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An efficient and effective region-based image retrieval framework Reporter: Francis 2005/5/12
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2 Outline 1. Introduction 2. Image content representation 3. Region-based retrieval 4. Relevance feedback 5. Learning region weighting 6. Experiments
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3 1.1 Region-based image retrieval RBIR attempt to overcome the drawback of global features by representing images at object-level. It has three issues: 1. How to compare two images: definition of image similarity measure 2. How to make it scalable: saving time or space 3. How to make it improve retrieval accuracy by interacting with users: the strategy of RF
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4 1.2 Our approach 1. The image similarity measure we adopt is the Earth Mover’s Distance (EMD) [26] 2. To be scalable, a region codebook is designed and utilized to save storages. 3. RF strategies: Only using positive feedback: QVM Using both positive and negative feedback: modified SVM
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5 1.3 Overview of our approach
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6 2. Image content representation Images are first segmented into homogeneous regions.(JSEG algorithm[6]) Visually similar regions are clustered to form a region codebook. Images are encoded in two ways: Compact representation that saves storage. Sparse and uniform representation enables effective RF techniques.
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7 2.1 Region properties 1. Visual features: we using color moment Simple, robust, effective. 2. Importance weight: Initial weight: the percentage of a region in an image. (discussed in 5) The sum of importance weights for an image should be normalized to 1.
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8 2.2 Compact and sparse representations
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9 3. Region–based retrieval
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10 3.1 image similarity measure Traditional measure: Euclidean distance Not considering the correlation between two codewords. Earth Mover’s Distance (EMD) [26]: A flexible similarity measure between multidimensional distributions.
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11 3.1 image similarity measure
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12 3.1 image similarity measure
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13 3.2 Indexing using modified inverted file Inverted file (IF) is the most common indexing structure used in information retrieval for simplicity and effectiveness. For each codeword, a list of images corresponding to the codeword is stored as the IF. When query a image, the codewords corresponding to the regions of the query are identified, then images that appear in the IF are regarded as candidates for further calculation.
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14 3.2 IF’s problem
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15 3.3 modified inverted file (MIT) It contains not only a list of images but also k most similar codewords sorted by their similarity to it (Using EMD). 意指 codeword 間可以比較 If region’s weight is w, we expand codewords to it. If k is large more expanded codewords and more accuracy results but more comparison time
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16 4. Relevance feedback Weighted query point movement: Using decaying factor α to reduce the effect of previous positive examples. αis set to be 0 in the first iteration and (1/m) after the second iteration. βis set to be 1/(n-m+1) and 1/[m(α-1)+n] after second iteration.
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17 4. Relevance feedback SVM with positive and negative example: Modified kernel is
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18 5. Learning region weighting Basis assumption in [14] is that important regions should appear more times in positive images and fewer times in negative images.
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19 5.1 Basic definition The region frequency is:
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20 5.1 Basic definition A region becomes less important for a query if it is similar to many images in the database. We define a measure called inverse image frequency:
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21 5.2 Defining region importance 111 The cumulation makes the RI more robust.
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22 6. Experiment results
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23 6. Experiment results
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24 6. Experiment results
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