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。有人能举例说明怎么做吗?
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,那么文档很少见。
答案 0 :(得分:14)
您可以非常轻松地定义自己的目标管理器。 jpeg_compress_struct
包含指向jpeg_destination_mgr
的指针,其中包含指向缓冲区的指针,缓冲区中剩余的空间计数以及指向函数的3个指针:
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,然后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
和term_destination
不执行任何操作。
以下是一些示例代码:
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;
答案 1 :(得分:13)
jpeg_mem_src
中定义了预定义函数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。
答案 2 :(得分:3)
我已经尝试过Mark的解决方案,并且在我的平台上它执行时始终会出现SEGMENTATION FALUT错误
cinfo->dest->term_destination = &my_term_destination;
我转向jpeglib源代码(jdatadst.c),发现了这个:
jpeg_mem_dest (j_compress_ptr cinfo, unsigned char ** outbuffer, unsigned long * outsize)
就在jpeg_stdio_dest()方法的正下方,我只是通过填写缓冲区的地址(char *)和缓冲区大小的地址(int)来尝试它。目标管理器自动为缓冲区分配内存,程序需要在使用后释放内存。
它成功运行在我的平台上,Beaglebone Black带有预装的Angstrom Linux。我的libjpeg版本是8d。
答案 3 :(得分:0)
您需要做的就是将FILE
之类的对象传递给jpeg_stdio_dest()
。
答案 4 :(得分:0)
unsigned char ***image_ptr
unsigned char* ptr;
unsigned char** image_buf;
for(int i=0;i<h;i++){
image_buf[i] = new unsigned char[w*o];
}
ptr = image_buf[0];
while (info.output_scanline < info.image_height) {
jpeg_read_scanlines(&info,&ptr,1);
ptr = image_buf[c];
c++;
}
*image_ptr = image_buf;
这就是你需要阅读的全部内容。
JSAMPROW row_pointer;
while (info.next_scanline < info.image_height) {
row_pointer = &image_buf[info.next_scanline][0];
(void) jpeg_write_scanlines(&info, &row_pointer, 1);
}
这就是你需要写的全部内容。