由于 Linux 系统的 FrameBuffer 机制，把屏幕上的每个点映射成一段线性内存空间，这样，程序就可以通过改变这段内存的值来改变屏幕上某一点的颜色。如果我们想把当前的显示内容保存起来，可能会想到如下命令：
# cat /dev/fb0 > fb_data.raw
反过来，可以将这些数据回显到 framebuffer 中：
# cat fb_data.raw > /dev/fb0
使用 clear 命令清除，可以恢复正常。
但是，用这用方法保存起来的数据是原始数据，只有专用软件才能打开，并且大小固定（如：8MB）。基于这些原因，我们找到一个不错的工具——gsnap，这个工具可以将 framebuffer 的数据保存为图片（png或jpeg格式）。下面我们介绍一下移植过程。
这里的移植很简单，因为源文件只有 gsnap.c，因此我们只需用相应平台的编译工具链进行编译链接即可。命令如下：
# $(CC) gsnap.c -ljpeg -lpng -o gsnap
显然，gsnap 需要用到 libjpeg 和 libpng 两个库。那么编译成功与否就跟这两个库有关了。如果你的目标平台还没有这些依赖库，那么就有必要下载相关源码进行编译安装，步骤遵循 configure、make、make install 三部曲。
由于我的目标平台已经包含 libjpeg 和 libpng，于是我尝试用上述命令进行编译，提示缺少头文件，所以编译不成功。然后我将 libjpeg 和 libpng 源码包中的头文件抽取出来，添加到 /usr/include。发现仍然缺少头文件，如下：
libpng 方面——找不到 pnglibconf.h，经检查发现将 scripts/pnglibconf.h.prebuilt 另存为
pnglibconf.h，并添加到 /usr/include 即可。
libjpeg 方面——找不到 jconfig.h，经检查发现将 jconfig.txt 另存为 jconfig.h，并添加到
/usr/include 即可。
不用担心，如果你遵循三部曲来安装这些库，上面的 pnglibconf.h 和 jconfig.h 都会在编译的过程中生成。
一旦编译成功，我们就可以运行 gsnap 来截取屏幕画面了。gsnap 的使用也很简单，格式为：
gsnap <jpeg|png file> <framebuffer dev>
例如：
# ./gsnap test.png /dev/fb0
我这里用的是 i.mx6q 的 yocto 1.5.3 系统，截图 test.png 如下：
以下是 gsnap.c 的源代码：
/*
* File:gsnap.c
* Author: Li XianJing <xianjimli@hotmail.com>
* Brief: snap the linux mobile device screen.
*
* Copyright (c) 2009 Li XianJing <xianjimli@hotmail.com>
*
* Licensed under the Academic Free License version 2.1
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/*
* History:
* ===================================================
* 2009-08-20 Li XianJing <xianjimli@hotmail.com> created
* 2011-02-28 Li XianJing <xianjimli@hotmail.com> suppport RGB888 framebuffer.
* 2011-04-09 Li XianJing <xianjimli@hotmail.com> merge figofuture's png output.
*
*/
#include <png.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <jpeglib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <linux/fb.h>
#include <linux/kd.h>
struct _FBInfo;
typedef struct _FBInfo FBInfo;
typedef int (*UnpackPixel)(FBInfo* fb, unsigned char* pixel,
unsigned char* r, unsigned char* g, unsigned char* b);
struct _FBInfo
{
int fd;
UnpackPixel unpack;
unsigned char *bits;
struct fb_fix_screeninfo fi;
struct fb_var_screeninfo vi;
};
#define fb_width(fb) ((fb)->vi.xres)
#define fb_height(fb) ((fb)->vi.yres)
#define fb_bpp(fb)((fb)->vi.bits_per_pixel>>3)
#define fb_size(fb) ((fb)->vi.xres * (fb)->vi.yres * fb_bpp(fb))
static int fb_unpack_rgb565(FBInfo* fb, unsigned char* pixel,
unsigned char* r, unsigned char* g, unsigned char* b)
{
unsigned short color = *(unsigned short*)pixel;
*r = ((color >> 11) & 0xff) << 3;
*g = ((color >> 5) & 0xff) << 2;
*b = (color & 0xff )<< 3;
return 0;
}
static int fb_unpack_rgb24(FBInfo* fb, unsigned char* pixel,
unsigned char* r, unsigned char* g, unsigned char* b)
{
*r = pixel[fb->vi.red.offset>>3];
*g = pixel[fb->vi.green.offset>>3];
*b = pixel[fb->vi.blue.offset>>3];
return 0;
}
static int fb_unpack_argb32(FBInfo* fb, unsigned char* pixel,
unsigned char* r, unsigned char* g, unsigned char* b)
{
*r = pixel[fb->vi.red.offset>>3];
*g = pixel[fb->vi.green.offset>>3];
*b = pixel[fb->vi.blue.offset>>3];
return 0;
}
static int fb_unpack_none(FBInfo* fb, unsigned char* pixel,
unsigned char* r, unsigned char* g, unsigned char* b)
{
*r = *g = *b = 0;
return 0;
}
static void set_pixel_unpacker(FBInfo* fb)
{
if(fb_bpp(fb) == 2)
{
fb->unpack = fb_unpack_rgb565;
}
else if(fb_bpp(fb) == 3)
{
fb->unpack = fb_unpack_rgb24;
}
else if(fb_bpp(fb) == 4)
{
fb->unpack = fb_unpack_argb32;
}
else
{
fb->unpack = fb_unpack_none;
printf("%s: not supported format.\n", __func__);
}
return;
}
static int fb_open(FBInfo* fb, const char* fbfilename)
{
fb->fd = open(fbfilename, O_RDWR);
if (fb->fd < 0)
{
fprintf(stderr, "can't open %s\n", fbfilename);
return -1;
}
if (ioctl(fb->fd, FBIOGET_FSCREENINFO, &fb->fi) < 0)
goto fail;
if (ioctl(fb->fd, FBIOGET_VSCREENINFO, &fb->vi) < 0)
goto fail;
fb->bits = mmap(0, fb_size(fb), PROT_READ | PROT_WRITE, MAP_SHARED, fb->fd, 0);
if (fb->bits == MAP_FAILED)
goto fail;
printf("---------------framebuffer---------------\n");
