Determining the size of a JPEG (JFIF) image

user_4528 0
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I need to find the size of a JPEG (JFIF) image. The image is not saved as a stand-alone file, so I can't use GetFileSize or any other API such this one (the image is placed in a stream and no other header is present, except the usual JPEG/JFIF header(s)).

I did some research and found out that JPEG images are composed of different parts, each part starting with a frame marker (0xFF 0xXX), and the size of this frame. Using this information I was able to parse a lot of information from the file.

The problem is, I cannot find the size of the compressed data, as it seems there is no frame marker for the compressed data. Also, it seems the compressed data follows the SOS (FFDA) marker and the image ends with the End Of Image (EOI) (FFD9) marker.

A way to accomplish this would be to search for the EOI marker from byte to byte, but I think the compressed data might contain this combination of bytes, right?

Is there an easy and correct way to find the total size of the image? (I would prefer some code/idea without any external library)

Basically, I need the distance (in bytes) between the Start of Image (SOI-FFE0) and End of Image (EOI-FFD9).

size jpeg
4 Answers

Maybe something like this

int GetJpgSize(unsigned char *pData, DWORD FileSizeLow, unsigned short *pWidth, unsigned short *pHeight)
{
  unsigned int i = 0;


  if ((pData[i] == 0xFF) && (pData[i + 1] == 0xD8) && (pData[i + 2] == 0xFF) && (pData[i + 3] == 0xE0)) {
    i += 4;

    // Check for valid JPEG header (null terminated JFIF)
    if ((pData[i + 2] == 'J') && (pData[i + 3] == 'F') && (pData[i + 4] == 'I') && (pData[i + 5] == 'F')
        && (pData[i + 6] == 0x00)) {

      //Retrieve the block length of the first block since the first block will not contain the size of file
      unsigned short block_length = pData[i] * 256 + pData[i + 1];

      while (i < FileSizeLow) {
        //Increase the file index to get to the next block
        i += block_length; 

        if (i >= FileSizeLow) {
          //Check to protect against segmentation faults
          return -1;
        }

        if (pData[i] != 0xFF) {
          return -2;
        } 

        if (pData[i + 1] == 0xC0) {
          //0xFFC0 is the "Start of frame" marker which contains the file size
          //The structure of the 0xFFC0 block is quite simple [0xFFC0][ushort length][uchar precision][ushort x][ushort y]
          *pHeight = pData[i + 5] * 256 + pData[i + 6];
          *pWidth = pData[i + 7] * 256 + pData[i + 8];

          return 0;
        }
        else {
          i += 2; //Skip the block marker

          //Go to the next block
          block_length = pData[i] * 256 + pData[i + 1];
        }
      }

      //If this point is reached then no size was found
      return -3;
    }
    else {
      return -4;
    } //Not a valid JFIF string
  }
  else {
    return -5;
  } //Not a valid SOI header

  return -6;
}  // GetJpgSize
user_2437830
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