Revenue maximizing envy-free fixed-price auctions with budgets

Traditional incentive-compatible auctions [6,16] for selling multiple goods to unconstrained and budgeted bidders can discriminate between bidders by selling identical goods at different prices. For this reason, Feldman et al. [7] dropped incentive compatibility and turned the attention to revenue maximizing envy-free item-pricing allocations for budgeted bidders. Envy-free allocations were suggested by classical papers [9,15]. The key property of such allocations is that no one envies the allocation and the price charged to anyone else. In this paper we consider this classical notion of envy-freeness and study fixed-price mechanisms which use nondiscriminatory uniform prices for all goods. Feldman et al. [7] gave an item-pricing mechanism that obtains 1/2 of the revenue obtained from any envy-free fixed-price mechanism for identical goods. We improve over this result by presenting an FPTAS for the problem that returns an (1 − ε)-approximation of the revenue obtained by any envy-free fixed-price mechanism for any ε > 0 and runs in polynomial time in the number of bidders n and 1/ ε even for exponential supply of goods m. Next, we consider the case of budgeted bidders with matching-type preferences on the set of goods, i.e., the valuation of each bidder for each item is either v i or 0. In this more general case, we prove that it is impossible to approximate the optimum revenue within O( min (n,m)1/2 − ε ) for any ε > 0 unless P = NP. On the positive side, we are able to extend the FPTAS for identical goods to budgeted bidders in the case of constant number of different types of goods. Our FPTAS gives also a constant approximation with respect to the general envy-free auction

A New Ranking Approach and a Revisited Ratio Test for Improving Content-Based Image Retrieval

Geometric Verification (GV) is the last step for most visual search systems. It consists of two parts: first, ratio test is used to find matches between feature descriptors; second, a geometric consistency check is applied. Both steps are computationally expensive, but all the attempts made to speed up the process deal with the geometric check part only. In this work, we focus indeed on ratio test. Using simple PCA and other tricks, a speed-up of an order of magnitude is achieved preserving good retrieval accuracy. Moreover, we propose a modified ranking approach which exploits distance information between descriptors and further improves retrieval performance

MPEG-7 Requirements to Highlight and Order Description Items

This document proposes some additional requirements to the MPEG-7 Requirements Document [2]. We think that some very important MPEG-7 requirements should deal with the concepts of (1) highlighting description items (e.g. images, sounds, events, objects etc.) which are most relevant to characterize a multimedia document and (2) of information ordering. In other words, due to a possible high amount of description items, an entity who will create descriptions of multimedia (MM) documents, according to MPEG-7 standard specifications (i.e. a description provider), shall highlight the items most representative for the kind of document being described in order to facilitate user needs (such as queries, navigation etc.). Besides, we consider the need of providing users with ordering mechanisms a very relevant issue to MPEG-7. Such ordering mechanisms can be derived by combining descriptor values (e.g., a set of key – frames ordered on the basis a color descriptor or a set of sounds ordered by means of their loudness descriptor). However the possible large variety of descriptors forming a description could consequently lead to a high number of ordering criteria for every description item of a subset of these items. Therefore we propose that the description provider should also highlight a reduced set of descriptors allowing to order some elements (e.g., key frames, events etc.) of a certain description in a proper manner for the MM document being described. The document is organized as follows: we explain the motivation behind our proposals in Section 2 and we present the new requirements in Section 3

