Lidu

Background

With the development of digital image processing in recent years， numerous image editing techniques have been developed so that realistic synthetic images can be produced conveniently without leaving noticeable visual artifacts. Although these image editing technologies can enrich the user experience, they also cause the problem for people to trust the authenticity of the images. JPEG is the most popular image format for cameras, and the modification of JPEG images may be re-saved to generate a new image, which would cause the double compression of the image. This project focuses on the detection of double compression and single compression to verify the authenticity of the image.

Introduction

JPEG Principle

JPEG (Joint Photographic Experts Group) is one of the most widely used standards for compressing image data. It is a lossy compression scheme. The procedure of the JPEG compression is like follows:

1. Given a three channel color image (RGB), convert it to YCbCr (luminance/chrominance) space.
2. The two chrominance channels (CbCr) are subsampled by a factor of two relative to the luminance channel (Y).
3. Each channel is divided into many 8*8 blocks, and all unsigned integers (range from 0 to 255) are converted to signed integers (range from -128 to 127).
4. DCT (Discrete Cosine Transform) is applied to each block in each channel, the lowest frequency resides in the upper-left corner, and the highest frequency resides in the lower-right corner of each block.The same position in each block is called a mode. The mode table is shown below.
5. For a pre-defined quantization table (also of the size 8*8), each DCT coefficient is quantized by the corresponding the step in the quantization table, and then rounded to the nearest integer. Usually, the steps for low frequencies are small and steps for high frequencies are big, so the high frequency can be removed without much effect on human vision.
6. Encode the quantized coefficients and the compression is completed.

Table 1

Double JPEG Compression

Double Compression can be regarded as a compression of a compressed image. For a compressed image, inverse quantization and inverse DCT are applied to decompress. After rounding all the values to integers, JPEG compression procedure is applied again, usually with quantization steps.