LEE- Bilgisayar Mühendisliği Lisansüstü Programı
Bu topluluk için Kalıcı Uri
Gözat
Yazar "Bayazıt, Uluğ" ile LEE- Bilgisayar Mühendisliği Lisansüstü Programı'a göz atma
Sayfa başına sonuç
Sıralama Seçenekleri
-
ÖgeCompression of geometry videos by 3D-SPECK wavelet coder(Lisansüstü Eğitim Enstitüsü, 2021) Bahçe Gülbak, Canan ; Bayazıt, Uluğ ; 723134 ; Bilgisayar MühendisliğiA geometry image represents a manifold surface in 3D space as an 2D array of 3D points. This involves 3 steps : First, cutting the manifold which essential defines the boundary of the square, second, defining the parametrization which defines the interior of the square and lastly, rasterizing and scan converting the geometry and applying compression to it. By representing manifold 3D objects using a global 2D parametrization (mapping) it is possible to use existing video techniques to represent 3D animations. 2D-SPECK coder, discovered by Islam and Pearlman, codes sets of DWT coefficients grouped within subbands. SPECK coder is different from the other schemes in that it does not use trees which span and also exploits the similarity accross different subbands. It makes use of sets in the form of blocks. The main idea is to exploit the clustering of energy in frequency and space in the hierarchical structures of wavelet transformed images. 3D-SPECK coder, is an extension of the 2D-SPECK algorithm for compressing 3D data with high coding efficiency. A geometry video is formed as a sequence of geometry images where each frame is a remeshed form of a frame of an animated mesh sequence. For efficiently coding geometry videos by exploiting temporal as well spatial correlation at multiple scales, this thesis proposes the 3D-SPECK algorithm which has been successfully applied to the coding of volumetric medical image data and hyperspectral image data in the past. The thesis also puts forward several postprocessing operations on the reconstructed surfaces that compensate for the visual artifacts appearing in the form of undulations due to the loss of high frequency wavelet coefficients, cracks near geometry image boundaries due to vertex coordinate quantization errors and serrations due to regular or quad splitting triangulation of local regions of large anisotropic geometric stretch. Experimental results on several animated mesh sequences demonstrate the superiority of the subjective and objective coding performances of the newly proposed approach to those of the commonly recognized animated mesh sequence coding approaches at low and medium coding rates.
-
ÖgeVisual attention and visual distortion sensitivity based regional rate allocation in JPEG2000(Lisansüstü Eğitim Enstitüsü, 2021) Pak, Mesut ; Bayazıt, Uluğ ; 662856 ; Bilgisayar MühendisliğiThis thesis study proposes a regional bit allocation method for improving the subjective quality for image encoding. This bit allocation method jointly uses the visual attention and visual distortion sensitivity levels of image regions for decreasing the perceptual distortions. Visual attention levels of image regions are estimated by using an exponential model of fixation durations. The human visual system is more sensitive to the distortions in structured regions than the distortions in complex textured regions. Therefore, a method for estimating distortion sensitivity, that distinguishes the structured regions from complex textures by using uniform distribution of gradient directions and connected sets of pixels having same gradient directions, is also proposed. The estimates for the visual attention level and the distortion sensitivity level are jointly used to modify the distortion contribution of each codeblock in bit allocation. The exponential model used to obtain the visual attention map of an image inputs the average of fixation duration maps of all viewer subjects. The fixation duration maps are based on eye-tracking experiments. For optimizing the perceptual quality by bit allocation, the encoder should decrease the perceptual distortions in visually conspicuous regions. The distortions in structured regions, such as object contours or letters, are very noticeable. Therefore, the distortion sensitivity estimation should determine the image regions containing true edges with significant lengths regardless of the edges' gradient magnitudes. The second visual distortion sensitivity issue is determining whether a region is complex textured or not. The complex textured regions can conceal distortions caused by lossy compression whereas in smooth regions such distortions are easily perceptible. The proposed method compares the entropy of the orientations of gradients within an image block against a threshold for classifying it as complex textured. The visual quality achieved by the proposed bit allocation method is compared with those achieved by well-known bit allocation methods (post-compression rate-distortion optimization, saliency map, foveation of fixations, and foveated just-noticeable-difference map) in order to validate the proposed method. Additionally, to assess the contribution of the use of visual distortion sensitivity to the perceived quality achieved by the proposed method, the reconstructed images resulting from bit allocation based on only the visual attention maps are also compared against those resulting from the proposed bit allocation method. The performance comparisons are primarily based on the paired comparison method developed by ITU-T that evaluates the subjective qualities of the images. In the pairwise comparative evaluation facilitated by a web page, the evaluator subjects are presented with the decoded images for different bit allocation methods pair by pair and vote for the differential quality of each pair. In addition to subjective evaluation, a more objective perceptual quality assessment method, known as Masked MS-SSIM, is also used. This assessment method calculates a similarity index between the reconstructed image and the original image for regions of interest. The plausibility of the subjective and objective comparison results are verified by using statistical hypothesis tests. In summary, the proposed bit allocation method has been experimentally shown to yield a substantially higher perceptual visual quality than the other well-known bit allocation methods. The method is conceivable for use in media server applications where the server processes the eye fixation data collected by the clients to obtain the visual attention map.