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The 

Guide 

68 

Cro
p

 a

nd

 Re
siz

e 

Cr

op

 

Crop allows you to specify the number of pixels 
from each edge of the video that will be 
discarded before encoding. 
 
The cropping area is fixed throughout the video. 

QUICK GUIDE 

Crop 

Crop top 

Crop bottom 

Cr

op

 le

ft

 

Cr

op

 ri

gh

t 

You should crop any borders surrounding a video before encoding it to avoid 
wasting bits. This will improve the overall quality of your video. 
 
Cropping is performed

 

before resizing by the encoder. The dimensions of the 

video after both cropping and resizing have been performed must be divisible  
by four. For optimal performance the dimensions should be divisible by 
sixteen. 

Cropping: 
 

-c 

<left>,<right>,<top>,<bottom> 

 

Left, right, top 

and 

bottom

 are the number of pixels from each edge of the 

image that will be discarded before encoding. 

Cropping in some applications, such as VirtualDub, can require colorspace 
conversions that reduce encoding performance and minimally degrade the 
video. Where crop and resize are the only filtering operations required 
greater encoding performance will be seen using the encoders crop filter. 

Содержание USBAV-708 INSTANT VIDEOMPX

Страница 1: ...Revision 1 0...

Страница 2: ...51 Slow 53 Slowest 54 Psychovisual Enhancement What is psychovisual enhancement 56 The DCT iDCT and Human Visual System 56 The psychovisual enhancement system 57 Experiencing PVE 58 Fast psychovisual...

Страница 3: ...hat is the DivX Certified Program 101 What s in a profile 102 DivX Certified Program requirements 103 Electrokompressiongraph What is the EKG 105 How the EKG works 106 Using the EKG 107 DivX Decoder A...

Страница 4: ...The Guide 4 Introduction...

Страница 5: ...s that are set to take the consumer market by storm in 2004 We will also show you how to configure the DivX decoder to improve the playback of DivX videos on your personal computer Before we begin it...

Страница 6: ...ocess of creating a new codec entirely from scratch In May 2000 Gej became co founder of DivXNetworks together with CEO Jordan Greenhall Director of Product Management Joe Bezdek R D director Darrius...

Страница 7: ...ation of the MPEG4 Video Standard supporting both simple profile and advanced simple profile encoding Essentially the Moving Pictures Expert Group creates standards and methods for achieving various o...

Страница 8: ...ch the logo shown in the encoding wizard to the logo on your player and you can t go wrong The DivX Certified Program already includes over 30 companies and DivXNetworks rigorously test every single c...

Страница 9: ...storically video transferred over the net has been small jumpy blocky and suffering from poor color definition DivX breaks down the barriers to transferring high quality video via the Internet Using a...

Страница 10: ...The Guide 10 Quick Start Guide...

Страница 11: ...learning the basics of VirtualDub while using it to create your very first DivX videos Until you have completed this part of the guide you should simply follow the instructions precisely and resist th...

Страница 12: ...ualDub main window from top to bottom Program menus Video display area currently empty Timeline and seek control Timeline control buttons Tool tips bar From left to right the timeline control buttons...

Страница 13: ...to DivX and also apply any filters should we choose to do so Because we will not be applying filtering here Normal Recompress might give us better performance Fast recompress gives the best performan...

Страница 14: ...ttings using the Restore Defaults button but you might want to take this opportunity to explore the encoder configuration dialogue before doing so When you are ready click Restore Defaults and then OK...

Страница 15: ...more CPU time with other applications but will also result in longer encoding times New to version 5 1 is the Feedback Window This window lets you monitor the inner workings of the DivX encoder in rea...

Страница 16: ...t DivX movie using The DivX Player bundled free with DivX or another media player application on your system If you would like The DivX Player to open your file by default as opposed to any other play...

Страница 17: ...ctual video output will be generated Enabling Write MV file allows the encoder to make use of a technique that accelerates successive Multipass passes Save your AVI file using a suitable file name You...

