RemoveDirt 0.9

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An Avisynth 2.5x filter for removing dirt from film clips

By Rainer Wittmann

The most recent version of this document is always available at

The binary and the source code are subject to the GNU General Public License. Last change May 7, 2005.


Version 0.8 of this plugin has been completely rewritten from scratch. It no more contains a RemoveDirt filter. It now only contains the filter RestoreMotionBlocks, which is the core element of the AVS script function RemoveDirt discussed below. The old RemoveDirt suffered from two problems. Firstly, fast motion of big objects was getting jerky after RemoveDirt filtering and secondly the essentially temporal approach of RemoveDirt was no more competitive with the spatial-temporal approach of RemoveDust (an AVS script function derived from my RemoveGrain package) as far as compression performance is concerned. If my theory about jerky RemoveDirt motion is correct, the new RemoveDirt script function will resolve this problem. The basic idea of the new script function is to use the old RemoveDirt for static aseas and a sharper version of RemoveDust for the motion areas, where RemoveDirt didn't do any cleaning before. Altogether the new RemoveDirt aims to achieve similar or better compression than RemoveDust with considerable more details in motion areas, where RemoveDust often destroyed fine details.  Because of the massive changes the old binary and the old source code will remain available for the time being.  If you have questions or suggestions about RemoveDirt or ResoterMotionBlocks, don't hesitate to post in the RemoveDirt thread of the doom9 forum rather than sending me email.

RemoveDirt supports the color spaces YV12 and planar YUY2. Thus if you want to use it with YUY2 clips, you have to convert from interleaved YUY2 to planar YUY2 before RestoreMotionBlocks and then back to interleaved YUY2 with the filters Interleaved2Planar and Planar2Interleaved from my SSETools plugin. As long as there is no official version of this plugin, it is contained in the RemoveGrain  package. Since RemoveGrain, Repair also support YUY2 only in planar form, this conversion has to be done only once before a script function, which is built from filters in these plugins.


The binary package of RemoveDirt contains two versions of the plugin. RemoveDirt.dll (dynamically linked, hence small), RemoveDirtSSE2.dll (only for SSE2 capable cpus, dynamically linked) and RemoveDirtS.dll (staticly linked, hence big). Try first one of the two small dlls and copy it to Avisynth's plugin directory. If it doesn't work, this is probably due to missing msvcr71.dll library. Either you install this library in C:\windows\system32 or you delete RemoveDirt.dll and replace it by RemoveDirtS.dll. Please put only one of the dlls into the plugin directory. If you fill the plugin directory with all kinds of superfluous dlls, then you only slow down the start of any application which uses Avisynth. There should be no conflict with other filters.


RestoreMotionBlocks(filtered, restore, neighbour, neighbour2, alternative, gmthreshold, mthreshold, pthreshold, cthreshold, noise, noisy, dist, tolerance, dmode, grey, show, debug)

The first five variables are clip variables. All clips must be of the same type (same width, height and color space). The number of frames is the minimum of the length of all these five clips. The first two variables are mandatory. "filtered" is usually an aggressively filtered clip, from which motion artifacts have to removed. If RestoreMotionBlocks identfies an 8x8 block as a motion block, it copies this block from the clip "restore" to the clip "filtered". This is the basic operation of RestoreMotionBlocks. To identify motion blocks RestoreMotionBlocks uses the clip "neighbour". The default value for neighbour is the "restore" clip. However, in the RemoveDirt script "neighbour" is different from "restore". The "neighbour2" is for using RemoveDirt in combination with motion compensation filters like MVtools (see MCRemoveDirt below). Finally, if the number of motion blocks exceeds the percentage specified in the "gmthreshold" variable, then RestoreMotionBlocks simply takes the frame from the clip "alternative". In this way, scene switches or global motion can be handled specifically. The clip "restore" is the default value  for "alternative". The default value for gmthreshold is 80, i.e. if 80% of the blocks are motion blocks, then the frame is taken from "alternative". "mthreshold" is similar as in the old RemoveDirt. However, because we now use the ordinary SAD for block comparison, the values should be somewhat higher, especially if the value of noise is low. The default value for "mthreshold" is 160. With the variable "noise" one can specify a noise level, which should be ignored by the motion detection. The default value of "noise" is 0. The variable "noisy" is used to specifiy the number of noisy pixels of an 8x8 block, which must be exceeded for a motion block. If noisy >= 0 and noise > 0, then the value of "mthreshold" is ignored. The variables "pthreshold" and "cthreshold" are the same as in the old RemoveDirt. Postprocessing is has not been changed in the new RemoveDirt. The default value for pthreshold is 10 and cthreshold inherits the value of pthreshold by default. Negative values are allowed for pthreshold and cthreshold, but are not very reasonable. The variables "dist" and "tolerance" are the same as in the old RemoveDirt plugin if dmode=0, the default. The default value for dist is 1 and the default value for tolerance is 12. If grey=true (false is the default value), then the chroma is ignored by RestoreMotionBlocks. The boolean variables "show" and "debug" are used for debugging (see section Debugging).

