Everyone is looking for the ultimate settings! The ones
that will make their DivX movie have the very best possible picture quality
for the smallest possible file size! I too have spent much time searching
this dark road. And now I feel its about time I gave everyone a few pointers
from my experience =). These are just a handful of tips that I have made
and should not be taken as though I am to telling you what settings you
should be using. Rather, I hope to save you some time and help you to
make good choices for yourselves.
INSIDE THE DIVX CODEC
Before we can decide on the best setting to use for our
movies, it would be good idea to start by looking at what all those settings
on the DivX codec do and what the differences are between the Fast Motion
and Low Motion codecs.
Most video formats that use compression will have
keyframes. These help the video player to seek to various parts
of the movie easily and to keep the picture quality good. They also
take up the most amount of memory of any video frame. Most codecs
will use one keyframe between every 5-10 seconds. The default setting
on the DivX codec is 1 keyframe every 10 seconds. This is fine most
of the time and changing it will not really increase the image quality
enough to notice. In the past people started making DivX's with
a keyframe every 9999 seconds to keep down the file size. The result
was that you couldn't resume watching the video from where you left
off! Every time you played it you had to watch it from the start
again! The position of the keyframe is important. If a keyframe
is put at the start of a scene change then all frames after it will
generally look better. In view of this fact some hacked DivX codec's
(such as VKI) have been designed to detect the scene changes and
put a keyframe there. This produces only a minor improvement of
There are two kinds of bitrate setting used on video compression.
Constant Bitrate (CBR) and Variable Bitrate (VBR).
Constant Bitrate: constant bitrate represents how
much memory per second a codec will use to encode a movie. Obviously,
the higher the bitrate the better the picture quality. Say, for example,
one second of video was 25 frames. If I set a bitrate of 1000 Kilobits
per Second (kbps) and encode 1 second, each frame would use 40 Kilobits
of memory each. If I used a bitrate of 2000 kbps each picture could use
80 Kilobits. Obviously an 80 kilobit frame will look much better than
a 40 kilobit frame.
Notice too that it doesn't matter what resolutions we use!
Lets say we encoded one second of video at 1000 kbps at a resolution of
352 x 288 pixels - the final filesize would be exactly 1000 kilobytes!
Again, if we encoded one second of video at 1000 kbps, but this time we
use a cinema resolution of 5000 x 5000 pixels; the final movie clip would
still only be 1000 kilobytes in size! I know it sounds strange,
but we set only '1000 kilobits' for one second of video and that is all
it will use!
Variable Bitrate: a variable bitrate codec sets its
own bitrate in accordance with the movie. To illustrate, mpeg compresion
works basically by saving only the differences between sequances of frames.
In non-action scenes there are very few differences between frames. Hence,
if the total amount of differences are "small" the kilobits
needed to store them will also be small. On the other hand, if there are
lots of differences between frames it will take "lots" of kilobits
to store them.
It is impossible to know how much memory a Variable Bitrate
encoded movie will take because it all depends on the amount of Fast Motion
and Low Motion scenes it has. Nevertheless, most variable bitrates codec
have limits. For example, DVD's are often encoded with a certain maximum
and minimum bitrates that will make sure they don't use too much memory
and go over the target filesize or use so little memory that it gives
a bad looking picture.
SMOOTHNESS & CRISPINESS SETTINGS
One trick people use on AVI files to reduce the filesize
is to use a smaller framerate. For example, NTSC movies use a framerate
of almost 30 frames per second (fps). If we only used 15 fps then we have
effectively halved the filesize! This doesn't look too bad either especially
on cartoons which are usually only produced at 15fps anyway!
But although this worked for uncompressed AVI's, no Mpeg
format could do this! Say, for example, we tried to encode an Mpeg-1 file
at 15 fps. The mpeg codec would automatically add extra frames to make
the video at least 23.976 fps which is the speed captured with a motion
picture camera! Then it would add a code at the start of the movie to
tell the video player to only play it back at 15fps! In other words, the
movie would still take the same amount of space as a 23.976 movie but
play back at 15 frames a second! When I first wrote my previous quality
guide I thought this rule must also applied to DivX, but this was not
quite correct. The DivX codec is able to "drop" frames in order
to save space!