Страница 18: ...file using the same file name as you used for the 1st pass It is normal practice in Multipass encoding to overwrite the output from the previous pass every time you save After an nth pass encoding ha...

Страница 19: ...ems MP3 audio is supported by DivX Certified hardware players CBR MP3 audio is least likely to introduce de synchronization between audio and video The MP3 codec distributed with Windows is the Fraunh...

Страница 20: ...ctly interleaved for stable playback performance particularly when content is to be stored on optical media such as CD R or DVD R or other media with high seek times Consider this AVI container with o...

Страница 21: ...ely locate the associated audio and resume playback more promptly Using VirtualDub you can access the Interleaving options from the Audio menu Ensure Enable audio video interleaving is enabled The rec...

Страница 22: ...decompressed and fully processed when saving including the application of any filters or compressors you have selected Both Direct stream copy and Full processing mode have exactly the same semantics...

Страница 23: ...The Guide 23 Forward...

Страница 24: ...me rate given in frames per second or fps If we were to slow a video down to such an extent that it were possible to see individual frames it would become evident there is generally little change in t...

Страница 25: ...ture is not perfect and DivX has to store some additional data to correct for the difference known as the residual Even so recreating the picture using motion vectors and residual consumes far less st...

Страница 26: ...ery important purpose All of the blocks in an I frame are stored as images thus decoding an I frame reveals a complete picture without dependency on reference frames For this reason I frames are also...

Страница 27: ...sequences for quantization with regards to image quality Examine the following two tables The first quantizes a series of number with a fixed quantizer of 3 The second quantizes the same series of num...

Страница 28: ...is because the range of results encompassed by lower quantizers is larger than that of higher quantizers In the tables results of data quantized at Q 3 could potentially include any value of interval...

Страница 29: ...e appropriate to a feature A key to the guide This indicates a feature is not compatible with the DivX Certified Program Use of the feature will mean your video will not be compatible with DivX Certif...

Страница 30: ...h feature description you will see the Quick Guide bar Here the Quick Guide bar indicates that there are performance tips and usage notes associated with the feature All other icons are grayed out Thi...

Страница 31: ...The Guide 31 Bitrate mode...

Страница 32: ...ven a video of any fixed duration encoding at a higher bitrate will lead to larger file sizes and better quality video while conversely encoding at a lower bitrate will lead to smaller file sizes but...

Страница 33: ...o 128 The number of kilobits per second for audio x 1 000 The number of bits in a kilobit x 3 600 The duration of the video in seconds from step 1 460 800 000 bits consumed by audio 4 Calculate the bi...

Страница 34: ...mplex in terms of image and motion it is impossible to maintain a constant bitrate throughout the file without introducing substantial quality differences from frame to frame Instead DivX distributes...

Страница 35: ...tent of all available modes in terms of video quality After running 1 Pass encoding your DivX file can be viewed immediately During 1 Pass encoding it is possible to modify the bitrate in real time by...

Страница 36: ...oint value between 1 0 and 31 0 specifying the fixed quantizer to be used 1 Pass mode is particularly useful when you are capturing from a live source and you desire control over the bitrate file size...

Страница 37: ...he short term history as video is received Multipass mode is designed to avoid this extremism by allowing the encoder to first analyze the entire video before encoding it This analysis is performed in...

Страница 38: ...numbers may appear to be out of order but in fact this is because they are in display order For example in the section of log shown the sequence reads 0 2 1 for a set of I P B frames respectively The...

Страница 39: ...ctional frames the value is associated with the bits required to encode the residual difference between source and motion compensated frames modulation Value Value is a floating point number represent...

Страница 40: ...he new optimized rate control strategy can again be written back into the log file and used to further refine the rate control strategy for a successive nth pass The log file will be updated during an...

Страница 41: ...to the level of correction required Once again taking an action movie as an example imagine encoding at a very limited bitrate in order to create video suitable for Internet distribution It might be d...

Страница 42: ...cy when it is possible to pass an identical source video through the encoder multiple times When performing more than 2 passes 1st nth in order to be effective in refining the rate control and hence q...

