Research On Screen Image Sequence Coding

With the wide spread application of GUI(Graphic User Interface) and the rapid development of networks,Real-time screen image sequence transmission technology become more and more important,and will be widely used in the fields of internet multimedia conference,products demo,telecommuting,remote education etc.Due to this technology, people can use local digital devices like PC and smart display set to interact with remote computers graphically,or to realize remote computer aided massive computing work.This makes a wide spread hardware and software resource share throughout networks.However,the large amount of original image data is a major obstacle for real-time screen image sequence transmission.It usually causes network congestion and transmission delay over Internet. Therefore,screen image sequence compression algorithms are essential for real-time transmission.Screen image sequence is a kind of compound image sequence,containing text,graphics and natural images.How to compress screen image sequence is a hard problem.An effective region classification and a proper hybrid encoding are very helpful to improve its compression efficiency.This dissertation focus on the screen image sequence coding and researches the methods of region-based classification and encoding,scalable encoding and coding artifacts removal.Firstly,this dissertation proposes a new block region character based screen image sequence coding scheme(BRC-SISC).It applies the inter-frame motion detection to eliminate the temporal redundant information.And according to the different characters on color and gradient,it classifies each block of image into three categories:text/graphics block,picture block and hybrid block.Text/graphics block is compressed with a lossless coding algorithm (PHDC),which integrates palette coding,Hextile coding and Deflate algorithm.Picture block is encoded by lossy JPEG.For hybrid block,first,a second classification is adopted to distinguish between text/graphics pixels and picture pixels.Then,each kind of pixels is filled into a layer block.Text/graphics layer block uses lossless coding method,and Picture layer block adopts lossy coding method.The proposed classification and coding methods could exactly distinguish between the text/graphics and picture information.And it achieves good compression ratio with low computation,which satisfies the real-time application requirement.Secondly,differences in consumer requirement,terminal processing capability and network bandwidth always exist among clients in screen image sequence transmission system. In order to meet the requirements of the clients in different performance conditions of computer and network,this dissertation proposes a block region character based scalable screen image sequence coding method(BRC-SSISC).First,based on the earlier text/graphics and picture classification result and the human vision character,a three-layer SNR scalable coding method is offered to provide high quality screen images for all the clients.Then,a compensatory coding method(CC) is applied to make sure clearer screen image could be received,when the performance of client becomes better or screen image sequence changes slower.Experiment results shows that the proposed scalable coding method is effective.And it also achieves good compression ratio with low computation,which satisfies the real-time application requirement.Finally,coding artifacts removal method is discussed.Influenced by the local region character of image and the human vision character,coding artifacts are manifested to varying degree in different areas.Therefore,this dissertation proposes a new spatial adaptive filtering of coding artifacts removal(SAFCAR).For de-blocking,it classifies the block boundary region into smooth region and texture region based on its gradient character,and then adopts different detecting standards and filtering range to remove the blocking artifacts.For de-ringing,besides the edge detection used to trace the strong ringing artifacts block,a complementary ringing detection method is proposed to locate the weak ringing artifacts block.After twice detections,a fuzzy filter,which is adaptive to the ringing strength,is applied to remove ringing artifacts.In both natural picture sequence and screen image sequence,the proposed post-filtering method has a good performance on coding artifacts removal and detail preservation.