从FrameBuffer中获取Android屏幕截图

我们知道，DDMS可以很容易的获取Android手机的屏幕截图，那么它是怎么做到的呢？

其实，android手机上有一个叫做FrameBuffer的设备，图像信息都是通过FrameBuffer写到手机屏幕上去的。因此可以通过读取此设备中的数据来获取当前正在显示的图像。当然DDMS也是这么做到的。

FrameBuffer对应的设备文件就是/dev/graphics/fb0。因此我们可以通过下面的代码读取屏幕图像数据。其中传入的参数fd为一个文件描述符，也可以是socket描述符。这样我们就可以把从fb中读取的屏幕图像信息传递给我们自己的应用，从而获取手机屏幕信息。

void framebuffer_service(int fd)
{
struct fb_var_screeninfo vinfo;
int fb, offset;
char x[256];

struct fbinfo fbinfo;
unsigned i, bytespp;

fb = open("/dev/graphics/fb0", O_RDONLY);
if(fb < 0) goto done;

if(ioctl(fb, FBIOGET_VSCREENINFO, &vinfo) < 0) goto done;
fcntl(fb, F_SETFD, FD_CLOEXEC);

bytespp = vinfo.bits_per_pixel / 8;

fbinfo.version = DDMS_RAWIMAGE_VERSION;
fbinfo.bpp = vinfo.bits_per_pixel;
fbinfo.size = vinfo.xres * vinfo.yres * bytespp;
fbinfo.width = vinfo.xres;
fbinfo.height = vinfo.yres;
fbinfo.red_offset = vinfo.red.offset;
fbinfo.red_length = vinfo.red.length;
fbinfo.green_offset = vinfo.green.offset;
fbinfo.green_length = vinfo.green.length;
fbinfo.blue_offset = vinfo.blue.offset;
fbinfo.blue_length = vinfo.blue.length;
fbinfo.alpha_offset = vinfo.transp.offset;
fbinfo.alpha_length = vinfo.transp.length;

/* HACK: for several of our 3d cores a specific alignment
* is required so the start of the fb may not be an integer number of lines
* from the base. As a result we are storing the additional offset in
* xoffset. This is not the correct usage for xoffset, it should be added
* to each line, not just once at the beginning */
offset = vinfo.xoffset * bytespp;

offset += vinfo.xres * vinfo.yoffset * bytespp;
printf("offset %d/n", offset);

if(writex(fd, &fbinfo, sizeof(fbinfo))) goto done;

lseek(fb, offset, SEEK_SET);
for(i = 0; i < fbinfo.size; i += 256) {
if(readx(fb, &x, 256)) goto done;
if(writex(fd, &x, 256)) goto done;
}

if(readx(fb, &x, fbinfo.size % 256)) goto done;
if(writex(fd, &x, fbinfo.size % 256)) goto done;

done:
if(fb >= 0) close(fb);
close(fd);
}