Страница 43: ...Multipass nth pass Bitrate is an integer value between 0 and 16 000 specifying the number of kilobits per second Quantizer is floating point value between 1 0 and 31 0 specifying the fixed quantizer...

Страница 44: ...stimation need take place only once For MV re use Write MV file must be enabled during the Multipass 1st pass and Read MV file during each Multipass nth pass QUICK GUIDE MV re use After the 1st pass i...

Страница 45: ...nsuring the encoder stores the image data for each frame with a consistent accuracy However when you fix the quantizer it is not possible to set or predict the bitrate and hence file size because the...

Страница 46: ...gher quantizer will produce smaller files and vice versa 1 Pass Quality based mode is particularly useful for capture when you intend to later re compress a video using Multipass mode In fact 1 Pass Q...

Страница 47: ...The Guide 47 Performance Quality...

Страница 48: ...encoder may fail to meet the average bitrate unless a very high average bitrate is specified Fastest mode is unsuitable for use during multipass encoding because the rate control algorithm used by th...

Страница 49: ...The Guide 49 Performance Quality Fastest Performance Quality pq mode Mode is one of 1 Fastest 5 Standard 64 Slow 192 Slowest...

Страница 50: ...d mode respects the Maximum Keyframe Interval and Scene Change Threshold settings described later Using these two settings it is possible to control the encoders decision process for frame type select...

Страница 51: ...ed or which of several possible motion vectors for a block yields the best balance of bits spent against quality With respect to the illustration opposite consider any decision the encoder might make...

Страница 52: ...curve Increasing Lambda dragging the slider to the left causes a sharper rise in the rate distortion curve meaning larger improvements are demanded for an increase in bit spend Decreasing Lambda drag...

Страница 53: ...the possible outcomes of various decisions made by the encoder QUICK GUIDE Slow In Slow mode the encoder uses the new Rate Distortion algorithm to make frame type decisions based upon the best balanc...

Страница 54: ...the possible outcomes of various decisions made by the encoder QUICK GUIDE Slowest In Slowest mode the encoder uses the new Rate Distortion algorithm to make frame type decisions based upon the best...

Страница 55: ...The Guide 55 Psychovisual Enhancement...

Страница 56: ...re error is least perceivable To understand how this works an explanation of the method DivX uses to encoded the image is necessary This is a complex subject that we will only touch on it briefly do n...

Страница 57: ...ge co efficients are manipulated so that fine details are enhanced If we were to encode in 1 Pass Quality Based mode i e at a fixed quantizer this enhancement would naturally increase the bits spent o...

Страница 58: ...choosing with Fast psychovisual enhancement mode enabled Begin encoding and click the Pause button on the encoder feedback window when a suitable image to work with appears Set Show picture to encode...

Страница 59: ...than Slow psychovisual enhancement mode When choosing which psychovisual enhancement mode to use do not equate speed with quality Each psychovisual enhancement mode produces slightly different results...

Страница 60: ...and lowers the probability that visible artifacts will be introduced as a consequence of the process When choosing which psychovisual enhancement mode to use do not equate speed with quality Each psy...

Страница 61: ...The Guide 61 Source pre processing...

Страница 62: ...complex and the motion search is more successful Pre processing is most effective when applied to video taken from an analogue source typical examples being VHS tape TV capture and camcorder footage...

Страница 63: ...to frame A good example of temporal artifacting is flickering video where even though there may be little noise within each individual frame there is temporal noise affecting the intensity of pixels o...

Страница 64: ...e two presets it may be advisable to process short sample clips under each to find the best balance between noise reduction and smoothing QUICK GUIDE Source pre processing Source pre processing is rec...

Страница 65: ...The Guide 65 Crop and Resize...

Страница 66: ...this functionality by providing its own crop and resize controls The encoders internal filters are optimized with regards to quality and performance and may offer better performance than those in your...

