Question

I receive images on the PC from two cameras. The stream is not continuous but takes place at event detection. Therefore, I have to receive around 8-10 image pairs after a particular time. Since I have to process the images, I want to first keep all the images in a large buffer and later process them in a batch. It is also acceptable to keep images in two large buffers (one for each camera). How do I keep these pair of images in one (or two) large buffer(s)? I am implementing this program in C.

Answer 1

Assumeing the following API to get the image:

int ImageGet(
  char * pBuffer, 
  size_t * pSize); /* returns 0 on success or any other value if no image is available or on error */

you could do the following:

#include <stdlib.h>
#include <string.h>

...

#define IMAGESIZE_MAX (1024*1024) 

char * pBuffer = NULL;
char * pBufferCurrent = pBuffer;
int iResult = 0;
size_t size = IMAGESIZE_MAX;
size_t sizeTotal = 0;

do 
{
  char * pBufferCurrent = realloc (
    pBuffer, 
    sizeTotal + sizeof(size) + size);

  if (!pBufferCurrent)
  {
    break;
  }

  pBuffer = pBufferCurrent;

  pBufferCurrent += sizeTotal;

  if ((iResult = ImageGet (
    pBufferCurrent + sizeof(size), 
    &size))
  {
    break; 
  }

  memcpy (
    pBufferCurrent,
    &size, 
    sizeof(size));

  sizeTotal += (sizeof(size) + size);
} while (1);

...

This stores your data sequentially:

[     size 1     ][  image data 1  ][     size 2     ][  image data 2  ]...
<-sizeof(size_t)-><-    size 1    -><-sizeof(size_t)-><-    size 2    ->

in the buffer pointed to by pBuffer.

---

To retrieve a reference to the data describing image N from the buffer pBuffer you could use the following method:

const char * BufferImageGetByIndex(
    const char * pBuffer,
    unsigned indexImage)
{
  for (unsigned indexImageCnt = 0;
    indexImageCnt < indexImage;
    ++ indexImageCnt)
  {
    pBuffer += sizeof(size_t) + *((size_t *) pBuffer);
  }

  return pBuffer;
}

Then to actually get an image you could do like so:

...
/* Get a reference to the data decribing **fourth** image in the buffer filled by the example above. */
const char * pBufferImage4 = *BufferImageGetByIndex(pBuffer, 3);

/* Get the size ... */
size_t sizeImage4 = *((size_t *) pBufferImage4);
/* ... and a reference to the image data. */
const char * pImage4 = pBufferImage4 + sizeof(size_t);
...