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1How
Rea
listic
is P
hoto
real
istic
?
2
How Realistic is Photorealistic?
Yaniv Lefel
Hagay Pollak
Based on the work of - Siwei Lyu and Hany Farid
3
Introduction
• Among the set of all possible images, natural images only occupy a tiny subspace.
• For instance, there are totally 256^(n^2) different 8-bit grayscale images of size nxn pixels. Natural images are sparsely distributed in the space of all possible images.
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Image space
•e.g. when n = 10 pixels, it results in 1.3x10^154 different images !!!
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Introduction (cont’)
• The regularities within natural images can be modeled statistically.
• Image statistical models are already in use by applications such as:Compression, de-noising, segmentation, texture synthesis, content-based retrieval and object/scene categorization.
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Motivation 1 Identify Computer Graphics
• Sophisticated computer graphics software can generate highly convincing photorealistic images able to deceive the human eye.
• Differentiating these two types of images is an important task to ensure the authenticity and integrity of photographs.
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Computer graphics example
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Motivation 2Identify Steg Images
• Image steganography hides messages in digital images in a non-intrusive way that is hard to detect visually.
• The task of generic steganalysis is to detect the presence of such hidden messages without the detailed knowledge of the embedding methods.
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Steganography example
• Steg is the message image embedded into the original image.• The rightmost image is the absolute value of the difference
between the original and steg image, normalized into 8 bit for display purposes.
Original
message
Steg |Original-Stego|
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How ? Example
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Motivation 3Identify Re-broadcasting
• Biometrics-based (e.g., face, iris, or voice) authentication and identification systems are vulnerable to the “rebroadcast” attacks. (e.g. using a high-resolution photograph of a human face).
• We need to differentiate a “live” image (captured in real time by a camera) and a “rebroadcast” one (a photograph).
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How to distinuish images ?
• Image properties ? – Image intensity histogram– Image frequency
Other method ?
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Using known methods
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Why wavelets
• Image representations based on multi-scale image decomposition (e.g., wavelets) decompose an image with basis functions partially localized in both space and frequency - a compromise between these representations.
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QMF - Quadrature Mirror Filter
• The QMF pyramid decomposition splits the image frequency space into three different scales, and within each scale, into three orientation subbands (Vertical, Horizontal and Diagonal).
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QMF diagram
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QMF
• The vertical, horizontal and diagonal subbands at scale i are denoted by Vi(x; y), Hi(x; y), and Di(x; y), respectively.
• Can be generated by convolving the image, I(x, y), with low-pass and high-pass filters.
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QMF decomposition – Example 1
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QMF decomposition – Example 2
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Example – QMF statistics
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Add some magic …• QMF
coefficients
• Magic Box
• Error coefficients
Simple but long (and out of scope)
mathematical procedure
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Technique Diagram
Feature vector
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Computing the Feature Vector
• 3 – Sub-bands (vertical, horizontal, diagonal).• 3 – Scales (levels of decompositions).• 4 – First order statistics (mean, variance, skewness
– asymmetry measure, kurtosis).• 3 – Colors (RGB)• 2 – marginal statistics (wavelet coefficients),
error statistics.• 216 = 3*3*4*3*2
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Image examples
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Feature vectors projected on 3D space
Natural image – Blue.Synthetic images - noise (Green), fractal (Black), and discs (Red)
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Learning and Testing CG\Steg\rebroadcast
• CG\steg\rebroadcast images are prepared.• Statistics is collected over natural images and
CG\steg\rebroadcast images (not using color).• A Machine learning system (e.g. FLD, LDA,
SVM) is then trained on some of the natural and some of the CG\steg\rebroadcast images.
• The remaining natural and CG\steg\rebroadcast images are used for testing.
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Natural vs. CG results (SVM)
All images
Train Succ [%]
Test Succ [%]
Natural 40000 32000 70.9 8000 66.8
CG 6000 4800 99.1 1200 98.8
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Training the system
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Photorealistic (CG) images
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The Impact of Color
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Correctly Classified Photorealistic
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Incorrectly Classified Photographic
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Natural vs. Steganography images
• A message consists of a 64x64 pixel region of a random image chosen from the same image database.
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Natural vs. Steganography results
All images
Train Succ [%]
Test Succ [%]
Natural 1000 750 99.5 250 98.9
Steg 1000 750 98.3 250 97.6
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Live vs. rebroadcast
• We collect statistics from natural images and the same images after having been printed on a laser printer and re-scanned with a scanner (printing and scanning are done at 72 dpi).
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Live vs. rebroadcast results
All images
Train Succ [%]
Test Succ [%]
Natural 1000 750 99.5 250 99.5
rebroadcast
200 150 100 50 99.8
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Live vs. rebroadcast (cont’)
• Remark: It is not surprising that printing significantly disturbs the image statistics. Detecting a rebroadcast image will become more difficult with printers improvement.
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Rebroadcasting example
Shown is the original iris images (top row) and the images after being printed and scanned (bottom row).
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Feature vectors projected on 3D space
Results from a four-way classifier of 1000 natural, 1000 steg, 500 graphic, and 200 rebroadcast images.
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More Applications
how many different artists ?
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More Applications
Forgery detection.
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Finally
• Statistical model.
• capture regularities that are inherent to photographic images.
• Distinguish tampered \ CG images and natural images.