Страница 67: ...fewer blocks and thus a greater bit spend per block giving higher overall quality but if the video dimensions are reduced too far then within each block the texture and motion may become so complex t...

Страница 68: ...ng by the encoder The dimensions of the video after both cropping and resizing have been performed must be divisible by four For optimal performance the dimensions should be divisible by sixteen Cropp...

Страница 69: ...ing is more complex than bilinear sampling calculating the color of each pixel in the resized image from 4x4 pixels in the source Instead of simply interpolating linearly between pixels bicubic sampli...

Страница 70: ...o calculate the correct vertical resolution 1 Aspect x Horizontal dimension Or 1 2 35 1 x 704 299 57 Recall that dimensions must be divisible by four or more optimally by sixteen 704 is already divisi...

Страница 71: ...The Guide 71 MPEG4 Tools...

Страница 72: ...directional frames can contain blocks that are intra forwards predicted backwards predicted or both forwards and backwards predicted This means that B frames reference not only the previous frame but...

Страница 73: ...issues relating to the sequence in which frames must be encoded and decoded Whereas forward prediction as used by P frames simply references the last frame that was decoded a backwards predicted bloc...

Страница 74: ...frame 1 and frame 3 frame 3 is decoded first It is now clear that in the re ordered sequence all frames can be decoded as every frame makes reference only to others that have been previously decoded...

Страница 75: ...ed from the second step of the Select Profile Wizard dialogue under MPEG4 Tools QUICK GUIDE Bi directional encoding Bi directional encoding is not performed when Performance Quality mode is set to Fas...

Страница 76: ...The Guide 76 Bi directional encoding Bi directional encoding Bi directional encoding b Enabled only when CLI parameter is present...

Страница 77: ...xel in any direction at a time The relative vector offsets from the origin of the current block during this search might be 0 0 0 1 0 2 1 0 and so on This scheme would be called whole pixel because th...

Страница 78: ...appear to move slowly in the distance where perspective causes space to appear compressed and fast near to the camera where perspective has the least significant effect In this scene the effect of pe...

Страница 79: ...ly when Disable profiles has been selected during the first step QUICK GUIDE Quarter pixel Quarter pixel motion search resolution can substantially increase encoding duration Support for Quarter pixel...

Страница 80: ...quire substantially reduced post processing leading to visible artifacting that can counter act the quality improvements offered by quarter pixel By increasing the accuracy of the motion vectors quart...

Страница 81: ...se all of the blocks representing a static scene would appear to move anti clockwise around the center of the picture If the camera were to zoom in all blocks would appear to move outwards from the ce...

Страница 82: ...tion QUICK GUIDE Support for Global motion compensation is not required by DivX Certified devices Although some hardware devices claim to support GMC this functionality is not required by the DivX Cer...

Страница 83: ...The Guide 83 Advanced...

Страница 84: ...tched in a reference frame it will be encoded as an intra block and instead of prediction via a motion vector its image will be stored One way to select where to place intra frames is therefore to set...

Страница 85: ...the seek target in order to resume playback at the desired frame Reducing the scene change threshold will cause an increase in the proportion of key frames to predicted frames throughout the video Key...

Страница 86: ...the nearest past key frame until the seek target in order to resume playback at the desired frame Reducing the maximum key frame interval will cause an increase in the proportion of key frames to pre...

Страница 87: ...The Guide 87 Interlacing...

Страница 88: ...t on odd lines by the television then the bottom field is broadcast and scanned out on the even lines In this way a complete picture is recreated An interlaced camera may capture all the even lines in...

Страница 89: ...d in the image interlacing artifacts would be present Greater motion causes greater artifacting and inconsistency in the texture between frames The motion search must be able to match blocks in the cu...

Страница 90: ...lect to de interlace all frames to progressive unless you have a specific reason for preserving interlacing Encoding as interlaced requires substantially higher bitrates in order to achieve equal perc...

Страница 91: ...The Guide 91 Interlacing Source interlace Source interlace d mode Mode is one of 1 Encode as progressive parameter may be omited 2 Encode as interlaced 3 De interlace all frames...

