How to get the width/height of jpeg file without using library?

Question

Firstly I want to say I tried many times to find the answer by using google search, and I found many results but I did not understand, because I don\'t know the idea of read

Answer 1

Here's some simple code I wrote which seems to work reliably.

#define MOTOSHORT(p) ((*(p))<<8) + *(p+1)
unsigned char cBuf[32];
int iBytes, i, j, iMarker, iFilesize;
unsigned char ucSubSample;
int iBpp, iHeight, iWidth;

         Seek(iHandle, 0, 0); // read the first 32 bytes
         iBytes = Read(iHandle, cBuf, 32);

         i = j = 2; /* Start at offset of first marker */
         iMarker = 0; /* Search for SOF (start of frame) marker */
         while (i < 32 && iMarker != 0xffc0 && j < iFileSize)
            {
            iMarker = MOTOSHORT(&cBuf[i]) & 0xfffc;
            if (iMarker < 0xff00) // invalid marker, could be generated by "Arles Image Web Page Creator" or Accusoft
               {
               i += 2;
               continue; // skip 2 bytes and try to resync
               }
            if (iMarker == 0xffc0) // the one we're looking for
               break;
            j += 2 + MOTOSHORT(&cBuf[i+2]); /* Skip to next marker */
            if (j < iFileSize) // need to read more
               {
               Seek(iHandle, j, 0); // read some more
               iBytes = Read(iHandle, cBuf, 32);
               i = 0;
               }
            else // error, abort
               break;
            } // while
         if (iMarker != 0xffc0)
            goto process_exit; // error - invalid file?
         else
            {
            iBpp = cBuf[i+4]; // bits per sample
            iHeight = MOTOSHORT(&cBuf[i+5]);
            iWidth = MOTOSHORT(&cBuf[i+7]);
            iBpp = iBpp * cBuf[i+9]; /* Bpp = number of components * bits per sample */
            ucSubSample = cBuf[i+11];
            }