libjpeg 可以从
FILE*
或缓冲区读取 JPEG 数据。我的数据来自std::istream
。我可以将整个 std::istream
读入缓冲区以与 libjpeg 一起使用,但如果可能的话,我宁愿让 libjpeg 直接从 std::istream
读取。这怎么办?
您只需要提供 istream 周围的包装器。例如定义一个结构体
struct JpegStream {
jpeg_source_mgr pub;
std::istream* stream;
byte buffer [4096];
}
那么你需要四种方法来对流进行操作:
void init_source (j_decompress_ptr cinfo)
{
auto src = (JpegStream*)(cinfo->src);
src->stream-> // seek to 0 here
}
boolean fill_buffer (j_decompress_ptr cinfo)
{
// Read to buffer
JpegStream* src = // as above
src->pub.next_input_byte = src->buffer;
src->pub.bytes_in_buffer = // How many yo could read
return eof() ? FALSE : TRUE;
}
void skip (j_decompress_ptr cinfo, long count)
{
// Seek by count bytes forward
// Make sure you know how much you have cached and subtract that
// set bytes_in_buffer and next_input_byte
}
void term (j_decompress_ptr cinfo)
{
// Close the stream, can be nop
}
以及将它们绑定到 JPEG 解压缩信息结构的一种方法:
void make_stream (j_decompress_ptr cinfo, std::istream* in)
{
JpegStream * src;
/* The source object and input buffer are made permanent so that a series
* of JPEG images can be read from the same file by calling jpeg_stdio_src
* only before the first one. (If we discarded the buffer at the end of
* one image, we'd likely lose the start of the next one.)
* This makes it unsafe to use this manager and a different source
* manager serially with the same JPEG object. Caveat programmer.
*/
if (cinfo->src == NULL)
{
/* first time for this JPEG object? */
cinfo->src = (struct jpeg_source_mgr *)
(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, POOL_PERMANENT, sizeof(JpegStream));
src = reinterpret_cast<JpegStream*> (cinfo->src);
}
src = reinterpret_cast<JpegStream*> (cinfo->src);
src->pub.init_source = init_source;
src->pub.fill_input_buffer = fill_buffer;
src->pub.skip_input_data = skip;
src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
src->pub.term_source = term;
src->stream = in;
src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
src->pub.next_input_byte = NULL; /* until buffer loaded */
}
调用
jpeg_create_decompress
后,调用您的 make_stream
函数。
扩展 Anteru 的出色答案(大约 13 年前!),我根据它生成了以下代码。它通过使用 JpegStream 结构中的静态成员函数将所有内容包装在一起,并完全实现所有功能(尽管我还没有看到
skip
被调用,所以我不能 100% 确定它可以正常工作)。
#define STREAM_BUFFER_SIZE 4096
struct JpegStream {
jpeg_source_mgr mgr;
std::istream* stream;
unsigned char buf[STREAM_BUFFER_SIZE];
size_t ofs;
static void initSource(j_decompress_ptr cinfo)
{
JpegStream* src = reinterpret_cast<JpegStream*>(cinfo->src);
src->stream->seekg(0);
src->ofs = 0;
}
static boolean fillBuffer(j_decompress_ptr cinfo)
{
// Read to buffer
JpegStream* src = reinterpret_cast<JpegStream*>(cinfo->src);
std::istream& stream = *src->stream;
stream.read((char*)src->buf, STREAM_BUFFER_SIZE);
src->mgr.next_input_byte = src->buf;
src->ofs = 0;
src->mgr.bytes_in_buffer = stream.gcount();
return stream ? TRUE : FALSE;
}
static void skip(j_decompress_ptr cinfo, long count)
{
// Seek by count bytes forward
JpegStream* src = reinterpret_cast<JpegStream*>(cinfo->src);
if (src->ofs + count < STREAM_BUFFER_SIZE)
{
src->ofs += count;
// Make sure you know how much you have cached and subtract that
// set bytes_in_buffer and next_input_byte
src->mgr.next_input_byte = &src->buf[src->ofs];
src->mgr.bytes_in_buffer = STREAM_BUFFER_SIZE - src->ofs;
}
else
{
std::istream& stream = *src->stream;
stream.seekg(count - STREAM_BUFFER_SIZE);
src->mgr.next_input_byte = NULL;
src->mgr.bytes_in_buffer = 0;
}
}
static void term(j_decompress_ptr /*cinfo*/)
{
// Close the stream, be we ignore.
}
static void attach(j_decompress_ptr cinfo, std::istream* stream)
{
/* The source object and input buffer are made permanent so that a series
* of JPEG images can be read from the same file by calling jpeg_stdio_src
* only before the first one. (If we discarded the buffer at the end of
* one image, we'd likely lose the start of the next one.)
* This makes it unsafe to use this manager and a different source
* manager serially with the same JPEG object. Caveat programmer.
*/
if (cinfo->src == NULL)
{
/* first time for this JPEG object? */
cinfo->src = (struct jpeg_source_mgr *)
(*cinfo->mem->alloc_small)((j_common_ptr) cinfo, JPOOL_PERMANENT, sizeof(JpegStream));
}
JpegStream* src = reinterpret_cast<JpegStream*>(cinfo->src);
src->mgr.init_source = initSource;
src->mgr.fill_input_buffer = fillBuffer;
src->mgr.skip_input_data = skip;
src->mgr.resync_to_restart = jpeg_resync_to_restart; /* use default method */
src->mgr.term_source = term;
src->stream = stream;
src->ofs = 0;
src->mgr.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
src->mgr.next_input_byte = NULL; /* until buffer loaded */
}
};
使用很简单:
std::istream stream;
struct jpeg_decompress_struct cinfo;
// Set up cinfo and stream
JpegStream::attach(&cinfo, &stream);