And here is where the DivX smoothness and crispiness
settings come in. The smoothness and crispiness settings seem to
be a pre-filter that softens noise in the picture. The softer the
picture the better it compresses. The sharper the picture the harder
it is to compress. Most Mpeg encoders offer some sort of prefiltering
like this. The DivX codec encodes each movie in small sections at
a time, each containing only so many blocks of information. If the
picture is too clear and sharp it will tend to 'drop' frames to
compensate to fit the movie into the block size. This frame dropping
produces a clean crisp image but a slightly jerky looking one. If
the picture is smoothed it will be able to compress better and keep
all the frames making it more blurry but also much smoother animation.
Before you start panicing and turning the smoothness to 0, this
frame dropping effect only really happens at very very low bitrates.
At the bitrates we would normally encode a DivX, dropped frames
are not a big problem. Nevertheless, while I used to say just use
100% crispiness for best quality, now I think keeping it to about
75 is safer. My tests have convinced me that you should not suffer
from any dropped frames or jerky playback this way.
Framerates and Bitrates: Finally this brings me to
a strange fact. If we encode a DivX at 30fps it will actually end up smaller
than the same DivX encoded at 25fps! I cannot really explain this, but
I have a theory. A 25fps movie encoded at 1000 kbps would (at a constant
birate) use 40 kilobits per frame. But a 30fps movie encoded at 1000 kbps
will use about 33 kilobits per frame. 30fps movies are basically 24fps
movies with "repeat" frames added to take up space. Now, since,
for the most part, Mpeg-4 only records the differences between frames
the total differences between one movie of 25fps and the same movie at
30fps is zero! But the codec is now still only using a bitrate of 33 per
frame. This is possibly what makes the smaller filesize, which in turn,
gives a lower picture quality.
WHATS THE DIFFERENCE?!
"Hi Folks !
Always the same question, always
the same answer... The LowMotion codec is a hack from version 4.1.00.4920
of the M$ MPEG4v3, the HighMotion codec is a hack from version 4.1.4917
of the M$ MPEGv3.
I really don't know the internal
difference between the low-motion and the high-motion. I NOTICE the difference
and decide to make two version, the low-motion which came from the beta
version and the high-motion witch is the version 4.1.00.3917, the newer
builds looks like be an low-motion style... The low-motion and the high-motion
are the same file but with differents builds ! As you know I'm not the
coder of this thing so I could not help you more... I mainly use the low-motion
which produce better half-toning results, and the high-motion when the
picture is very moving.
The final bitrate is extremly related
to the CONTENTS of the video, a fast moving, very detailled picture is
harder to code than a almost still, very clean picture... I think that
the requested bitrate is a MAXIMUM bitrate, and the differences are in
the way the encoder try to match this bitrate, the low-motion seems to
allocate more bit to color and is 'tighter' to the requested bitrate,
the high-motion codec allocate more bit to the luminance but is not as
'tight' as the low-motion is.
As you probably guessed the above quote is from Gej the
person who made the DivX codec! All Mpeg-4 codec's are just hacks of the
Micro$oft ASF codec. This includes SmR (nAVI) and even the Angel Potion
codec (according to Avery Lee) which claims to be the first one that isn't
a hack. To be fair the Angel Potion does seem to perform a little differently
from the rest and is perhaps a highly altered ASF. But either way, at
the time of writing this article it was filled with bugs and so is not,
in my opinion, a good choice to use. When I say they are "hacks"
the only real differences are they allow just about any program to use
the codec to encode Mpeg-4 files. There was no alteration to the codec
at all - its just like making a hack for a time limited trial, the hacked
program itself remains unchanged. So there is no real difference in quality
between various Mpeg-4 hacks - all look basically the same quality!
This brings us to the big question of which is better, ASF
or DivX! And why bother to hack it anyway? Well, Micro$oft decided no
one could use Mpeg-4 unless it was encoded with their crappy encoder.
ASF files were only permited to be produced at very small resolutions.
ASF's when compared to pure Mpeg-4 files would add almost 100MB's of extra
data to the average 650MB movie! This was due to bulky overheads and all
the stuff designed to make them stream over the inernet. ASF's didn't
allow MP3 audio and almost always ended up with serious audio synchronizatioin
problems. Later versions of Media Encoder only allowed usage of the Fast
Motion codec which produced bad image quality for low motion scenes! In
short, DivX solved all these problems and anyone who says DivX is just
a joke and ASF's look better just doesnt know what they are talking about!
FAST MOTION VS LOW MOTON CODECS
Gej was correct in his observations of the Fast and Low
Motion codecs. BOTH codecs are variable bitrate which means the final
filesizes are very hard to predict.
