Hoi-Kok Cheung

Fast global motion estimation for sprite generation

In this paper, an efficient and fast global motion estimation method is proposed. This method is a generic one that can accommodate various global motion models, ranging from a simple translation model to an eight parameters perspective model. At the first stage, a simple filter is implemented to classify different parts of the frames. Since pixels from homogeneous regions and highly textured regions do not contribute much in global motion estimation, it is proposed that they-be excluded from processing. The remained pixels are fed into a three-level hierarchical motion estimation system and a dynamic sprite is eventually generated. From the experimental results, we can conclude that the proposed global motion estimation system manages to speed up the processing time for sprite generation while maintaining quality in the reconstructed frames.

New Block-Based Motion Estimation for Sequences with Brightness Variation and Its Application to Static Sprite Generation for Video Compression

In this brief, a new local motion estimator is proposed which can accurately estimate motion activities under varying strong brightness conditions. The proposed estimator makes use of a new block division technique which manages practically to get rid of the adverse influence caused by brightness changes between frames. We also propose a new static sprite coding system using the proposed local motion estimator. The system is characterized not only with the features of accurate motion estimation under varying brightness conditions, but also possesses the capability of coding the brightness variability of the background scene using a single layered sprite image. Experimental results show that the resulting static sprite coding system improves the PSNR by 6.32 dB as compared with the conventional static sprite coding system when the background scenes of the video sequences involve strong brightness variations in the spatial and time domains.

Accurate distance estimation using camera orientation compensation technique for vehicle driver assistance system

This paper proposes a camera orientation compensation technique to help compensating unwanted camera orientation so as to accurately estimate the distance of a preceding vehicle in a vehicle driver assistance system. We propose a method in computing a homography to compensate the unwanted camera orientation misalignment. Distance between the preceding vehicle and the digital camera can then be accurately estimated. We will demonstrate that this approach be most suitable for vehicle driver assistance systems, and it also further expands the extent of automobile safety.

Local affine motion prediction for H.264 without extra overhead

Conventional coding system using translation only motion estimation and compensation system cannot efficiently handle complex inter-frame motion activity including scaling, rotation and various forms of distortion. In this paper, we propose a local affine motion prediction method, which manages to enhance the inter-frame image prediction quality using the conventional motion vectors. Our method is characterized with the fact that no extra bit has to be sent to the decoder for proper decoding. Experimental results show that our method manages to achieve a maximum average bit rate reduction of 1.7% compared to the conventional inter-frame prediction method using translation only motion compensation techniques, at no cost on quality degradation.

Retinex based motion estimation for sequences with brightness variations and its application to H.264

Conventional motion estimation does not take inter-frame brightness variations into consideration, which causes inefficient video coding for sequences involving brightness variations. H.264 provides a specific mode called weighted prediction targeting to improve the coding efficiency for this case. In this paper, we propose a Retinex based motion estimation scheme which effectively removes the inter-frame de-correlation factor resulting from brightness variations. We also propose to use some DCT techniques to generate the Retinex images for both current and reference images and apply conventional motion estimation and compensation procedures for coding. We applied the scheme to the H.264 testing the efficiency in the multiple reference frame motion compensation environment. Experimental results show that our proposed scheme outperforms the H.264 system with weighted prediction enabled. It allows the system to use a smaller number of reference frames for coding, e.g. 2, to achieve a similar (or slightly better) compression efficiency of the H.264 system using 5 reference frames.

An efficient retinex-like brightness normalization method for coding camera flashes and strong brightness variation in videos

Conventional hybrid video coding systems rely on the assumption that the brightness is constant. This does not take inter-frame brightness variations into consideration during motion estimation and compensation processes. Under the influence of inter-frame lighting variations like camera flashes, video motion activities are not accurately estimated and the pixel prediction is poor which directly increases the bits for prediction error coding. In this paper, we propose an efficient algorithm based on the retinex-like system which allows inter-frame brightness being normalized before applying the conventional motion estimation and compensation. Experimental results show that our approach is superior to all similar approaches in the literature and demonstrate that our proposed system is very robust against the inter-frame brightness variations. Further experimental works have been done using the verification models of the MPEG-4 and the H.264 on sequences with brightness variations, results of which show that our proposed system outperforms these coding systems, including the weighted prediction feature in H.264, which were specifically designed for this purpose.

A novel illumination compensation scheme for sprite coding

A novel illumination compensation scheme for sprite coding is presented in this paper. Inter-frame pixel discrepancy due to the effect of frustrating illumination conditions is one of the major challenges for most correlation-based global motion estimators. Our proposed system employs a lighting compensation technique, by means of some frame division operations, to eliminate the effect of illumination variation before motion estimation is conducted. The background information is then expressed in terms of our proposed mosaic based representation which can efficiently handle sequences involving frustrating illumination condition. This effectively increases the robustness of the global motion estimator. Meanwhile, this allows the normalization of the lighting conditions of each image in the sequence with respect to a particular base frame and gives rise to a sprite image characterized with global illumination homogeneity. Experimental results show that our proposed technique manages to increase the coding efficiency of video sequences which involve a strong variation of lighting conditions (up to 10 dB improvement), and is suitable for the sprite coding application in MPEG-4.

Constrained one-bit transform for retinex based motion estimation for sequences with brightness variations

In this paper, we propose a DCT based binary matching approach for fast motion estimation. The proposed approach is suitable to be applied to our previously proposed retinex based coding system which is characterized with fast motion estimation and the capability of accurately estimating motions for sequences having inter-frame brightness variations. We propose to apply the DCT techniques to the transformation of images to the binary bit plane and to the scaled retinex domain, which is more computationally efficient compared to the conventional convolution based bit transformation approach. Experimental results show that our proposed DCT based bit transform has a very close prediction quality performance(less than 0.1dB drop) to that of the convolution based approach while our system can avoid the extra convolution procedure.

Windowing technique for the DCT based retinex algorithm to handle videos with brightness variations coded using the H.264

Conventional block based motion estimators assume constant inter-frame object brightness. Pixel discrepancy is resulted primarily from motion factor without considering the influence of brightness changes. In this paper, we propose a simple and efficient windowing technique using the Hamming window and integrate it to our previously proposed algorithm. The algorithm is based on retinex approach using the DCT technique and designed to handle brightness variations. The new technique manages to greatly reduce the influence of ripple effect and further increase the compression efficiency without adding any extra overhead bits to the bit-stream. We applied the scheme to H.264 for testing. Experimental results show that the retinex based approach is an effective technique to handle inter-frame brightness variations and outperforms the H.264 system with weighted prediction enabled for sequence involving brightness variations. With our proposed windowing technique using the Hamming window, the coding efficiency can be further improved by a maximum of 0.17 dB.

New block-based motion estimation for sequences with illumination variation and its application to video mosaicking

A new block-based local motion estimator independent of illumination variation is presented. Conventional motion estimators rely on an assumption that the difference between images results solely from motion activities. This hypothesis ignores another possibility, that variation in the contents of an image can be a result of variation in lighting conditions. A local motion estimator, which is independent of the adverse influence of the distinct illumination condition, is presented. The search space of the estimator is effectively restricted to dimension two, which is identical to the conventional block-based motion estimation. To show the accuracy of our proposed estimator, it was applied to sprite generation, which is relatively sensitive to the accuracy of the motion estimator. Experimental results show that our proposed system gives better quality reconstructed images in terms of PSNR as compared with a system using the conventional PDS estimator. Our estimator is particularly suitable for handling video sequences involving a strong variation of lighting conditions and is suitable for sprite coding application in MPEG-4.