Страница 92: ...The Guide 92 Video Buffer Verifier...

Страница 93: ...ations can often be overlooked without any significant consequences However when encoding for broadcast over an IP network e g the Internet or for a hardware device with fixed sustainable throughput i...

Страница 94: ...for doing so The default VBV parameters are The default initial occupancy is 75 of the buffer Profile Bitrate Buffer Initial occupancy Handheld 128000 262144 196608 Portable 768000 1048576 786432 Hom...

Страница 95: ...the maximum sustainable read bitrate in bits per second of the target system Buffer size is the size in bits of the buffer associated with the video stream in the target system Initial occupancy is th...

Страница 96: ...The Guide 96 Profiles...

Страница 97: ...m of profiles for DivX Certified devices those that have been rigorously test by DivXNetworks to ensure they meet the high standards the certification program demands Associated with each profile are...

Страница 98: ...fully choosing encoder options Certified devices are not required to support Global Motion Compensation or Quarter pixel under any profile and when profiles are disabled you should take care to set th...

Страница 99: ...The Guide 99 Profiles Profiles Profiles profile number Number is one of 0 Disable profiles 1 Handheld profile 2 Portable profile 3 Home Theatre profile 4 High definition profile...

Страница 100: ...The Guide 100 DivX Certified Program...

Страница 101: ...ly tested by DivXNetworks before being awarded certification Only devices that say DivX Certified and carry one of the four official certification logos have been tested by DivXNetworks as capable of...

Страница 102: ...evice from the Select Profile Wizard prior to encoding you ensure the video stream will always play correctly on the device What s in a profile Maximum resolution 176 x 144 15 fps 352 x 240 30 fps 352...

Страница 103: ...ww DivX com divx drdivx DivX Certified Program requirements Feature Required All DivX 3 11 movies on 1 CD anything under 1 mbps average bitrate Yes All DivX 4 content Yes DivX 5 content with no GMC an...

Страница 104: ...The Guide 104 E K G...

Страница 105: ...the quality of a particular section of video For example when encoding a movie in its entirety it is likely the viewer will be less concerned with the quality of the titles than that of the movie its...

Страница 106: ...code a particular frame at Q 6 but a modulation of 2 0 was specified the frame would instead be encoded at Q 12 A modulation of 1 0 would leave the original quantizer unchanged 1 0 x Q Q and a modulat...

Страница 107: ...owed by the encoded AVI file The EKG will display a preview of the video and graph the data from the log file Hover the mouse over the Legend bar to see the graph legend Click the Swap button to displ...

Страница 108: ...the grab bar After you release the grab bar the new modulation for the frame will be set and reflected in the orange modulation line that runs throughout the graph You can select multiple frames to mo...

Страница 109: ...The Guide 109 DivX Decoder...

Страница 110: ...in media players that are based upon Microsoft s DirectShow platform You can check that the genuine DivX decoder is playing your DivX videos simply by playing any DivX video in a DirectShow based med...

Страница 111: ...hm Ringing appears as an outline or shadowing around contrasting edges in the picture Ringing is harder to see than blocking and requires more computation to correct By default the encoder will apply...

Страница 112: ...s warmth to the image and masks the artificial smoothing effect that can result from low bitrate encoding by applying a film grain effect to the picture The film effect is particularly useful in count...

Страница 113: ...r slower CPUs With smooth playback disabled b frames are decoded at the same time as the frames they reference and then displayed at the appropriate time If the frame sequence were 1I 2P 3B 4P 5B the...

Страница 114: ...lows you modify the pixel aspect ratio of the display Double Buffering Double buffering causes the video card to reserve additional memory for receiving video data Double buffering improves the smooth...

Страница 115: ...The Guide 115 Acknowledgements...

Страница 116: ...ng J r me Rota Darrius Thompson Thanks also to Tom Huntington Corporate Communications Manager for his editorial review and Scott Green Art Director for his cover artwork and other illustrations Alast...

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