Android原生(Native)C(JNI/NDK)开发之二：framebuffer篇

为方便以后学习和工作，现转载一批文章，方便以后使用。

来源：http://blog.sina.com.cn/s/blog_4a0a39c30100auh9.html 作者：风子

如对Android原生(Natvie)C开发还任何疑问，请参阅《Android原生(Native)C开发之一：环境搭建篇》：http://blog.sina.com.cn/s/blog_4a0a39c30100auh9.html(或者http://android-sz.iteye.com/blog/828246)

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 framebuf.c 如下：

C代码

#include<unistd.h>
#include<stdio.h>
#include<fcntl.h>
#include<linux/fb.h>
#include<sys/mman.h>
inlinestaticunsignedshortintmake16color(unsignedcharr,unsignedcharg,unsignedcharb)
{
return(
(((r>>3)&31)<<11)|
(((g>>2)&63)<<5)|
((b>>3)&31));
}
intmain(){
intfbfd=0;
structfb_var_screeninfovinfo;
structfb_fix_screeninfofinfo;
longintscreensize=0;
char*fbp=0;
intx=0,y=0;
intguage_height=20,step=10;
longintlocation=0;
//Openthefileforreadingandwriting
fbfd=open("/dev/graphics/fb0",O_RDWR);
if(!fbfd){
printf("Error:cannotopenframebufferdevice.\n");
exit(1);
}
printf("Theframebufferdevicewasopenedsuccessfully.\n");
//Getfixedscreeninformation
if(ioctl(fbfd,FBIOGET_FSCREENINFO,&finfo)){
printf("Errorreadingfixedinformation.\n");
exit(2);
}
//Getvariablescreeninformation
if(ioctl(fbfd,FBIOGET_VSCREENINFO,&vinfo)){
printf("Errorreadingvariableinformation.\n");
exit(3);
}
printf("sizeof(unsignedshort)=%d\n",sizeof(unsignedshort));
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);
//Figureoutthesizeofthescreeninbytes
screensize=vinfo.xres*vinfo.yres*vinfo.bits_per_pixel/8;;
//Mapthedevicetomemory
fbp=(char*)mmap(0,screensize,PROT_READ|PROT_WRITE,MAP_SHARED,
fbfd,0);
if((int)fbp==-1){
printf("Error:failedtomapframebufferdevicetomemory.\n");
exit(4);
}
printf("Theframebufferdevicewasmappedtomemorysuccessfully.\n");
//settoblackcolorfirst
memset(fbp,0,screensize);
//drawrectangle
y=(vinfo.yres-guage_height)/2-2;//Wherewearegoingtoputthepixel
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;
*((unsignedshortint*)(fbp+location))=255;
}
y=(vinfo.yres+guage_height)/2+2;//Wherewearegoingtoputthepixel
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;
*((unsignedshortint*)(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;
*((unsignedshortint*)(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;
*((unsignedshortint*)(fbp+location))=255;
}
//Figureoutwhereinmemorytoputthepixel
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;//Someblue
*(fbp+location+1)=15+(x-100)/2;//Alittlegreen
*(fbp+location+2)=200-(y-100)/5;//Alotofred
*(fbp+location+3)=0;//Notransparency
}else{//assume16bpp
unsignedcharb=255*x/(vinfo.xres-step);
unsignedcharg=255;//(x-100)/6Alittlegreen
unsignedcharr=255;//Alotofred
unsignedshortintt=make16color(r,g,b);
*((unsignedshortint*)(fbp+location))=t;
}
}
//printf("x=%d,temp=%d\n",x,temp);
//sleeptoseeit
usleep(200);
}
//cleanframebuffer
munmap(fbp,screensize);
close(fbfd);
return0;
}

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);

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);

效果图如下：

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