jpeg库下载地址:
http:
交叉编译三部曲:
A ./configure --host=arm-linux-gcc --prefix=/home/flying/jpeg-install
B make
C make install
jpeg库是解压jpeg/jpg图片或将图片压缩为jpeg/jpg格式所用到的库,使用起来比较简单!
jpeg/jpg解码过程:
1. 创建jpeg对象和错误处理对象
struct jpeg_decompress_struct cinfo;
struct jpeg_error_mgr jerr;
2. 将错误处理对象绑定到jpeg对象上
EXTERN(struct jpeg_error_mgr *) jpeg_std_error
JPP((struct jpeg_error_mgr * err));
3. 初始化jpeg对象
#define jpeg_create_compress(cinfo) \
jpeg_CreateCompress((cinfo), JPEG_LIB_VERSION, \
(size_t) sizeof(struct jpeg_compress_struct))
4. 指定解压数据源(有两种方式)
a. EXTERN(void) jpeg_mem_src JPP((j_decompress_ptr cinfo, const unsigned char * inbuffer, unsigned long insize));
参数一: jpeg对象地址
参数二: 储存jpeg数据源缓冲区
参数三: 缓冲区数据源大小
b. EXTERN(void) jpeg_stdio_src JPP((j_decompress_ptr cinfo, FILE * infile));
参数一: jpeg对象地址
参数二: 要解压的jpeg图片的文件指针
5. 填充cinfo对象的缺省信息,常见的可用信息包括图像的
宽: cinfo.image_width,
高: cinfo.image_height,
色彩空间: cinfo.jpeg_color_space,
颜色通道数: cinfo.num_components等。
EXTERN(int) jpeg_read_header JPP((j_decompress_ptr cinfo,
boolean require_image));
6. 为解压设定参数**(依情况而定)**
比如解压的图片与原图的缩放比例为1/2,这可以设置参数
cinfo.scale_num = 1
cinfo.scale_denom = 2
7. 开始解压
EXTERN(boolean) jpeg_start_decompress JPP((j_decompress_ptr cinfo));
在完成解压缩操作后,会将解压后的图像信息填充至cinfo结构中。比如,输出图
像宽度cinfo.output_width,输出图像高度cinfo.output_height,每个像
素中的颜色通道数cinfo.output_components(比如灰度为1,全彩色为3)
等。
一般情况下,这些参数是在jpeg_start_decompress后才被填充到cinfo中
的,如果希望在调用jpeg_start_decompress之前就获得这些参数,可以通过
调用jpeg_calc_output_dimensions()的方法来实现。
8. 取数据
解压出来的数据是按照行取出的,从左到右,从上到下的顺序,每个像
素对应的各颜色或灰度通道数据是依次存储,24位RGB图解压后的颜色
通道排序是R,G,B,R,G,B......
