使用libjpeg写入内存缓冲区而不是文件？

Question

我发现这个函数使用libjpeg写入文件:

int write_jpeg_file( char *filename )
{
    struct jpeg_compress_struct cinfo;
    struct jpeg_error_mgr jerr;

    /* this is a pointer to one row of image data */
    JSAMPROW row_pointer[1];
    FILE *outfile = fopen( filename, "wb" );

    if ( !outfile )
    {
        printf("Error opening output jpeg file %s\n!", filename );
        return -1;
    }
    cinfo.err = jpeg_std_error( &jerr );
    jpeg_create_compress(&cinfo);
    jpeg_stdio_dest(&cinfo, outfile);

    /* Setting the parameters of the output file here */
    cinfo.image_width = width;  
    cinfo.image_height = height;
    cinfo.input_components = bytes_per_pixel;
    cinfo.in_color_space = color_space;
    /* default compression parameters, we shouldn't be worried about these */
    jpeg_set_defaults( &cinfo );
    /* Now do the compression .. */
    jpeg_start_compress( &cinfo, TRUE );
    /* like reading a file, this time write one row at a time */
    while( cinfo.next_scanline < cinfo.image_height )
    {
        row_pointer[0] = &raw_image[ cinfo.next_scanline * cinfo.image_width *  cinfo.input_components];
        jpeg_write_scanlines( &cinfo, row_pointer, 1 );
    }
    /* similar to read file, clean up after we're done compressing */
    jpeg_finish_compress( &cinfo );
    jpeg_destroy_compress( &cinfo );
    fclose( outfile );
    /* success code is 1! */
    return 1;
}

我实际上需要将jpeg压缩图像写入内存缓冲区,而不将其保存到文件中,以节省时间.有人可以举个例子来说明怎么做吗？

我一直在网上搜索一段时间,但如果有任何例子也难以获得,那么文档非常罕见.

Answer 1

您可以非常轻松地定义自己的目标管理器.它jpeg_compress_struct包含一个指向a的指针jpeg_destination_mgr,它包含一个指向缓冲区的指针,缓冲区中剩余的空间计数,以及3个指向函数的指针:

init_destination (j_compress_ptr cinfo)
empty_output_buffer (j_compress_ptr cinfo)
term_destination (j_compress_ptr cinfo)

在第一次调用jpeg库之前需要填写函数指针,让这些函数处理缓冲区.如果您创建的缓冲区大于您期望的最大可能输出,则这变得微不足道; init_destination只需填写缓冲区指针和计数,empty_output_buffer并且term_destination什么都不做.

这是一些示例代码:

std::vector<JOCTET> my_buffer;
#define BLOCK_SIZE 16384

void my_init_destination(j_compress_ptr cinfo)
{
    my_buffer.resize(BLOCK_SIZE);
    cinfo->dest->next_output_byte = &my_buffer[0];
    cinfo->dest->free_in_buffer = my_buffer.size();
}

boolean my_empty_output_buffer(j_compress_ptr cinfo)
{
    size_t oldsize = my_buffer.size();
    my_buffer.resize(oldsize + BLOCK_SIZE);
    cinfo->dest->next_output_byte = &my_buffer[oldsize];
    cinfo->dest->free_in_buffer = my_buffer.size() - oldsize;
    return true;
}

void my_term_destination(j_compress_ptr cinfo)
{
    my_buffer.resize(my_buffer.size() - cinfo->dest->free_in_buffer);
}

cinfo->dest->init_destination = &my_init_destination;
cinfo->dest->empty_output_buffer = &my_empty_output_buffer;
cinfo->dest->term_destination = &my_term_destination;

Answer 2

有一个预定义的函数jpeg_mem_src定义jdatasrc.c.最简单的用法示例:

unsigned char *mem = NULL;
unsigned long mem_size = 0;
struct jpeg_compress_struct cinfo;
struct jpeg_error_mgr jerr;
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_compress(&cinfo);
jpeg_mem_dest(&cinfo, &mem, &mem_size);

// do compression

// use mem buffer

不要忘记释放缓冲区.