printf("%s: \n width : %8d\n height: %8d\n bpp : %8d\n r(%2d, %2d)\n g(%2d, %2d)\n b(%2d, %2d)\n",
fbfilename, fb_width(fb), fb_height(fb), fb_bpp(fb),
fb->vi.red.offset, fb->vi.red.length,
fb->vi.green.offset, fb->vi.green.length,
fb->vi.blue.offset, fb->vi.blue.length);
printf("-----------------------------------------\n");
set_pixel_unpacker(fb);
return 0;
fail:
printf("%s is not a framebuffer.\n", fbfilename);
close(fb->fd);
return -1;
}
static void fb_close(FBInfo* fb)
{
munmap(fb->bits, fb_size(fb));
close(fb->fd);
return;
}
static int snap2jpg(const char * filename, int quality, FBInfo* fb)
{
int row_stride = 0;
FILE * outfile = NULL;
JSAMPROW row_pointer[1] = {0};
struct jpeg_error_mgr jerr;
struct jpeg_compress_struct cinfo;
memset(&jerr, 0x00, sizeof(jerr));
memset(&cinfo, 0x00, sizeof(cinfo));
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_compress(&cinfo);
if ((outfile = fopen(filename, "wb+")) == NULL)
{
fprintf(stderr, "can't open %s\n", filename);
return -1;
}
jpeg_stdio_dest(&cinfo, outfile);
cinfo.image_width = fb_width(fb);
cinfo.image_height = fb_height(fb);
cinfo.input_components = 3;
cinfo.in_color_space = JCS_RGB;
jpeg_set_defaults(&cinfo);
jpeg_set_quality(&cinfo, quality, TRUE);
jpeg_start_compress(&cinfo, TRUE);
row_stride = fb_width(fb) * 2;
JSAMPLE* image_buffer = malloc(3 * fb_width(fb));
while (cinfo.next_scanline < cinfo.image_height)
{
int i = 0;
int offset = 0;
unsigned char* line = fb->bits + cinfo.next_scanline * fb_width(fb) * fb_bpp(fb);
for(i = 0; i < fb_width(fb); i++, offset += 3, line += fb_bpp(fb))
{
fb->unpack(fb, line, image_buffer+offset, image_buffer + offset + 1, image_buffer + offset + 2);
}
row_pointer[0] = image_buffer;
(void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
}
jpeg_finish_compress(&cinfo);
fclose(outfile);
jpeg_destroy_compress(&cinfo);
return 0;
}
static int snap2png(const char * filename, int quality, FBInfo* fb)
{
FILE *outfile;
if ((outfile = fopen(filename, "wb+")) == NULL)
{
fprintf(stderr, "can't open %s\n", filename);
return -1;
}
/* prepare the standard PNG structures */
png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,0,0,0);
png_infop info_ptr = png_create_info_struct(png_ptr);
/* setjmp() must be called in every function that calls a PNG-reading libpng function */
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(outfile);
return -1;
}
/* initialize the png structure */
png_init_io(png_ptr, outfile);
//
int width = 0;
int height = 0;
int bit_depth = 8;
int color_type = PNG_COLOR_TYPE_RGB;
int interlace = 0;
width = fb_width(fb);
height = fb_height(fb);
png_set_IHDR (png_ptr, info_ptr, width, height, bit_depth, color_type,
(!interlace) ? PNG_INTERLACE_NONE : PNG_INTERLACE_ADAM7,
PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
/* write the file header information */
png_write_info(png_ptr, info_ptr);
png_bytep row_pointers[height];
png_byte* image_buffer = malloc(3 * width);
int i = 0;
int j = 0;
unsigned char* line = NULL;
for( ; i < height; i++ )
{
line = (char*)fb->bits + i * width * fb_bpp(fb);
for(j = 0; j < width; j++, line += fb_bpp(fb))
{
int offset = j * 3;
fb->unpack(fb, line, image_buffer+offset, image_buffer+offset+1, image_buffer+offset+2);
}
row_pointers[i] = image_buffer;
png_write_rows(png_ptr, &row_pointers[i], 1);
}
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(outfile);
return 0;
}
int main(int argc, char* argv[])
{
FBInfo fb;
const char* filename = NULL;
const char* fbfilename = NULL;
if(argc != 3)
{
printf("\nUsage: %s [jpeg|png file] [framebuffer dev]\n", argv[0]);
printf("Example: %s fb.jpg /dev/fb0\n", argv[0]);
printf("-----------------------------------------\n");
printf("Powered by broncho(www.broncho.cn)\n\n");
return 0;
}
filename = argv[1];
fbfilename = argv[2];
memset(&fb, 0x00, sizeof(fb));
if (fb_open(&fb, fbfilename) == 0)
{
if(strstr(filename, ".png") != NULL)
{
snap2png(filename, 100, &fb);
}
else
{
snap2jpg(filename, 100, &fb);
}
fb_close(&fb);
}
return 0;
}