Crate dct_io 

Read and write quantized DCT coefficients in baseline JPEG files.

This crate provides direct access to the quantized DCT coefficients stored in the entropy-coded data of a baseline JPEG. It is useful for steganography, watermarking, forensic analysis, and JPEG-domain signal processing where you need to read or modify coefficients without fully decoding the image to pixel values.

What this crate does

JPEG compresses images by dividing them into 8×8 pixel blocks, applying a Discrete Cosine Transform (DCT) to each block, quantizing the resulting coefficients, and then entropy-coding them with Huffman coding. This crate parses the entropy-coded stream, decodes the Huffman symbols, reconstructs the quantized coefficient values, and lets you read or modify them before re-encoding everything back into a valid JPEG byte stream.

What this crate does NOT do

• It does not decode pixel values (no IDCT, no dequantisation).
• It does not support progressive JPEG (SOF2), lossless JPEG (SOF3), or arithmetic coding (SOF9). Passing such files returns an error.
• It does not support JPEG 2000.

Supported JPEG variants

• Baseline DCT (SOF0) — the most common variant
• Extended sequential DCT (SOF1) — treated identically to SOF0
• Grayscale (1 component) and colour (3 components, typically YCbCr)
• All standard chroma subsampling ratios (4:4:4, 4:2:2, 4:2:0, etc.)
• EXIF and JFIF headers
• Restart markers (DRI / RST0–RST7)

Example

use dct_io::{read_coefficients, write_coefficients};

let jpeg = std::fs::read("photo.jpg").unwrap();

let mut coeffs = read_coefficients(&jpeg).unwrap();

// Flip the LSB of every eligible AC coefficient in the first component.
for block in &mut coeffs.components[0].blocks {
    for coeff in block[1..].iter_mut() {
        if *coeff != 0 {
            *coeff ^= 1;
        }
    }
}

let modified = write_coefficients(&jpeg, &coeffs).unwrap();
std::fs::write("photo_modified.jpg", modified).unwrap();

Structs

ComponentCoefficients

Quantized DCT coefficients for a single component (Y, Cb, or Cr).

ComponentInfo

Metadata for a single image component, as read from the SOF marker.

JpegCoefficients

Quantized DCT coefficients for all components in a JPEG image.

JpegInfo

Image metadata extracted from a JPEG without decoding the entropy stream.

Enums

DctError

Errors returned by this crate.

Functions

block_count

Return the number of 8×8 DCT blocks per component in a JPEG.

eligible_ac_count

Count the number of AC coefficients with |v| >= 2 across all components.

inspect

Inspect a JPEG and return image metadata without decoding the entropy stream.

read_coefficients

Decode the quantized DCT coefficients from a baseline JPEG.

write_coefficients

Re-encode a JPEG with modified DCT coefficients.