在 OpenCV 中使用 libjpeg 将 IplImage 压缩为 JPEG

Question

所以我有这个问题。 我有一个 IplImage，我想将其压缩为 JPEG 并用它做一些事情。我使用 libjpeg。 我找到了很多答案“阅读示例和文档”等等，然后就这样做了。并成功为此编写了一个函数。

FILE* convert2jpeg(IplImage* frame) 
{
FILE* outstream = NULL;
outstream=malloc(frame->imageSize*frame->nChannels*sizeof(char))

unsigned char *outdata = (uchar *) frame->imageData;
struct jpeg_error_mgr jerr;
struct jpeg_compress_struct cinfo;
int row_stride;
JSAMPROW row_ptr[1];

jpeg_create_compress(&cinfo);
jpeg_stdio_dest(&cinfo, outstream);

cinfo.image_width = frame->width;
cinfo.image_height = frame->height;
cinfo.input_components = frame->nChannels;
cinfo.in_color_space = JCS_RGB;

jpeg_set_defaults(&cinfo);
jpeg_start_compress(&cinfo, TRUE);
row_stride = frame->width * frame->nChannels;

while (cinfo.next_scanline < cinfo.image_height) {
    row_ptr[0] = &outdata[cinfo.next_scanline * row_stride];
    jpeg_write_scanlines(&cinfo, row_ptr, 1);
}

jpeg_finish_compress(&cinfo);
jpeg_destroy_compress(&cinfo);

return outstream;
}

现在这个函数直接来自示例（除了分配内存的部分，但我需要它，因为我没有写入文件），但它仍然不起作用。 它在 jpeg_start_compress(&cinfo, TRUE); 部分死亡？

有人可以帮忙吗？

Answer 1

我已经能够使用他们网站上提供的最新 jpeglib 找到解决方案。
新方法：jpeg_mem_dest(&cinfo, outbuffer, outlen);

bool ipl2jpeg(IplImage *frame, unsigned char **outbuffer, long unsigned int *outlen) {
unsigned char *outdata = (uchar *) frame->imageData;
struct jpeg_compress_struct cinfo = {0};
struct jpeg_error_mgr jerr;
JSAMPROW row_ptr[1];
int row_stride;

*outbuffer = NULL;
*outlen = 0;

cinfo.err = jpeg_std_error(&jerr);
jpeg_create_compress(&cinfo);
jpeg_mem_dest(&cinfo, outbuffer, outlen);

cinfo.image_width = frame->width;
cinfo.image_height = frame->height;
cinfo.input_components = frame->nChannels;
cinfo.in_color_space = JCS_RGB;

jpeg_set_defaults(&cinfo);
jpeg_start_compress(&cinfo, TRUE);
row_stride = frame->width * frame->nChannels;

while (cinfo.next_scanline < cinfo.image_height) {
    row_ptr[0] = &outdata[cinfo.next_scanline * row_stride];
    jpeg_write_scanlines(&cinfo, row_ptr, 1);
}

jpeg_finish_compress(&cinfo);
jpeg_destroy_compress(&cinfo);

return true;

}

Answer 2

我使用以下代码进行内存到内存压缩使其工作。

#include <stdio.h>
#include "jpeg/jpeglib.h"

    /*
This a custom destination manager for jpeglib that
enables the use of memory to memory compression.

See IJG documentation for details.
*/
typedef struct {
struct jpeg_destination_mgr pub; /* base class */
JOCTET* buffer; /* buffer start address */
int bufsize; /* size of buffer */
size_t datasize; /* final size of compressed data */
int* outsize; /* user pointer to datasize */
int errcount; /* counts up write errors due to
buffer overruns */
} memory_destination_mgr;

typedef memory_destination_mgr* mem_dest_ptr;

/* ------------------------------------------------------------- */
/* MEMORY DESTINATION INTERFACE METHODS */
/* ------------------------------------------------------------- */


/* This function is called by the library before any data gets written */
METHODDEF(void)
init_destination (j_compress_ptr cinfo)
{
mem_dest_ptr dest = (mem_dest_ptr)cinfo->dest;

dest->pub.next_output_byte = dest->buffer; /* set destination buffer */
dest->pub.free_in_buffer = dest->bufsize; /* input buffer size */
dest->datasize = 0; /* reset output size */
dest->errcount = 0; /* reset error count */
}

/* This function is called by the library if the buffer fills up

I just reset destination pointer and buffer size here.
Note that this behavior, while preventing seg faults
will lead to invalid output streams as data is over-
written.
*/
METHODDEF(boolean)
empty_output_buffer (j_compress_ptr cinfo)
{
mem_dest_ptr dest = (mem_dest_ptr)cinfo->dest;
dest->pub.next_output_byte = dest->buffer;
dest->pub.free_in_buffer = dest->bufsize;
++dest->errcount; /* need to increase error count */

return TRUE;
}

/* Usually the library wants to flush output here.

