101,-100,-98,-96,-94,-93,-91,-89,-87,-85,-84,-82,-80,-78,-76,-75,-73,-71,-69,-68,-66,-64,-62,-60,-59,-57,
-55,-53,-52,-50,-48,-46,-44,-43,-41,-39,-37,-36,-34,-32,-30,-28,-27,-25,-23,-21,-20,-18,-16,-14,-12,-11,-9,-7,-5,-4,
-2,0,2,4,5,7,9,11,12,14,16,18,20,21,23,25,27,28,30,32,34,36,37,39,41,43,44,46,48,50,52,53,55,57,59,60,62,64,66,68,69,
71,73,75,76,78,80,82,84,85,87,89,91,93,94,96,98,100,101,103,105,107,109,110,112,114,116,117,119,121,123,125,126,128,
130,132,133,135,137,139,141,142,144,146,148,149,151,153,155,157,158,160,162,164,165,167,169,171,173,174,176,178,180,
181,183,185,187,189,190,192,194,196,197,199,201,203,205,206,208,210,212,213,215,217,219,221,222,224,226};
typedef struct VideoBuffer {
void *start;
size_t length;
} VideoBuffer;
VideoBuffer framebuf[BUFFER_COUNT];
//查表法，把YUV422转换为RGB32
void process(unsigned long *fb_buf, unsigned char *v_buf)
{
int r,g,b;
int y0,u,y1,v;
y0=(int)*v_buf;
v=(int)*(v_buf+1);
y1=(int)*(v_buf+2);
u=(int)*(v_buf+3);
r=y0+rvarrxyp[v];
g=y0-guarrxyp[u]-gvarrxyp[v];
b=y0+buarrxyp[u];
if (r>255) r=255;
if (r<0) r=0;
if (g>255) g=255;
if (g<0) g=0;
if (b>255) b=255;
if (b<0) b=0;
*fb_buf=(b<<16)+(g<<8)+r;
r=y1+rvarrxyp[v];
g=y1-guarrxyp[u]-gvarrxyp[v];
b=y1+buarrxyp[u];
if (r>255) r=255;
if (r<0) r=0;
if (g>255) g=255;
if (g<0) g=0;
if (b>255) b=255;
if (b<0) b=0;
*(fb_buf+1)=(b<<16)+(g<<8)+r;
}
int main()
{
int i, ret;
// 打开摄像头设备
int fd;
fd = open(CAMERA_DEVICE, O_RDWR, 0);
if (fd < 0) {
printf("Open %s failed\n", CAMERA_DEVICE);
return -1;
}
// 获取驱动信息
struct v4l2_capability cap;
ret = ioctl(fd, VIDIOC_QUERYCAP, &cap);
if (ret < 0) {
printf("VIDIOC_QUERYCAP failed (%d)\n", ret);
return ret;
}
printf("Capability Informations:\n");
printf(" driver: %s\n", cap.driver);
printf(" card: %s\n", cap.card);
printf(" bus_info: %s\n", cap.bus_info);
printf(" version: %u.%u.%u\n", (cap.version>>16)&0XFF, (cap.version>>8)&0XFF,cap.version&0XFF);
printf(" capabilities: %08X\n", cap.capabilities);
// 设置视频格式
struct v4l2_format fmt;
memset(&fmt, 0, sizeof(fmt));
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.width = VIDEO_WIDTH;
fmt.fmt.pix.height = VIDEO_HEIGHT;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
ret = ioctl(fd, VIDIOC_S_FMT, &fmt);
if (ret < 0) {
printf("VIDIOC_S_FMT failed (%d)\n", ret);
return ret;
}
//获取视频格式
ret = ioctl(fd, VIDIOC_G_FMT, &fmt);
if (ret < 0) {
printf("VIDIOC_G_FMT failed (%d)\n", ret);
return ret;
}
// Print Stream Format
printf("Stream Format Informations:\n");
printf(" type: %d\n", fmt.type);
printf(" width: %d\n", fmt.fmt.pix.width);
printf(" height: %d\n", fmt.fmt.pix.height);
char fmtstr[8];
memset(fmtstr, 0, 8);
memcpy(fmtstr, &fmt.fmt.pix.pixelformat, 4);
printf(" pixelformat: %s\n", fmtstr);
printf(" field: %d\n", fmt.fmt.pix.field);
printf(" bytesperline: %d\n", fmt.fmt.pix.bytesperline);
printf(" sizeimage: %d\n", fmt.fmt.pix.sizeimage);
printf(" colorspace: %d\n", fmt.fmt.pix.colorspace);
printf(" priv: %d\n", fmt.fmt.pix.priv);
