Distinguishing Photographic Images and Photorealistic Computer Graphics Using Visual Vocabulary on Local Image Edges Rong Zhang,Rand-Ding Wang, and Tian-Tsong Ng

Definition photographic images (PIM): generated from natural scene by digital imaging tools, also called as natural images. photorealistic computer graphics (PRCG): created by a variety of rendering software with high photorealism.

Outline Introduction Data Sets Image Classification Based on Image Edge Vocabulary Experimental Results Conclusions and Future Work

Introduction Acquisition of PIM

Introduction Creating of PRCG By skilled artists or professional programmers Using artificial models Virtual scene Simplified generation process due to time-cost and computation complexity

We are attempting to identify natural images and photorealistic computer graphics. Basic idea: To exploit the statistical property of local edge patches in images.

Outline Introduction Data Sets Image Classification Based on Image Edge Vocabulary Experimental Results Conclusions and Future Work

Data Sets We collect two data sets, namely PIM data set and PRCG data set, respectively consisting of 1000 PIM and 900 PRCG.

Data Sets Considerations To explore the essential properties of natural images, we don’t take those images from Internet, which may undergo various unknown post-processing and compression at various quality factors. We collect all images in PIM set with high quality JPEG format from 8 consumer-end cameras and without any experience of post-process outside the cameras.

Considerations PRCG set contains 800 images from Columbia PRCG data set and 100 CG images with high visual realism from website

Outline Introduction Data Sets Image Classification Based on Image Edge Vocabulary Experimental Results Conclusions and Future Work

Image Classification Based on Image Edge Vocabulary Image Edge Vocabulary It is derived from bag-of-words model in text categorization and bag-of-visual-words model in visual categorization. A visual word corresponds to a cluster center and the vocabulary is constructed by a set of cluster centers. The basic idea of the bag-of-visual-words helps us efficiently capture the significant difference in statistical distribution of geometrical structure of local edge patches between PIM and PRCG.

Presentation of 3×3 Edge Patches Convert color to grayscale. Each patch is regarded as 9-tuple of real number (log of gray values), i.e. a vector in Detect edge Define neighborhood blocks around edge points Randomly select ×3 local patches

Image Classification Based on Image Edge Vocabulary Data preprocessing Define contrast ||X|| D (D-norm) : where i ~j represents the 4-connected neighborhood.

Image Classification Based on Image Edge Vocabulary Data preprocessing Subtracting the mean and contrast normalizing lead to a new vector: where

Image Classification Based on Image Edge Vocabulary Data preprocessing Make change of basis with 2-dimensional Discrete Cosine Transform (DCT) basis corresponding to image patches: where

Image Classification Based on Image Edge Vocabulary Data preprocessing v is located on 7-sphere in a 8-D Euclidean space:

Image Classification Based on Image Edge Vocabulary Data preprocessing Calculate the angular distance between two points :

Image Classification Based on Image Edge Vocabulary Construction of Visual Vocabulary 17,520 Voronoi cells with roughly the same size and efficiently covering the 7–sphere are selected as the sampling point set: Where O i is the sampling point in the ith lattice and is a 8-D vector.

Image Classification Based on Image Edge Vocabulary Construction of Visual Vocabulary Now, the problem to observe the distribution of data points on the 7-sphere is converted to the one, which we should calculate the possibilities that data points fall into the corresponding Voronoi tessellations. we can use the histograms with 17,520 bins to respectively describe the possibility distribution of geometrical structure of edge patches of PIM and PRCG.

Construction of Visual Vocabulary Figure: Probabilities of edge patches in Voronoi cells that are sorted according to decreasing probability: (left) 200K edge patches from 200 PIM images; (right) 98,599 edge patches from 100 CG images collected by ourselves.

Image Classification Based on Image Edge Vocabulary Construction of Visual Vocabulary We find a smaller set of Voronoi cells can pick up the majority of the patches for both PIM and PRCG. We look upon the sampling points corresponding to those Voronoi cells with larger possibility as key sampling points.

Construction of Visual Vocabulary We construct image edge vocabulary based on these key sampling points.

Image Classification Based on Image Edge Vocabulary Image Classification

Outline Introduction Data Sets Image Classification Based on Image Edge Vocabulary Experimental Results Conclusions and Future Work

Experimental Results We use the remaining 800 PIM and 800 PRCG images from our data sets to evaluate the proposed method. The 1,600 images are used for evaluating our method through 10-fold cross-validation.

Experimental Results Image Classification We determine vocabulary size according to different possibility thresholds. Table: Classification accuracy of different vocabulary sizes As a performance- cost tradeoff On the same datasets, the results of Farid’s method

Experimental Results Generalization Capability To verify the generalization capability, we remove all (50) images taken by the Samsung camera from the PIM samples and train the classifier with the remaining 750 PIM images and 800 Columbia PRCG images. Only one image is incorrectly classified (classification accuracy 98%) with the proposed method while all images fail to be correctly detected with Farid’s. This result may indicate that the visual vocabulary based on local edge patch can characterize the general property of a special image source better. We need more experiments to ensure it.

Experimental Results Compression Attack 1600 images are compressed respectively with quality factor 90, 70, and 40. Table: Comparative experimental results of the proposed approach and Farid’s([5] ) on datasets with different JPEG compression factors.

Outline Introduction Data Sets Image Classification Based on Image Edge Vocabulary Experimental Results Conclusions and Future Work

Conclusions we have proposed a new approach of PIM and PRCG classification based on the idea of bag-of-visual-words. By projecting the image patch data onto a 7-dimensional sphere with a series of transforms, we observe the distribution of data points in individual Voronoi lattice. Then, visual vocabulary is constructed through determining the key sampling points corresponding to Voronoi cells. And then, a given image is represented as a histogram of visual words. Finally, we employ SVM classifier.

Conclusions Our experimental results demonstrate the efficient discrimination of the features. It is revealed that the intrinsic difference between PIM and PRCG may be captured by the geometry structure of local edge patches. Our conclusions is of great significance for digital image forensic as well as photorealism evaluation for computer graphics.

Future work To modify the proposed method, we are considering a closer analogy to document retrieval. We wish to make sense of the visual vocabulary set. We have had attempts to evaluate the generalization and resistance to compression of the proposed approach. More experiments are being done on a wider range of images.

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