Indexing Audio-Visual Sequences by Joint Audio and Video Processing

The focus of this work is oriented to the creation of a content-based hierarchical organisation of audio-visual data (a description scheme) and to the creation of meta-data (descriptors) to associate with audio and/or visual signals. The generation of efficient indices to access audio-visual databases is strictly connected to the generation of content descriptors and to the hierarchical representation of audio-visual material. Once a hierarchy can be extracted from the data analysis, a nested indexing structure can be created to access relevant information at a specific level of detail. Accordingly, a query can be made very specific in relationship to the level of detail that is required by the user. In order to construct the hierarchy, we describe how to extract information content from audio-visual sequences so as to have different hierarchical indicators (or descriptors), which can be associated to each media (audio, video). At this stage, video and audio signals can be separated into temporally consistent elements. At the lowest level, information is organised in frames (groups of pixels for visual information, groups of consecutive samples for audio information). At a higher level, low-level consistent temporal entities are identified: in case of digital image sequences, these consist of shots (or continuous camera records) which can be obtained by detecting cuts or special effects such as dissolves, fade in and fade out; in case of audio information, these represent consistent audio segments belonging to one specific audio type (such as speech, music, silence, ...). One more level up, patterns of video shots or audio segments can be recognised so as to reflect more meaningful structures such as dialogues, actions, ... At the highest level, information is organised so as to establish correlation beyond the temporal organisation of information, allowing to reflect classes of visual or audio types: we call these classes idioms. The paper ends with a description of possible solutions to allow a cross-modal analysis of audio and video information, which may validate or invalidate the proposed hierarchy, and in some cases enable more sophisticated levels of representation of information content

Progressive Medical Image Compression using a Diagnostic Quality Measure on Regions-of-Interest

Dealing with lossy compression of medical images requires particular attention whether for still images, video or volumetric slice-sets. In this work we propose an approach based on a selective allocation of coding resources that is directly related to the diagnostic task. We introduce the concepts of Region of Diagnostic Interest (RODI) and Diagnostic Quality as key links between the radiological activities and responsibilities and the functioning of a selective coding algorithm. The coding engine is a modied version of Shapiro's EZW algorithm and the coded bit-stream is fully progressive. The RODI selectivity corresponds to the choice of a set of subband weighting masks that depends on a small set of parameters handled and validated by the radiologist in a very natural manner. In conclusion, we present some experimental results that give interesting insights in favor of using lossy compression in a controlled fashion by a competent physician

Two-Way Interactive Refinement of Segmented Medical Volumes

For complex medical image segmentation tasks which also require high accuracy, prior information must usually be generated in order to initialize and confine the action of the computational tools. This can be obtained by task oriented specialization layers operating within automatic segmentation techniques or by advanced exploitation of user- data interaction, in this case the segmentation technique can conserve generality and results can be inherently validated by the user itself, in the measure he is allowed to effectively steer the process towards the desired result. In this paper we present a highly accurate and still general morphological 3D segmentation system where rapid convergence to the desired result is guaranteed by a two-way interactive segmentation-refinement loop, where the flow of prior information is inverted (from computing tools to the user) in the refinement phase in order to help the user to quickly select most effective refinement strategies

Efficient (Piecewise) Linear Minmax Approximation of Digital Signals

In this work efficient geometric algorithms are provided for the linear approximation of digital signals under the uniform norm. Given a set of n points (xi, yi)i=1..n, with xi < xj if i < j, we give a new method to find the optimum linear approximation in O(n). Given also an error bound, we demonstrate how to construct in O(n) a non continuous piecewise solution such that the number k of segments is optimal. Furthermore we show that for such number of segments, the solution that is l∞ optimal can also be found in O(n) provided that n/k = O(1)

On Unique Decodability

In this paper we propose a revisitation of the topic of unique decodability and of some fundamental theorems of lossless coding. It is widely believed that, for any discrete source X, every "uniquely decodable" block code satisfies E[l(X_1 X_2 ... X_n)]>= H(X_1,X_2,...,X_n), where X_1, X_2,...,X_n are the first n symbols of the source, E[l(X_1 X_2 ... X_n)] is the expected length of the code for those symbols and H(X_1,X_2,...,X_n) is their joint entropy. We show that, for certain sources with memory, the above inequality only holds when a limiting definition of "uniquely decodable code" is considered. In particular, the above inequality is usually assumed to hold for any "practical code" due to a debatable application of McMillan's theorem to sources with memory. We thus propose a clarification of the topic, also providing an extended version of McMillan's theorem to be used for Markovian sources.Comment: Accepted for publication, IEEE Transactions on Information Theor