EECE 541 Multimedia Systems Project Proposal: Logo ... - Courses

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are replaced by the logo, giving rise to an opaque overlapping of the logo over the input image. A logo often occupies a small portion of a frame, and is static over a frame sequence. <strong>Logo</strong>s often appear in a corner of a frame (e.g. the top left corner, bottom right corner). They, however, can be anywhere in a slice for H.264 compressed video, since slice partition is rather flexible in H.264 standard. Moreover, logos may present only in groups of successive frames, as opposite to all frames in a video sequence. III. VIDEO CODING AND TRANSCODING Having introduced some logo-related terminology, in this section, we will present several essential concepts for video coding and video transcoding. We first introduce three commonly used transcoding structures in what follows. A. Basic Transcoding Structures One straightforward transcoding structure is the cascaded form. For the cascaded structure as shown in Figure 1, the decoder decodes the compressed video stream completely, and the encoder re-encodes the reconstructed video into the target format. The cascaded architecture achieves high video quality, but it is computationally very expensive. Therefore, the cascaded structure is not often used, especially in the real-time transcoding. It is better to re-use the information contained in the original bit stream to simplify the architecture. Figure 1. Cascaded architecture in pixel domain. Open-loop structure is another commonly used transcoding structure. In the open-loop system, the bit stream is first variable-length decoded (VLD) to reconstruct the quantized discrete cosine transform (DCT) coefficients, motion vectors, prediction modes, and other macroblock-level information. The quantized coefficients are then inverse quantized and modified according to the transcoding requirements. Finally, the modified data is re-quantized and variable length coded to achieve the new output format. Figure 2 shows a requantization transcoder as an open-loop structure example. 4
Figure 2. Open-loop architecture. Open-loop systems are relatively simple. They do not include motion estimation, motion compensation, DCT, and inverse DCT (IDCT). Therefore, open-loop structures are computationally very efficient, but they suffer from low video quality due to the drift problem. The drift problem is caused by the mismatch between the actual reference frame used for motion estimation in the encoder and the degraded reference frame used for motion compensation in the decoder. The drift problem accumulates and causes severe degradation to the video quality. To avoid the high complexity problem in the cascaded architecture and the poor video quality problem in the open-loop structure, closed-loop systems provide a good tradeoff between quality and computational complexity. In closed-loop structure, significant complexity saving can be achieved while still maintaining acceptable video quality. Closed-loop systems provide drift compensation for re-quantized data. They aim at eliminating the mismatch between predictive and residual components by approximating the cascaded transcoding architecture. In the simplified closed-loop structure as shown in Figure 3, only one reconstruction loop is required with one DCT and one IDCT. Some architecture inaccuracy is introduced due to the non-linear nature in which the reconstruction loops are combined. However, it has been found that the approximation has little effect on the video quality. Figure 3. Closed-loop architecture. 5
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EECE 541 Multimedia Systems Project Proposal: Logo ... - Courses

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