How RestoreMotionBlocks works

To use the above variables properly, one has to understand how RestoreMotionBlocks works. It consists of three phases. For the first phase only the clip "neighbour" is used. Each frame is divided into a grid of 8x8 blocks. If  n is the number of the current frame, then for each block of this grid RestoreMotionBlocks looks at the luma of this block in neighbour(n-1) and neighbour(n+1). Note that we don't use the frame neighbour(n). There are three comparison methods (the old RemoveDirt has only one). If noise= 0, then simply the SAD of each block in  neighbour(n-1) and neighbour(n+1) is computed. If it is >= mthreshold, the block is identified as a motion block of frame n. This is the fasted method and a similar method was used in the old RemoveDirt. Its key disadvantage is that it may easily misled by noise. If noise >=0, then instead of SUM(|y-x|) RestoerMotionBlocks calculates SUM(| |y-x|-noise |). In particular, differences with absolute value <= noise are ignored. If it is >= mthreshold, then this block is identified as a motion block. We call this the noise adjusted SAD. From the way how the noise adjusted SAD is calculated, it is clear, that "mthreshold" should be decreased if  "noise" is increased.  If noise >= 0 and noisy >= 0, then RestoreMotionblocks counts the number of pixels of a block, for which the absolute difference between neighbour(n-1) and neighbour(n+1) is >= noise. If this number is >=  value of "noisy", then the block is identified as a motion block. We call this the NPC (= noisy pixel counting) method. The value of mthreshold is ignored, if NPC is selected. Note that a block has 64 pixels. Thus, if noisy > 64, then there can't be any motion blocks. In my view NPC is clearly the best method. It has likely about half the speed of SAD and about the same speed as NSAD.  Noise=-1 and noisy=-1 are the default values. Thus SAD is the default method for the first phase. I ran most of my RemoveDirt tests with noise=8 or 10 and noisy= 12. In the sequel the motion blocks found in the first phase are called phase 1 motion blocks.  In the second phase, for each block all the motion blocks found in the first phase which have a distance <= dist are counted. If the result is >= (tolerance /100) * (the number of all first phase blocks with distance <= dist) , then this block is called a motion neighbour  block. For instance, if dist = 1 and tolerance= 12 (the default values), then there are 9 blocks with a distance <= 1. Since 1 < (12/100)*9 < 2, there must be at least 2 phase 1 motion blocks among the 9 neighbour blocks such that the block is marked as a motion neighbour block. If dmode= 0, then all the motion neighbour blocks become phase 2 motion blocks. Thus if dmode=0 the number of motion blocks is increased quite a bit. If dmode= 2, then quite the opposite happens: a phase 1 motion block only becomes a phase 2 motion block, if it is also a motion neighbour block. In particular, there are less phase 2 motion blocks than phase1 motion blocks. For instance, if dist=1, tolerance= 2, dmode= 2, then a single phase 1 motion block is dicarded if there exists no further phase 1 motion block with a distance less than 1. Dmode=1 is just in the middle between dmode=0 and dmode= 2: the motion neighbour blocks become the phase 2 motion blocks. Thus, if dmode=1, the phase 1 motion blocks are only relevant for detecting motion neighbour blocks. After this task is completed the phase 1 information is discarded. If dist=0 or dmode=2 gmthreshold should be lowered to 60 or even 50. The third phase, called the postprocessing phase, starts with restoring the phase 2 motion blocks by copying them from the clip "restore" to the clip "filtered". All phase 2 motion blocks become also phase 3 motion blocks. Then the edges between motion and non-motion blocks are inspected. To this end the SAD of the two adjacent border line segments is calculated twice (these line segments are either horizontal or vertical and are 8 pixels long). It is calculated first in the clip "restore" and then in the clip "filtered". In the clip "filtered" the two blocks are from two different sources, one block, the motion block was restored from the clip "restore" and the non-motion block is from the original clip "filtered". Since the frames of the clip "restore" are not changed at all, both blocks are from the same source and should therefore fit together. If the edge SAD in the clip "filtered" > (edge SAD in the clip restore) + pthreshold, then the block is marked as a new (additional) phase 3 motion block and the block is restored by copying it from "restore" to "filtered", because the two blocks in "filtered" don't fit together well enough compared with the two blocks in "restore". In other words,  in this phase, it is checked whether a restored block fits to the yet unrestored blocks. If it doesn't, the not yet restored blocks, which do not fit well, are marked as phase 3 motion blocks and are restored as well. This procedure is repeated until there are no more blocks, which can be tested. If the value of the grey variable is false, then the same is done for the luma and the chroma (for the chroma the variable cthreshold is used instead of pthreshold). If grey= true, then postprocessing is only done for the luma.

