text_src/15.00__vp8-bitstream__loop-filter.txt - webm/bitstream-guide - Git at Google



 ### Chapter 15: Loop Filter                                  {#h-15-00}


 Loop filtering is the last stage of frame reconstruction and the next-to-last stage of the decoding process. The loop filter is applied to the entire frame after the summation of predictor and residue described in Chapter 14.

 The purpose of the loop filter is to eliminate (or at least reduce) visually objectionable artifacts associated with the semi-independence of the coding of macroblocks and their constituent subblocks.

 As was discussed in Chapter 5, the loop filter is "integral" to decoding, in that the results of loop filtering are used in the prediction of subsequent frames. Consequently, a functional decoder implementation must perform loop filtering exactly as described here. This is in distinction to any postprocessing that may be applied only to the image immediately before display; such postprocessing is entirely at the option of the implementor (and/or user) and has no effect on decoding per se.

 The baseline frame level parameters controlling the loop filter are defined in the frame header (Chapter 9.4) along with a mechanism for adjustment based on a macroblock's prediction mode and/or reference frame. The first is a flag selecting the type of filter (normal or simple), the other two are numbers (`loop_filter_level` and `sharpness_level`) that adjust the strength or sensitivity of the filter. As described in Chapters 9.3 and 10, `loop_filter_level` may also be overridden on a per-macroblock basis using segmentation.

 Loop filtering is one of the more computationally-intensive aspects of VP8 decoding. This is the reason for the existence of the optional, less-demanding simple filter type.

 Note carefully that loop filtering must be skipped entirely if `loop_filter_level` at either the frame header level or macroblock override level is `0`.  In no case should the loop filter be run with a value of `0`; it should instead be skipped.

 We begin by discussing the aspects of loop filtering that are independent of the controlling parameters and type of filter chosen.


	### Chapter 15: Loop Filter {#h-15-00}


	Loop filtering is the last stage of frame reconstruction and the next-to-last stage of the decoding process. The loop filter is applied to the entire frame after the summation of predictor and residue described in Chapter 14.

	The purpose of the loop filter is to eliminate (or at least reduce) visually objectionable artifacts associated with the semi-independence of the coding of macroblocks and their constituent subblocks.

	As was discussed in Chapter 5, the loop filter is "integral" to decoding, in that the results of loop filtering are used in the prediction of subsequent frames. Consequently, a functional decoder implementation must perform loop filtering exactly as described here. This is in distinction to any postprocessing that may be applied only to the image immediately before display; such postprocessing is entirely at the option of the implementor (and/or user) and has no effect on decoding per se.

	The baseline frame level parameters controlling the loop filter are defined in the frame header (Chapter 9.4) along with a mechanism for adjustment based on a macroblock's prediction mode and/or reference frame. The first is a flag selecting the type of filter (normal or simple), the other two are numbers (`loop_filter_level` and `sharpness_level`) that adjust the strength or sensitivity of the filter. As described in Chapters 9.3 and 10, `loop_filter_level` may also be overridden on a per-macroblock basis using segmentation.

	Loop filtering is one of the more computationally-intensive aspects of VP8 decoding. This is the reason for the existence of the optional, less-demanding simple filter type.

	Note carefully that loop filtering must be skipped entirely if `loop_filter_level` at either the frame header level or macroblock override level is `0`. In no case should the loop filter be run with a value of `0`; it should instead be skipped.

	We begin by discussing the aspects of loop filtering that are independent of the controlling parameters and type of filter chosen.