Android原生(Native)C开发 -- framebuffer篇

虽然现在能通过交叉环境编译程序，并push到Android上执行，但那只是console台程序，是不是有些单调呢？下面就要看如何通过Linux的 framebuffer 技术在Android上画图形，关于Linux的framebuffer技术，这里就不再详细讲解了，请大家google一下。

操作framebuffer的主要步骤如下：

1、打开一个可用的FrameBuffer设备；

2、通过mmap调用把显卡的物理内存空间映射到用户空间；

3、更改内存空间里的像素数据并显示；

4、退出时关闭framebuffer设备。

下面的这个例子简单地用framebuffer画了一个渐变的进度条，代码 framebuffer.c 如下：

#include <unistd.h>

#include <stdio.h>

#include <stdlib.h>

#include <fcntl.h>

#include <linux/fb.h>

#include <sys/mman.h>

inline static unsigned short int make16color(unsigned char r, unsigned char g, unsigned char b)

{

return (

(((r >> 3) & 31) << 11) |

(((g >> 2) & 63) << 5)  |

((b >> 3) & 31)        );

}

int main() {

int fbfd = 0;

struct fb_var_screeninfo vinfo;

struct fb_fix_screeninfo finfo;

long int screensize = 0;

char *fbp = 0;

int x = 0, y = 0;

int guage_height = 20, step = 10;

long int location = 0;

// Open the file for reading and writing

fbfd = open("/dev/graphics/fb0", O_RDWR);

if (!fbfd) {

printf("Error: cannot open framebuffer device.\n");

exit(1);

}

printf("The framebuffer device was opened successfully.\n");

// Get fixed screen information

if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {

printf("Error reading fixed information.\n");

exit(2);

}

// Get variable screen information

if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {

printf("Error reading variable information.\n");

exit(3);

}

printf("sizeof(unsigned short) = %d\n", sizeof(unsigned short));

printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel );

printf("xoffset:%d, yoffset:%d, line_length: %d\n", vinfo.xoffset, vinfo.yoffset, finfo.line_length );

// Figure out the size of the screen in bytes

screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;;

// Map the device to memory

fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,

fbfd, 0);

if ((int)fbp == -1) {

printf("Error: failed to map framebuffer device to memory.\n");

exit(4);

}

printf("The framebuffer device was mapped to memory successfully.\n");

//set to black color first

memset(fbp, 0, screensize);

//draw rectangle

y = (vinfo.yres - guage_height) / 2 - 2;       // Where we are going to put the pixel

for (x = step - 2; x < vinfo.xres - step + 2; x++) {

location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +

(y+vinfo.yoffset) * finfo.line_length;

*((unsigned short int*)(fbp + location)) = 255;

}

y = (vinfo.yres + guage_height) / 2 + 2;       // Where we are going to put the pixel

for (x = step - 2; x < vinfo.xres - step + 2; x++) {

location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +

(y+vinfo.yoffset) * finfo.line_length;

*((unsigned short int*)(fbp + location)) = 255;

}

x = step - 2;

for (y = (vinfo.yres - guage_height) / 2 - 2; y < (vinfo.yres + guage_height) / 2 + 2; y++) {

location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +

(y+vinfo.yoffset) * finfo.line_length;

*((unsigned short int*)(fbp + location)) = 255;

}

x = vinfo.xres - step + 2;

for (y = (vinfo.yres - guage_height) / 2 - 2; y < (vinfo.yres + guage_height) / 2 + 2; y++) {

location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +

(y+vinfo.yoffset) * finfo.line_length;

*((unsigned short int*)(fbp + location)) = 255;

}

// Figure out where in memory to put the pixel

for ( x = step; x < vinfo.xres - step; x++ ) {

for ( y = (vinfo.yres - guage_height) / 2; y < (vinfo.yres + guage_height) / 2; y++ ) {

location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +

(y+vinfo.yoffset) * finfo.line_length;

if ( vinfo.bits_per_pixel == 32 ) {

*(fbp + location) = 100;        // Some blue

*(fbp + location + 1) = 15+(x-100)/2;     // A little green

*(fbp + location + 2) = 200-(y-100)/5;    // A lot of red

*(fbp + location + 3) = 0;      // No transparency

} else { //assume 16bpp

unsigned char b = 255 * x / (vinfo.xres - step);

unsigned char g = 255;     // (x - 100)/6 A little green

unsigned char r = 255;    // A lot of red

unsigned short int t = make16color(r, g, b);

*((unsigned short int*)(fbp + location)) = t;

}

}

//printf("x = %d, temp = %d\n", x, temp);

//sleep to see it

usleep(200);

}

//clean framebuffer

munmap(fbp, screensize);

close(fbfd);

return 0;

}

好，然后我们建立Android.mk和相应的目录架构（如下图):



jni目录里面Android.mk里面的内容如下图:



最后确认你已经安装好ndk-build， 并且在你的path当中,然后用ndk-build来生成可执行文件:



注意，在Android环境，framebuffer设备不是象linux一样的 /dev/fb0，而是 /dev/graphics/fb0，

fbfd = open(”/dev/graphics/fb0″, O_RDWR);

复制代码

打开framebuffer设备，

fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,fbfd, 0);

将设备map到一块内存，然后就可以操作这块内存空间来显示你想画的图形了。

最后别忘了关闭设备：

munmap(fbp, screensize);

close(fbfd);

用adb push把 生成的文件"libs/armeabi/framebuffer" push 到 模拟器(已经启动)或者你的手机(已经链接上usb)里面，然后运行，如下图：



作者：Aries @ 米狗族

网站：http://www.meegozu.com/thread-370-1-1.html

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