1.环境准备

安装libjpeg库和libpng库

2.定义结构体

//使用该结构体存储图片数据
typedef struct img_pic_data pic_data;

struct img_pic_data
{
    int width, height;
    int bit_depth;
    int quality;
    int flag;
    int col_type;
    unsigned char *rgba;
};

3.加载jpeg图片到结构体

int LoadJPG(char *file, pic_data *data)
{
    struct jpeg_decompress_struct cinfo;
    struct jpeg_error_mgr jem;
    FILE *jpeg_fp;
    JSAMPARRAY buffer;
    int row_stride;
    int pos;
    int i;

    //打开jpeg格式图片
    if((jpeg_fp = fopen(file, "rb")) == NULL)
    {
        printf("can't open %s\n", file);
        return -1;
    }

    //初始化并申请解码器
    cinfo.err = jpeg_std_error(&jem);
    jpeg_create_decompress(&cinfo);

    //指定图片文件信息
    jpeg_stdio_src(&cinfo, jpeg_fp);

    //读取头部信息
    jpeg_read_header(&cinfo, TRUE);

    //开始解码
    jpeg_start_decompress(&cinfo);

    row_stride = cinfo.output_width * cinfo.output_components;
    data->width = cinfo.output_width;
    data->height = cinfo.output_height;
    data->bit_depth = 8;
    data->flag = 1;
    data->col_type = PNG_COLOR_TYPE_RGB_ALPHA;
    pos = 0;

    //给一行数据分配内存
    buffer = (*cinfo.mem->alloc_sarray)((j_common_ptr)&cinfo, JPOOL_IMAGE, row_stride, 1);

    //一行一行读取
    while(cinfo.output_scanline < cinfo.output_height)
    {
        jpeg_read_scanlines(&cinfo, buffer, 1);
        for(i=0; i<cinfo.output_width; i++)
        {
            data->rgba[4*pos+0] = buffer[0][3*i+0];
            data->rgba[4*pos+1] = buffer[0][3*i+1];
            data->rgba[4*pos+2] = buffer[0][3*i+2];
            data->rgba[4*pos+3] = 255;
            pos++;
        }
    }

    //读取结束
    jpeg_finish_decompress(&cinfo);

    //释放编码器对象
    jpeg_destroy_decompress(&cinfo);

    //关闭jpeg格式图片
    fclose(jpeg_fp);

    return 0;
}

4.将图像数据写入png文件

int PutPNG(char *file, pic_data *data)
{
    FILE *pic_fp;
    png_structp png_ptr;
    png_infop info_ptr;
    png_bytep *row_pointers;
    int color_type, channels;
    int temp;
    int i, j;
    int size, pos = 0;

    //打开png空文件
    pic_fp = fopen(file, "wb");
    if(pic_fp == NULL)
    {
        printf("can't open %s\n", file);
        return -1;
    }

    temp = data->width * 4;

    png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if(!png_ptr)
    {
        printf("png_create_write_struct failed\n");
        return -1;
    }

    info_ptr = png_create_info_struct(png_ptr);
    if(!info_ptr)
    {
        printf("png_create_info_struct failed\n");
        return -1;
    }

    if(setjmp(png_jmpbuf(png_ptr)))
    {
        printf("write error on init io\n");
        return -1;
    }

    png_init_io(png_ptr, pic_fp);

    if(setjmp(png_jmpbuf(png_ptr)))
    {
        printf("write error on write header\n");
        return -1;
    }

    png_set_IHDR(png_ptr, info_ptr, data->width, data->height, data->bit_depth, data->col_type, PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);

    png_write_info(png_ptr, info_ptr);

    if(setjmp(png_jmpbuf(png_ptr)))
    {
        printf("write error on writing bytes\n");
        return -1;
    }

    if(data->flag == 1)
    {
        pos = 0;
        row_pointers = (png_bytep *)malloc(data->height * sizeof(png_bytep));
        for(i=0; i<data->height; i++)
        {
            row_pointers[i] = (png_bytep)malloc(temp * sizeof(unsigned char));
            for(j=0; j<temp; j+=4)
            {
                row_pointers[i][j+0] = data->rgba[pos*4 + 0];
                row_pointers[i][j+1] = data->rgba[pos*4 + 1];
                row_pointers[i][j+2] = data->rgba[pos*4 + 2];
                row_pointers[i][j+3] = data->rgba[pos*4 + 3];
                pos++;
            }
        }

        png_write_image(png_ptr, row_pointers);

        if(setjmp(png_jmpbuf(png_ptr)))
        {
            printf("write error on write end\n");
            return -1;
        }

        png_write_end(png_ptr, NULL);

        for(i=0; i<data->height; i++)
        {
            free(row_pointers[i]);
        }

        free(row_pointers);
    }

    png_destroy_write_struct(&png_ptr, &info_ptr);

    fclose(pic_fp);

    return 0;
}

5.通过main函数输入jpeg文件和png文件名

int main(int argc, char **argv)
{
    FILE *file;
    pic_data pic_data;
    char rgba_data[16*300*300];
    char jpeg_name[256];
    char png_name[256];
    int ret;

    memset(jpeg_name, 0x00, sizeof(jpeg_name));
    memset(png_name, 0x00, sizeof(png_name));

    if(argc != 3)
    {
        printf("入参个数错误!\n");
        return -1;
    }

    strncpy(jpeg_name, argv[1], 255);
    strncpy(png_name, argv[2], 255);

    pic_data.rgba = rgba_data;

    ret = LoadJPG(jpeg_name, &pic_data);
    if(ret < 0)
    {
        printf(" LoadJPG error!\n");
        return -1;
    }

    ret = PutPNG(png_name, &pic_data);
    if(ret < 0)
    {
        printf(" PutPNG error!\n");
        return -1;
    }

    return 0;
}

6.加上头文件

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <jpeglib.h>
#include <png.h>

7.编译

$ gcc -o examle example.c -L$HOME/local/prior/lib -ljpeg -lpng

8.运行

$ ./examle a.jpg b.png