Finally, if the percentage of all phase 3 motion blocks with respect to all blocks exceeds the value of gmthreshold, then the "filtered" frame is discarded and replaced by the corresponding frame in "alternative".  In this way, we can give special treatments for sharp scene switches and scenes with a moving or zooming camera.

Black&White Clips

If grey=true the chroma of the clip "filtered" is not touched by RestoreMotionBlocks. Also for postprocessing only the luma is used. This is slightly faster than grey=false. If you use grey=false for b&w clips, then it not only takes longer but also the quality may degrade, because chroma noise may trigger false postprocessing. Thus "grey=true" is highly recommended for b&w clips.


The boolean variable debug and show are used for debugging. If show=true, then the blocks, which are marked as motion blocks in the first phase are colored red, those found in the second phase are colored green and finally the motion blocks marked by postprocessing are colored blue. In this way, one can easily check whether the above variables were selected appropriately. if debug=true, then RestoreMotionBlocks sends  output of the following kind to the debugview  utility:

[348] [21495] RemoveDirt: motion blocks =  942(14%), 1652(25%),  635( 9%), loops = 31
[348] [21496] RemoveDirt: motion blocks = 1745(26%), 2330(35%),   64( 0%), loops = 3
[348] [21497] RemoveDirt: motion blocks = 1480(22%), 1973(30%),   45( 0%), loops = 4
[348] [21498] RemoveDirt: motion blocks = 1081(16%), 1915(29%),   65( 1%), loops = 2
[348] [21499] RemoveDirt: motion blocks = 1403(21%), 2380(36%),  235( 3%), loops = 10
[348] [21500] RemoveDirt: motion blocks = 2618(40%), 2204(34%),   59( 0%), loops = 5
[348] [21501] RemoveDirt: motion blocks =  986(15%), 2065(31%),   75( 1%), loops = 3
[348] [21502] RemoveDirt: motion blocks = 1214(18%), 2291(35%),   78( 1%), loops = 3
[348] [21503] RemoveDirt: motion blocks = 1348(20%), 2179(33%),   57( 0%), loops = 4
[348] [21504] RemoveDirt: motion blocks =  961(14%), 1957(30%),   71( 1%), loops = 3
[348] [21505] RemoveDirt: motion blocks = 1833(28%), 2201(33%),   38( 0%), loops = 3
[348] [21506] RemoveDirt: motion blocks = 1644(25%), 2183(33%),   53( 0%), loops = 5
[348] [21507] RemoveDirt: motion blocks = 1420(21%), 2541(39%),  132( 2%), loops = 5
[348] [21508] RemoveDirt: motion blocks = 2238(34%), 2229(34%),  104( 1%), loops = 4
[348] [21509] RemoveDirt: motion blocks = 1351(20%), 2294(35%),  181( 2%), loops = 6
[348] [21510] RemoveDirt: motion blocks =  931(14%), 1800(27%),  229( 3%), loops = 5

The first number in brackets on the left hand side is the id of the process, which runs the script, the second number in brackets is the frame number. The first number (with percentages in brackets) after "motion blocks ="  is the number of phase 1 motion blocks, the second is the difference between phase 2 and phase 1 motion blocks (always >=0 if dmode=0, always <= 0 if dmode= 2) and the third is the difference between phase 3 and phase 2 motion blocks (always >= 0). Finally the number after "loops =" is the number of postprocessing loops used for this frame. Debug=true can be used to monitor RestoreMotionBlocks in an encoding process. Of course, show=true can only be used before an encoding process to find the right values for the various variables.