printf(" raw_date: %s\n", fmt.fmt.raw_data);
//请求分配内存
struct v4l2_requestbuffers reqbuf;
reqbuf.count = BUFFER_COUNT;
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_MMAP;
ret = ioctl(fd , VIDIOC_REQBUFS, &reqbuf);
if(ret < 0) {
printf("VIDIOC_REQBUFS failed (%d)\n", ret);
return ret;
}
VideoBuffer* buffers = calloc( reqbuf.count, sizeof(*buffers) );
struct v4l2_buffer buf;
for (i = 0; i < reqbuf.count; i++)
{
buf.index = i;
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
//获取内存空间
ret = ioctl(fd , VIDIOC_QUERYBUF, &buf);
if(ret < 0) {
printf("VIDIOC_QUERYBUF (%d) failed (%d)\n", i, ret);
return ret;
}
//映射到用户空间
framebuf[i].length = buf.length;
framebuf[i].start = (char *) mmap(0, buf.length, PROT_READ|PROT_WRITE, MAP_SHARED, fd, buf.m.offset);
if (framebuf[i].start == MAP_FAILED) {
printf("mmap (%d) failed: %s\n", i, strerror(errno));
return -1;
}
//内存入队列
ret = ioctl(fd , VIDIOC_QBUF, &buf);
if (ret < 0) {
printf("VIDIOC_QBUF (%d) failed (%d)\n", i, ret);
return -1;
}
printf("Frame buffer %d: address=0x%x, length=%d\n", i, (unsigned int)framebuf[i].start, framebuf[i].length);
}
// 启动视频流
enum v4l2_buf_type type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ret = ioctl(fd, VIDIOC_STREAMON, &type);
if (ret < 0) {
printf("VIDIOC_STREAMON failed (%d)\n", ret);
return ret;
}
struct fb_var_screeninfo vinfo;
struct fb_fix_screeninfo finfo;
int x,y;
int fd_fb;
long int screen_size = 0;
unsigned long *fbp32 = NULL;
//打开LCD设备
fd_fb = open(FB_DEVICE, O_RDWR);
if (!fd_fb)
{
printf("Error: cannot open framebuffer device.\n");
return(1);
}
if (ioctl(fd_fb, FBIOGET_FSCREENINFO, &finfo))
{
printf("Error reading fixed information.\n");
return(2);
}
//获取LCD属性信息
if (ioctl(fd_fb, FBIOGET_VSCREENINFO, &vinfo))
{
printf("Error 2 reading variable information.\n");
return(3);
}
//一屏的字节数
screen_size = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
printf("LCD: %dx%d, %dbpp, screen_size = %ld\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel, screen_size );
//映射framebuffer到用户空间
fbp32 = (unsigned long *)mmap(0, screen_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd_fb, 0);
if ((int)fbp32 == -1)
{
printf("Error: failed to map framebuffer device to memory.\n");
return(4);
}
printf("start camera testing...\n");
//开始视频显示
while(1)
{
//内存空间出队列
ret = ioctl(fd, VIDIOC_DQBUF, &buf);
if (ret < 0)
{
printf("VIDIOC_DQBUF failed (%d)\n", ret);
return ret;
}
if(vinfo.bits_per_pixel == 32)
{
for(y = 0; y < fmt.fmt.pix.height; y++)
{
for(x = 0; x < fmt.fmt.pix.width; x++)
{
//YUV422转换为RGB32
process(fbp32+y * vinfo.xres + x, framebuf[buf.index].start+y * fmt.fmt.pix.width*2+x*2);
x++;
}
}
}
// 内存重新入队列
ret = ioctl(fd, VIDIOC_QBUF, &buf);
if (ret < 0)
{
printf("VIDIOC_QBUF failed (%d)\n", ret);
return ret;
}
}//while(1)
//释放资源
for (i=0; i< 4; i++)
{
munmap(framebuf[i].start, framebuf[i].length);
}
munmap(fbp32, screen_size);
//关闭设备
close(fd);
close(fd_fb);
return 0;
}
这里有一个问题，我的摄像头是支持320×240，但无法实现该格式的视频输出，也不能设置更高的视频分辨率，不知何故？