The Improved Rate Control Method For Encoding Hybrid Screen Content

Screen codec technology that is for video recorded from screen is aspecial video codec technology. In the time of modern distance education,teachers in class need to record and transmit computer screen to remoteterminals. So the screen codec technology is widely used in distanceeducation modes such as online classroom.Previous studies divide screen image sequences into two scenes thatare fast scene and slow scene according the content of screen, and adoptdifferent bit rate control method for different scenes. But this method hastwo shortages. The first one is that the subjective quality is very badbecause of the lag of the scene recognition algorithm in switching scenes.The second one is that in the fast scene, the quality of key frames isunacceptable for hybrid scenes screen content because the target bit rateallocated is not enough.To overcome the lag caused by scene recognition algorithm, thisarticle improves the scene recognition method by storing and analyzingseveral frames before the codec begins. The new scene recognition methodcan accurately recognize the switching frame and reduce loss of imagequality.Hybrid scenes screen content refers the screen contains static text andimage as well as video at the same time. This article proposes a new ratecontrol method to overcome the shortages of previous studies takingadvantages of that the human eyes have different sensitivity on differentcontent.Also we have implemented a video recognition algorithm, which canrecognize the accurate position of the video in the screen according the changed region provided by Mirror Driver. So, we can divide screen intotwo regions that are video playing region and non-video playing region.Combined with the scene recognition algorithm, the new rate controlmethod based on implemented CRF(Constant Rate Factor) andVBV(Video Buffer Verifier) improve the previous studies both on frameand macro block layer: for the frame layer, if the current frame is P frameand it is in a certain range of I frame, then the frame adopt P_SKIP modeto reduce frame rate of non-video playing region so that it can compensatethe whole quality of I frame and enhance whole quality of screen videosequence; for the macro block layer, the new rate control will make acertain adjustments on QP and its variation range to improve the subjectivequality of whole frame based on the position of the macro block(videoplaying region or non-video playing region).The experimental results show that the new rate control methodscores40%more than x264’s original CRF and VBV method in subjectiveevaluation.