I will calculate output buffer size here.
Note that results become incorrect, once
empty_output_buffer was called.
This situation is notified by errcount.
*/
METHODDEF(void)
term_destination (j_compress_ptr cinfo)
{
mem_dest_ptr dest = (mem_dest_ptr)cinfo->dest;
dest->datasize = dest->bufsize - dest->pub.free_in_buffer;
if (dest->outsize) *dest->outsize += (int)dest->datasize;
}

/* Override the default destination manager initialization
provided by jpeglib. Since we want to use memory-to-memory
compression, we need to use our own destination manager.
*/
GLOBAL(void)
jpeg_memory_dest (j_compress_ptr cinfo, JOCTET* buffer, int bufsize, int* outsize)
{
mem_dest_ptr dest;

/* first call for this instance - need to setup */
if (cinfo->dest == 0) {
cinfo->dest = (struct jpeg_destination_mgr *)
(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
sizeof (memory_destination_mgr));
}

dest = (mem_dest_ptr) cinfo->dest;
dest->bufsize = bufsize;
dest->buffer = buffer;
dest->outsize = outsize;
/* set method callbacks */
dest->pub.init_destination = init_destination;
dest->pub.empty_output_buffer = empty_output_buffer;
dest->pub.term_destination = term_destination;
}

/* ------------------------------------------------------------- */
/* MEMORY SOURCE INTERFACE METHODS */
/* ------------------------------------------------------------- */

/* Called before data is read */
METHODDEF(void)
init_source (j_decompress_ptr dinfo)
{
/* nothing to do here, really. I mean. I'm not lazy or something, but...
we're actually through here. */
}

/* Called if the decoder wants some bytes that we cannot provide... */
METHODDEF(boolean)
fill_input_buffer (j_decompress_ptr dinfo)
{
/* we can't do anything about this. This might happen if the provided
buffer is either invalid with regards to its content or just a to
small bufsize has been given. */

/* fail. */
return FALSE;
}

/* From IJG docs: "it's not clear that being smart is worth much trouble"
So I save myself some trouble by ignoring this bit.
*/
METHODDEF(void)
skip_input_data (j_decompress_ptr dinfo, INT32 num_bytes)
{
/* There might be more data to skip than available in buffer.
This clearly is an error, so screw this mess. */
if ((size_t)num_bytes > dinfo->src->bytes_in_buffer) {
dinfo->src->next_input_byte = 0; /* no buffer byte */
dinfo->src->bytes_in_buffer = 0; /* no input left */
} else {
dinfo->src->next_input_byte += num_bytes;
dinfo->src->bytes_in_buffer -= num_bytes;
}
}

/* Finished with decompression */
METHODDEF(void)
term_source (j_decompress_ptr dinfo)
{
/* Again. Absolute laziness. Nothing to do here. Boring. */
}

GLOBAL(void)
jpeg_memory_src (j_decompress_ptr dinfo, unsigned char* buffer, size_t size)
{
struct jpeg_source_mgr* src;

/* first call for this instance - need to setup */
if (dinfo->src == 0) {
dinfo->src = (struct jpeg_source_mgr *)
(*dinfo->mem->alloc_small) ((j_common_ptr) dinfo, JPOOL_PERMANENT,
sizeof (struct jpeg_source_mgr));
}

src = dinfo->src;
src->next_input_byte = buffer;
src->bytes_in_buffer = size;
src->init_source = init_source;
src->fill_input_buffer = fill_input_buffer;
src->skip_input_data = skip_input_data;
src->term_source = term_source;
/* IJG recommend to use their function - as I don't know ****
about how to do better, I follow this recommendation */
src->resync_to_restart = jpeg_resync_to_restart;
}

在你的主压缩函数中，将 jpeg_stdio_dest 替换为

int numBytes = 0; //size of jpeg after compression
char * storage = new char[150000]; //storage buffer
JOCTET *jpgbuff = (JOCTET*)storage; //JOCTET pointer to buffer
jpeg_memory_dest(&cinfo,jpgbuff,150000,&numBytes);