SCSelect is a special filter, which distinguishes between scene begins, scene ends and global motion. The output of SCClense is used as an "alternative" clip for RestoreMotionBlocks. It can hardly used for other purposes, because it can only make proper decisions if there are a lot of motion blocks. Only if the percentage of motion blocks is > gmthreshold, then RestoreMotionBlocks chooses a frame from the clip specified with the alternative variable and then there are always a lot of motion blocks, if gmthreshold is not too small (gmthreshold >= 30 should be sufficiently large). SCSelect yields nonsense results if there are only few motion blocks. SCSelect is used as follows:

SCSelect(clip input, clip scene_begin, clip scene_end, clip global_motion, float dfactor, bool debug, bool planar)

The first four clip variables are mandatory and have no name. All four clips must have the same color space, width and height. The first clip is the clip, on which SCSelect bases its decision. Usually it should be the same clip, which was specified with the "neighbour" variable in RestoreMotionBlocks. If SCSelect realises a scene begin, it selects its output frame from the clip scene_begin. If SCSelect realises a scene end, it selects its output frame from the clip scene_end. If SCSelect realises a global motion, it selects its output frame from the clip global motion. Thus SCSelect doesn't produce any new frames. It only makes a selection from three different sources. Dfactor is the key variable for scene switch sensitivity. The higher dfactor the less scene begins and scene ends and the more global motion frames are detected. Dfactor=4.0 is the default value. SCSelect works with YV12 and planar YUY2. If planar YUY2 is used, then planar=true must be specified. If debug=true, then SCSelect sends output of the following type to the debugview  utility:

[3416] [67865] SCSelect: global motion
[3416] [67866] SCSelect: global motion
[3416] [67870] SCSelect: global motion
[3416] [67871] SCSelect: global motion
[3416] [67873] SCSelect: global motion
[3416] [67874] SCSelect: global motion
[3416] [67877] SCSelect: global motion
[3416] [68318] SCSelect: global motion
[3416] [68319] SCSelect: global motion
[3416] [68557] SCSelect: scene end
[3416] [68558] SCSelect: scene begin
[3416] [69481] SCSelect: scene end
[3416] [69482] SCSelect: scene begin
[3416] [70240] SCSelect: scene end
[3416] [70241] SCSelect: scene begin
[3416] [70406] SCSelect: global motion
[3416] [70407] SCSelect: global motion
[3416] [70408] SCSelect: global motion
[3416] [70409] SCSelect: global motion
[3416] [70410] SCSelect: global motion
[3416] [72032] SCSelect: global motion
[3416] [72164] SCSelect: global motion
[3416] [72165] SCSelect: global motion					

To describe the basic idea  behind SCSelect let SAD(n)  be the SAD difference between the frames input(n) and input(n+1). Now, if SAD(n) > dfactor * SAD(n-1), then SCSelect recognises a scene end and pulls the frame from the clip scene_end. If SAD(n-1) > dfactor * SAD(n), then SCSelect recognises a scene begin and pulls the frame from the clip scene_begin. If both SAD(n) <= dfactor * SAD(n-1) and SAD(n-1) <= dfactor * SAD(n), then SCSelect recognises a global motion and pulls the frame from the clip global_motion. From this dexcription it is clear that dfactor must be > 1 for getting reasonable results. The above algorithm is optimised such that often only one and not two SADs are calculated for one requested frame. However, there are certain shortcomings. If a scene ends with global motion, then SCSelect often can't detect the scene end. If a scene begins with global motion, then SCSelect often can't detect the scene begin. These two effects are usually responsible if lonely scene begins and scene ends are detected by SCSelected, otherwise each scene begin should be preceded by a scene end. By refining the above algorithm we could avoid lonely scene begins and scene ends, but there is one situation, where even such a refinement fails. Namely if the scene ends with global motion and the new scene starts with global motion. Then a sharp scene switch can only be detected reliably with a good motion analysis, which would result in an extreme slow down of the filter.