我们可以使用此函数取出循环取出数据:
extern JDIMENSION jpeg_read_scanlines(j_decompress_ptr, JSAMPARRAY,
JDIMENSION);
参数二: 一般来说就是unsigned char *,表示保存数据的缓冲区
参数三: 表示要读取的行数
9. 解压完毕释放资源
boolean jpeg_finish_decompress(j_decompress_ptr cinfo);
后续不需要使用jpeg对象:
void jpeg_destroy_decompress(j_decompress_ptr cinfo);
如果在后续还需使用jpeg对象,则使用:
void jpeg_abort_decompress(j_decompress_ptr cinfo);
实例:
#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <linux/fb.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <stdbool.h>
#include "jpeglib.h"
#include "my_err.h"
struct image_info
{
int width;
int height;
int pixel_size;
};
void write_jpeg_to_lcd(unsigned char *lcdmem, unsigned char *rgb_buffer
, struct fb_var_screeninfo *vinfo, struct image_info *image_info
, size_t xoffset, size_t yoffset)
{
assert(lcdmem);
assert(rgb_buffer);
assert(vinfo);
assert(image_info);
int x, y;
size_t r_offset = vinfo->red.offset / 8;
size_t g_offset = vinfo->green.offset / 8;
size_t b_offset = vinfo->blue.offset / 8;
for (y = 0; (y < vinfo->yres - yoffset) && (y < image_info->height); ++y) {
for (x = 0; (x < vinfo->xres - xoffset) && (x < image_info->width); ++x) {
size_t image_offset = (x * image_info->pixel_size) + (image_info->width * image_info->pixel_size * y);
size_t lcd_offset = ((x + xoffset) * vinfo->bits_per_pixel / 8) + ((y + yoffset) * vinfo->xres * vinfo->bits_per_pixel /8);
memcpy(lcdmem + lcd_offset + r_offset, rgb_buffer + image_offset, 1);
memcpy(lcdmem + lcd_offset + g_offset, rgb_buffer + image_offset + 1, 1);
memcpy(lcdmem + lcd_offset + b_offset, rgb_buffer + image_offset + 2, 1);
}
}
}
void read_image_from_file(int fd, unsigned char *jpeg_buffer, size_t image_size)
{
assert(jpeg_buffer);
int nread = 0;
while (image_size > 0) {
nread = read(fd, jpeg_buffer, image_size);
if (nread < 0) {
if (EINTR == errno) {
continue;
} else {
err_sys("read error");
}
}
image_size -= nread;
jpeg_buffer += nread;
}
}
int main(int argc, char **argv)
{
if (2 != argc) {
err_quit("Usage: %s <*.jpeg/jpg>\n", argv[1]);
}
int image_fd;
if ((image_fd = open(argv[1], O_RDONLY)) < 0) {
err_sys("open error");
}
struct stat sbuf;
if (fstat(image_fd, &sbuf) < 0) {
err_sys("stat error");
}
unsigned char *jpeg_buffer = calloc(1, sbuf.st_size);
read_image_from_file(image_fd, jpeg_buffer, sbuf.st_size);
struct jpeg_decompress_struct cinfo;
struct jpeg_error_mgr jerr;
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_decompress(&cinfo);
jpeg_mem_src(&cinfo, jpeg_buffer, sbuf.st_size);
if (JPEG_HEADER_OK != jpeg_read_header(&cinfo, true)) {
err_sys("read head error");
}
jpeg_start_decompress(&cinfo);
struct image_info *image_info = calloc(1, sizeof(struct image_info));
if (image_info == NULL) {
err_sys("calloc error for image_info");
}
image_info->width = cinfo.output_width;
image_info->height = cinfo.output_height;
image_info->pixel_size = cinfo.output_components;
int row_stride = image_info->width * image_info->pixel_size;
size_t rgb_size = row_stride * image_info->height;
unsigned char *rgb_buffer = calloc(1, rgb_size);
if (rgb_buffer == NULL) {
err_sys("rgb_buffer error");
}
while (cinfo.output_scanline < image_info->height) {
unsigned char *buffer_array[1];
buffer_array[0] = rgb_buffer + cinfo.output_scanline * row_stride;
jpeg_read_scanlines(&cinfo, buffer_array, 1);
}
jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);
free(jpeg_buffer);
int lcd_fd;
if ((lcd_fd = open("/dev/fb0", O_RDWR)) < 0 ) {
err_sys("open error");
}
struct fb_var_screeninfo vinfo;
ioctl(lcd_fd, FBIOGET_VSCREENINFO, &vinfo);
size_t mem_size = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
unsigned char *lcdmem = mmap(0, mem_size, PROT_READ|PROT_WRITE, MAP_SHARED, lcd_fd, 0);
if (lcdmem == MAP_FAILED) {
err_sys("mmap error");
}
write_jpeg_to_lcd(lcdmem, rgb_buffer, &vinfo, image_info, 100, 100);
free(rgb_buffer);
munmap(lcdmem, mem_size);
close(lcd_fd);
close(image_fd);
return EXIT_SUCCESS;
}
参考博客:
http://blog.csdn.net/xipiaoyouzi/article/details/53257720
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