Fast Motion Codec
The Fast Motion codec is the hardest codec to predict a
final filesize. You could encode one movie at 6000 kbps and another at
900 kbps with the Fast Motion codec and still end up with two movies of
almost the same size! Or the one at 6000 kbps could end up double the
size of the 900 kbps one! There is just no way to tell! When we set the
Fast Motions bitrate we are setting the Maximim bitrate! This means
it will always use an average bitrate of about 300 kbps until it reaches
a high action scene and only then will it increase the bitrate to the
maximum level! This is why people get confused, because it doesn't use
the bitrate they put in until it finds an action scene!
Low Motion Codec
The Low Motion codec, on the other hand, uses a Minimum
bitrate. This means that it will hardly ever go higher than what we set
it. So if we set it to 800 kbps it would use 800 kbps on most scenes and
only use a very little more or less depending on the action. This means
the Low Motion codec is a lot more predictable.
COMPARING THE TWO CODECS
The following examples will make clear the strengths and
weaknesses of the two codec's.
Fast Motion codec
Here is a high-action scene compressed with the Fast Motion codec
Here is a non-action scene compressed with the Fast Motion
codec at 6000kbps:
Low Motion codec
The following is a non-action scene encoded with the Low Motion
codec at 600 kbps:
Here is a high-action scene compressed with the Low Motion
codec at 600kbps:
From the above examples it is clear that the Low Motion
codec always looks better on non-action scenes even if it uses a bitrate
as low as 600 kbps. It is noteworthy too, though, that when the Low Motion
codec goes below 600 kbps it will not look significantly better than the
Fast Motion codec unless the Fast Motion codec is set to the same bitrate
The Fast Motion codec also has an upper limit of about 2000
kbps. It doesn't seem to make much noticable difference to quality whether
we choose Fast Motion at 2000kbps or 6000kbps! It does, however, make
significant demands on filesize.
Finally, as we can see from the examples below, when the
Low Motion codec is set to between 1000-1500 kbps it starts to look overall
better quality than the Fast Motion codec no matter what we set it to!
Fast Motion 6000 kbps
Fast Motion 2000 kbps
Low Motion 1500 kbps
The Fast Motion codec saves the most space and does very
well on high action scenes. If you are going to use it for movies always
set it to 2000 kbps. There is little point using any other setting and
it has the best quality to size ratio. You might consider using the Fast
Motion codec for putting long movies on a single CD.
The Low Motion codec set at 600 kbps will do better job
than the Fast Motion codec on just about all low action scenes. It cannot
compete with the Fast Motion codec on action scenes until we set a bitrate
of between 1000-1500kbps. After which, it starts to produce much better
results than the Fast Motion codec.
Final Note: At very small resolutions such as 320
x 240 the Fast Motion codec does a very bad job. It is probably better
to use the Low Motion codec all the time in such cases. But, as always,
do some testing and see what you think.
QUALITY AND RESOLUTION
All Mpeg formats, including DivX Mpeg-4, break up the picture
into 16 x 16 blocks called macroblocks. Each block is allocated a certain
amount of memory based on the bitrate we enter into the codec. The more
the bitrate the better the quality. Lets see what happens to the macroblocks
when we allocate less and less bitrate to them. The best way to see this
is to enlarge the video so we can see better, like this:
Take a look at the images below. The first one was given
the highest bitrate and the lowest was given the least bitrate. As you
can see, the less bitrate we give it the more simplified the macrblocks
Ideally we would love to encode all movies at 720 x 576
full DVD resolutions. But at this resolution the image is broken up into
1,620 macroblocks. It has an effective resolution of 45 x 36 blocks.
Lets say we have decided that to fit a movie on a single
CD as DivX we needed to use a bitrate of 800 kbps. But when we look at
the picture at 640 x 480 we can still see many of these macroblock artifacts.
We cannot increase the bitrate or it will not fit. Many think if we make
the image big (i.e. full sized) the blocks will be smaller. This is wrong!
Larger images means more blocks which means less memory is given to each
block. When each block has less memory allocated to it, it becomes simplier
and simplier until in the end most of the blocks become nothing more than
The only solution is to make the image smaller. Smaller
images means less blocks which means more memory is allocated to each
block and each block looks more like the original picture. A resolution
of 480 x 384 would give us 30 x 24 macroblocks, which is 900 less macroblocks.
This can make a big difference in quality!