RemoveDirt has now become an AVS script function, which involves RestoreMotionBlocks and various filters from my RemoveGrain package (version 0.9 or higher is necessary). During the tests I used the following script:

function RemoveDirt(clip input, bool "_grey", int "repmode") 
    _grey=default(_grey, false)
	repmode=default(repmode, 16)
	clensed=Clense(input, grey=_grey, cache=4)
	sbegin = ForwardClense(input, grey=_grey, cache=-1)
	send = BackwardClense(input, grey=_grey, cache=-1)
	alt=Repair(SCSelect(input, sbegin, send, clensed, debug=true), input, mode=repmode, modeU = _grey ? -1 : repmode ) 
	restore=Repair(clensed, input, mode=repmode, modeU = _grey ? -1 : repmode)
	corrected=RestoreMotionBlocks(clensed, restore, neighbour=input, alternative=alt, gmthreshold=70, dist=1, dmode=2, debug=false, noise=10, noisy=12, grey=_grey)
	return RemoveGrain(corrected, mode=clmode, modeU = _grey ? -1 : clmode )

Let us discuss this script in some detail. Firstly, we apply the brutal temporal clenser from the RemoveGrain package to obtain the clip "clensed". Then we use the filters ForwardClense and BackwardClense from RemoveGrain to construct the clip "alt", which is then used as the "alternative" variable in the subsequent RestoreMotionBlocks. While Clense does a lot of cleaning it certainly creates a lot of artifacts in motion areas. In the script function RemoveDust, the clip "clensed" is repaired entirely by the Repair filter from the RemoveGrain package. In RemoveDirt this repair is only made in motion areas. The static areas are not repaired. Since the clip is used only for restoring motion areas, we can use the much stronger Repair mode 16 (in RemoveDust usually modes 2 or 5 are used), which restores thin lines destroyed by clense. Finally, because there may be some left over from temporal cleaning especially when grain is dense, we use the spatial denoiser RemoveGrain(mode=17) to remove these dirt or grain rests.

Optimal Usage

1. If possible, crop after RestoreMotionBlocks. Modern codecs divide the frames in the same way as RemoveDirt into a grid of 8x8 pixel blocks to perform the crucial discret cosine transform for such blocks. Now if the clip is cropped after RemoveDirt, then the grid of RemoveDirt and the codec are likely to be different resulting in inferior compression. There is one exception, though: cropping afterwards does not hurt, if all four sides are cropped by a multiple of 8. For instance, crop(8,64,0,-72) is ok. On the other hand, one should crop after RemoveGrain/Repair if possible, because this filters cannot process the boundary pixels. Thus the optimal solution is to crop afterwards and then only by multiples of 8, which unfortunately is not always possible.

2. Crop only with "align=true". RestoreMotionBlocks heavily uses SSE/SSE2 instructions. If you crop without "align= true" before RestoreMotionBlocks, then the data on the frames may not be properly aligned and RemoveDirt will execute substantially slower. This is particularily important for the SSE2 version. As a consequence you should always crop with Avisynth and not with DVD2AVI or DGIndex.

3. Telecined movies must be inverse telecined before RemoveDirt. If a film is telecined some fields are doubled in order to increase the frame rate from 24fps to 30fps. Hence on such doubled fields the basic property of dirt, described above, is no more valid and no temporal cleaner can ever spot dirt on such doubled fields. On the other hand, after an inverse telecine usually every fourth frame is composed of fields, which originate from two different frames. Visually these two fields fit together well but both are from a different compression context, which can mislead RemoveDirt to false motion detection. In extreme cases, one field may be from an I- or P-frame, while the other is from a B-frame. But even if the fields are from from frames of identical type, the different compression context has a substantial effect. Consequently RemoveDirt performes less well on inverse telecined movies than on natively progressive movies. By the same reason also compression of inverse telcined movies is worse than of natively progressive movies. We in Europe should thank god every day that we are not getting telecined. However, here in Germany we have digital tv broadcasters, which like to comb progressive films (about 5% of all progressive movies from ARD and especially ZDF are combed). Fortunately these idiots are not able to double fields, so RemoveDirt should work, but on combed films the dirt is always split over two frames which clearly hurts RemoveDirt. On the other hand, if these combed films are uncombed, then we have the compression context problem for any frame and not only for any fourth frame. Stepping through the video with the builtin filter Bob() one can decide with near absolute certainty, whether the video is truely progressive, interlaced, telecined, field blended or progressive with a field shift.

4. Put other filters after RemoveDirt. Except those filters mentioned before, like crop and inverse telecine, all other filters should be put after RemoveDirt in the Avisynth script, because most filters have a negative rather than a positive impact on dirt detection.