Search code examples
imagejpegemgucvsteganographyimage-conversion

A code for step by step JPEG compression

The JPEG compression steps are as follows:

Raw image data -> forward DCT -> Quantization -> Entropy encoding -> JPEG image

There are numbers of converters and APIs out there and the converting process is a single API call. I was unable to find a step by step code. My question is where can I find a code for each individual step, or can I perform these individual steps one by one and produce a standard JPEG image? I am using EmguCV for my image steganography project.

Solution

  • where can I find a code for each individual step

    If a C library might be a candidate for you, you should have a look at jpec a lightweight JPEG encoder written in C - note that it only supports grayscale images.

    The main encoding function (jpec_enc_run) is easy to read and provides each above step via internal functions:

    /* open = write JPEG headers */
jpec_enc_open(e);

while (jpec_enc_next_block(e)) {
  /* compute the DCT for the current 8x8 block */
  jpec_enc_block_dct(e);

  /* quantize the DCT coefficients for the current block */
  jpec_enc_block_quant(e);

  /* re-order the quantized coefficients with the zig-zag pattern */
  jpec_enc_block_zz(e);

  /* perform entropy coding of the current block and write to the global buffer*/
  e->hskel->encode_block(e->hskel->opq, &e->block, e->buf);
}

/* close = write JPEG end of image marker */
jpec_enc_close(e);

    can I perform these individual steps one by one and produce a standard JPEG image

    This is not available out-of-the box with jpec, but you should be able to modify it for that purpose quite easily (by exposing and/or adapting the low-level internal functions).