150000 是一个静态大小的缓冲区，你可能会有超过它的图像，因此进行相应的分配。

Answer 3

我让内存压缩发挥作用。请参阅以下内容

#define OUTPUT_BUF_SIZE 4096 /* choose an efficiently fwrite’able size */

/* Expanded data destination object for memory output */

typedef struct {
struct jpeg_destination_mgr pub; /* public fields */

unsigned char ** outbuffer; /* target buffer */
unsigned long * outsize;
unsigned char * newbuffer; /* newly allocated buffer */
JOCTET * buffer; /* start of buffer */
size_t bufsize;
} my_mem_destination_mgr;

typedef my_mem_destination_mgr * my_mem_dest_ptr;

void
init_mem_destination (j_compress_ptr cinfo)
{
/* no work necessary here */
}

boolean
empty_mem_output_buffer (j_compress_ptr cinfo)
{
size_t nextsize;
JOCTET * nextbuffer;
my_mem_dest_ptr dest = (my_mem_dest_ptr) cinfo->dest;

/* Try to allocate new buffer with double size */
nextsize = dest->bufsize * 2;
nextbuffer = (JOCTET *)malloc(nextsize);

if (nextbuffer == NULL)
ERREXIT1(cinfo, JERR_OUT_OF_MEMORY, 10);

memcpy(nextbuffer, dest->buffer, dest->bufsize);

if (dest->newbuffer != NULL)
free(dest->newbuffer);

dest->newbuffer = nextbuffer;

dest->pub.next_output_byte = nextbuffer + dest->bufsize;
dest->pub.free_in_buffer = dest->bufsize;

dest->buffer = nextbuffer;
dest->bufsize = nextsize;

return TRUE;
}

void
term_mem_destination (j_compress_ptr cinfo)
{
my_mem_dest_ptr dest = (my_mem_dest_ptr) cinfo->dest;

*dest->outbuffer = dest->buffer;
*dest->outsize = dest->bufsize – dest->pub.free_in_buffer;
}

void
jpeg_mem_dest (j_compress_ptr cinfo,
unsigned char ** outbuffer, unsigned long * outsize)
{
my_mem_dest_ptr dest;

if (outbuffer == NULL || outsize == NULL) /* sanity check */
ERREXIT(cinfo, JERR_BUFFER_SIZE);

/* The destination object is made permanent so that multiple JPEG images
* can be written to the same buffer without re-executing jpeg_mem_dest.
*/
if (cinfo->dest == NULL) { /* first time for this JPEG object? */
cinfo->dest = (struct jpeg_destination_mgr *)
(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,
sizeof(my_mem_destination_mgr));
}

dest = (my_mem_dest_ptr) cinfo->dest;
dest->pub.init_destination = init_mem_destination;
dest->pub.empty_output_buffer = empty_mem_output_buffer;
dest->pub.term_destination = term_mem_destination;
dest->outbuffer = outbuffer;
dest->outsize = outsize;
dest->newbuffer = NULL;

if (*outbuffer == NULL || *outsize == 0) {
/* Allocate initial buffer */
dest->newbuffer = *outbuffer = (unsigned char*)malloc(OUTPUT_BUF_SIZE);
if (dest->newbuffer == NULL)
ERREXIT1(cinfo, JERR_OUT_OF_MEMORY, 10);
*outsize = OUTPUT_BUF_SIZE;
}

dest->pub.next_output_byte = dest->buffer = *outbuffer;
dest->pub.free_in_buffer = dest->bufsize = *outsize;
}
//*******************************************************************************************

要使用此功能，请在 main 中执行类似的操作

/************/

unsigned long outlen;
unsigned char *outbuffer;

jpeg_mem_dest (&cinfo,&outbuffer,&outlen );
printf(“outlen is %lu\n”,(long unsigned int)outlen);

Answer 4

在与 libJpeg 斗争了 2 天（指针、内存步进和拉扯头发）之后，我放弃了并使用了最喜欢的保存到磁盘加载到内存的方法，所以如果有人感兴趣的话，这里是方法：

char* convert2jpeg(IplImage* frame, int* frame_size) {

FILE* infile = NULL;
struct stat fileinfo_buf;

if (cvSaveImage(name_buf, frame) < 0) {
    printf("\nCan't save image %s", name_buf);
    return NULL;
}

if (stat(name_buf, &fileinfo_buf) < 0) {
    printf("\nPLAYER [convert2jpeg] stat");
    return NULL;
}

*frame_size = fileinfo_buf.st_size;
char* buffer = (char *) malloc(fileinfo_buf.st_size + 1);

if ((infile = fopen(name_buf, "rb")) == NULL) {
    printf("\nPLAYER [convert2jpeg] fopen %s", name_buf);
    free(buffer);
    return NULL;
}

fread(buffer, fileinfo_buf.st_size, 1, infile);
fclose(infile);

return buffer;
}

我希望有人发现这很有用。我希望 OpenCV 开发人员中的某个人看到这个线程并在 OpenCV 中实现直接到缓冲区 JPEG 转换，从而避免我们的痛苦和 1 次保存/加载到磁盘操作。