Android原生(Native)C開發之二 framebuffer篇

Android原生(Native)C開發之二 framebuffer篇

如對Android原生(Natvie)C開發還任何疑問，請參閱http://emck.avaw.com/?p=205

雖然如今能經過交叉環境編譯程序，並push到Android上執行，但那只是console臺程序，是否是有些單調呢？下面就要看如何經過Linux的 framebuffer 技術在Android上畫圖形，關於Linux的framebuffer技術，這裏就再也不詳細講解了，請你們google一下。

操做framebuffer的主要步驟以下：

一、打開一個可用的FrameBuffer設備；
二、經過mmap調用把顯卡的物理內存空間映射到用戶空間；
三、更改內存空間裏的像素數據並顯示；

四、退出時關閉framebuffer設備。

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

#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設備不是象linux同樣的 /dev/fb0，而是 /dev/graphics/fb0 ，

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

打開framebuffer設備，

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

將設備map到一塊內存，而後就能夠操做這塊內存空間來顯示你想畫的圖形了。

最後別忘了關閉設備：

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

效果圖以下：

Android framebuffer截圖