Old 19-01-2003   #1
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Quarter Pel, GMC, Bidirectionel Encoding etc

Hey everyone!

I just have a few questions about the DivX 5.02 codec regarding it's advanced functions like GMC, Quarter Pel etc. Can someone explain what these things do (i tried to find it on doom9.org and divx.com and i did but it was very brief and not very clear).

Well what i'd like to know now what exactly Quarter Pel, GMC (Global Motion Compensation) and Bidirectional encoding does. Also can someone explain what Psychovisual enhancements do, and which setting to use; Light, Normal and Strong. For the decoding part: what does "Film Effect" mean?

tnx in advance
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Old 20-01-2003   #2
Mgz
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here we go folk
http://www.divx.com/support/divx/guide.php


Quote:
Global Motion Compensation
Global Motion Compensation (GMC) helps to improve complex scenes where zooming and panning are present. The ability to reduce the required data from one frame to the next can be reduced since there is a commonality within panning and zooming scenes that can be used to more efficiently compensate for what is more normally a group of blocks in such scenes.

Quarter Pel
As explained in the "B-frames" summary, data is reduced when the difference between two frames (prediction error) is transmitted instead of the entire image being sent. The difference in a successive frames composition is generally computed on a macroblock-by-macroblock basis (16x16 pels) or on a block by block basis (8x8 pels). For example, a part of an image located in a block at grid location (1,1) may move to grid location (1,2) in the next frame. As you may realize an image in one block will likely need more accuracy than just the ability to move on a limited block by block basis with an accuracy that is limited to an integer pixel unit (1,1). DivX has increased the previous accuracy of using a half pel (1.5, 1.5) to include the ability of using "Quarter Pel" (1.25, 1.75) accuracy with the Codec release. Quarter Pel performs a specific filtering on each block to produce a virtual block that should represent how the original block should appear if it is moved a 1/4 of a pixel unit.

Psychovisual Enhancements
By exploiting what we know about the Human Visual System (HVS) we have increased the efficiency of allocation video data helping to increase the perception of quality in video. For example, if the human visual system has very low sensitivity to a specific type of characteristic in an image we may decrease the amount of data located at this location and re-allocate this data to a location within an image where the human visual system is much more sensitive. The Psychovisual enhancements are applied to both a frame and macroblock basis. One of the important factors in evaluating Psychovisual Modeling is to NOT just compare a single frame but to compare a full sequence. An image may look worse or better when a single frame is examined but the key to reducing data is to reduce data in a way that the human visual system does not notice over a video sequence running at a full frame rate (e.g. 30 frames per second). Psychovisual modeling is a fairly new field when applied to real videos or movies. This area is full of possibilities we have only just started on and will continue to explore.
I thinks that's all I want to know about this ,but I don't know is it enough for you
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Old 20-01-2003   #3
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tnx for your explanation, guess i didn't search long enough

Does anyone has first-hand experience in using these functions, and could you recommend using Quarter Pel, GMC and Bidirectional encoding? And does it save you a lot of space? For the psychovisual enhancement; i heard some people saying that it screwed their video!! Does anyone has experience with this function? And still the decoding part; what does "Film effect" mean?

Last edited by theunbeatable; 20-01-2003 at 22:14.
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Old 21-01-2003   #4
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Using GMC and B-Frames is probably a good idea, Using Q-Pel probably wont help that much, but it makes the video slow in decoding, so it's porbably not worth it.

Psy enhancements are no longer broken, so it should be fine to use, though I wouldn't recommend it except at low bitrates.

Film effect adds the sort of effects you see in the movies, like grain. It doesn't improve the video, but people sometimes like them better. Sort of like how some people prefer LP's to CD's because they are used to the artifacts of them.
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Old 21-01-2003   #5
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tnx for your help SirDavidguy!
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