Android原生(Native)C開發之二：framebuffer篇

 原文：http://blog.sina.com.cn/s/blog_4a0a39c30100ax1v.html

 

如對Android原生(Natvie)C開發還任何疑問，請參閱《Android原生(Native)C開發之一：環境搭建篇》：http://blog.sina.com.cn/s/blog_4a0a39c30100auh9.html

 

雖然現在能通過交叉環境編譯器，並push到Android上執行，但那隻是console台程式，是不是有些單調呢？下面就要看如何通過Linux的
framebuffer
技術在Android上畫圖形，關於Linux的framebuffer技術，這裡就不再詳細講解了，請大家google一下。

 

操作framebuffer的主要步驟如下：

1、開啟一個可用的FrameBuffer裝置；
2、通過mmap調用把顯卡的實體記憶體空間映射到使用者空間；
3、更改記憶體空間裡的像素資料並顯示；

4、退出時關閉framebuffer裝置。

 

下面的這個例子簡單地用framebuffer畫了一個漸層的進度條，代碼 framebuf.c 如下：

#include <unistd.h>
#include <stdio.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;
}

注意，在Android環境，framebuffer裝置不是象linux一樣的 /dev/fb0，而是 /dev/graphics/fb0 ，

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

開啟framebuffer裝置，

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

 

將裝置map到一塊記憶體，然後就可以操作這塊記憶體空間來顯示你想畫的圖形了。

最後別忘了關閉裝置：

   
munmap(fbp, screensize);
   
close(fbfd);

 

如下：