The above methodology is nothing new to DivX, the same has
always applied to all Mpeg formats since they were designed. Because of
the way it is compressed the picture resolution is not the key factor
in determining the image quality. Smaller pictures actually look better
than larger ones at the same bitrate. The technique for making the best
quality image is simple mathematics:
1. Use the highest bitrate you can.
2. If the image shows too many macroblocks shrink the resolution
3. Check it again. If there are still too many macroblocks shrink
it a little more until they are not noticable. Its unlikely you will
get rid of all macroblocks on a single CD DivX but you can get rid
of most of them.
Commercial Video CD's (VCD) are considered by experts to
be almost VHS quality video. Yet they only use a resolution of 352 x 240!
This is possible even though a TV resolution is something like
576 lines. It is hard to compare an analog image to a digital image by
mere resolution. For one thing VHS uses signal compression, yes, that's
right, VHS is also a compressed format! But the response curve of VHS
places -3 dB at around 2 MHz of analog luminance bandwidth is equivalent
to 200 samples / line. VHS chroma is considerably less dense in the horizontal
direction than MPEG source video. And from a sampling density perspective,
VHS is superior only in the vertical direction, but when taking into account
interfield magnetic tape crosstalk and the TV monitor Kell factor, not
by all that much. Well, that what I read anyway! But the conclusion of
the matter is using small resolutions than TV lines can still produce
video's almost as good as VHS.
I have only a couple of rules I always follow for deciding
video resolutions. I never make it larger than 640 x 480 (which is higher
than TV resoloutions) and I never go lower than 240 pixels high unless
CROPPING THE VIDEO
Considering the previous facts about macroblocks, it makes
sense that cropping out the black bars found at the top, bottom and sometimes
at the sides of a movie would allow the codec to allocate more memory
to image quality. But the amount of memory allocated to a pitch black
area is quite negligable. So the most vital thing to remember for optimal
compression is to always crop a few pixels into the image so as to delete
all of the black bars. Mpeg compression works best on blurry images; so
if a hard black line is seen at the edge of your cropped movie it will
not compress as effectively. In fact, if you cannot crop into the image
then I'd say don't bother cropping at all, because the memory saved is
Warning on Cropping
It is only fair to warn you that, athough in my opinion cropping improves
the image quality of the DivX rip by a lot, before you decide that cropping
is the best way to go, you must consider these four facts:
1. A cropped movie is sometimes harder to convert into another
format. This is because you may need to re-add the black bars to the
top and bottom of the movie first or the movie may be stretched out
2. A cropped DivX movie will play at the wrong aspect ratio
in PowerDVD and some other Video players. On the other hand, Media
Player, MicroDVD and many other players will play back a cropped movie
3. VCD's and SVCD's cannot be cropped if they are to be played
in a standalone DVD player because it will not accept them.
4. Finally, Mpeg-4 files (and DVD files for that matter)
have problems playing back on some hardware if they are not encoded
in sizes that can be divided by 32. This means the Matrox G400 or
the Nvidia GeForce would probably have problems outputing it to TV.
This TV out problem is associated with the Mpeg-4 codec and does not
apply to most other codecs.
As an example:
A 528 pixels wide size divided by 32 = 16.5.
This is not a multiple of 32 and so may have trouble.
But a 576 pixel wide size divided by 32 = 18.
This is a multiple of 32 and will play back perfectly.
DON'T BLAME IT ON THE BITRATE!
If you find you start to encode a movie and it looks like the picture
below it is nothing to do with the compresion. It wouldn't matter if you
used 6000kbps or 10kbps it wouldn't get rid of these lines. The problem
is that Flask, or whatever decoder you are using is unable to decode your
Since TV screens are built out of lines, the DVD has a code inside it
that tells it how to output those lines to the screen and in what order.
If the order of the "fields" is incorrect you will get the annoying
combing effect we see above. So far there is not complete solution to
this. You can try resizing the picture smaller or you can use Flask Mpeg's
deinterlace filter. There is also a super slow annoying way to fix this
using DVD2AVI. For more information this interlace subject check out the
information in my appendix called: "Video Formats: NTSC & PAL / Telecine".
As you know, DVD's have a content scrambling system (CSS) which makes
it difficult to copy. These require code keys to decrypt. If you use the
wrong key you will end up with a corrupted file. Most ripping softare
automatically finds and uses these keys now, but it is always a good idea
to check your decoded DVD by playing it on your computer DVD player, just
to make sure it is correct before you convert it to something else. You
will know if it's corrupt because you will get loads garbage or green
/ pink blocks like in the picture below:
As you can see, it is not only 'dead people' this kid sees =o).
Duplication of links or content is strictly
prohibited. (C) NICKY PAGE 2000