Artifacts

[Asterix]

What is an artifact ?

-Asterix

[Dr Mordrid]

Any distortion of the video or audio data is the basic definition. This can include a wide range of defects. Some basic ones for video;

Dropouts: These are usually due to a bad tape. These days mostly used in reference to DV where such problems can cause large pixelizations in the video or beeps/tones in the audio.

Re-compression: Most compression schemes like DV, MJPeg or MPEG use mathematical constructs called "discrete cosine transforms" to help encode the data.

Putting it VERY simply: DCT based compressions purposefully throw away some of the original color and detail content to increase compression efficiency. Because of this DCT compressing the same data more than once (re-compression) can cause visual defects due to the accumulation of reproduction errors caused by the missing data.

Generally speaking the higher the compression ratio (or the lower the data rate) of the original data the worse the recompression errors can be.

The most common occurances of re-compression are when transcoding to another codec (i.e. MJPeg-to-MPEG) or when applying effects, titles or graphic overlays in a video editor. This is why it is often advantageous to use uncompressed sources like YUY2 or RGB for those sequences to which you plan on adding extensive effects. Once all the effects are finished you then do a single compression to output the finished sequence.

The most common types of re-compression errors are;

Blocking: Like it sounds. Square or rectangular defects, often appearing in irregularly shaped groups.

Quilting: Again like it sounds. Large rectangular blocks over large areas, or even the whole video frame. Some call it "checkerboarding".

Gibbs: AKA "mosquito" artifacts. Most often seen as an in-motion "halo" around persons, titles or areas of a single color.

Re-compression artifacts can be made much worse by a high color or brightness contrast between adjacent areas of the video frame.

Field: Seen in interlaced video. There are two common types;

Motion: if there is some fast motion occuring between the two fields these show up as the video looking like it had a comb run through it.

Field order reversal: this is caused if video captured with one field order (i.e. Field B) is displayed using the other (Field A). It gives the video a severe flicker.

Bandwidth excursion: The most common type is caused by a sharp horizontal brightness changes across the video frame.

These changes can contain significant energy at frequencies above the allowed band. This causes a "ringing" in the video signal that then overshoots it's allowed bandwidth. This results in "sync buzz", an annoying 60 Hz buzz you get in the audio when the overmodulated carrier bleeds into the sound subcarrier.

You'll most often see/hear this with poorly designed black & white titles.

Another annoying cause is when some moron weatherman wears a black & white checkerboard or striped sport coat.

Ewww....

Dr. Mordrid


[This message has been edited by Dr Mordrid (edited 04 September 2000).]

[dchip]

A confirmed arti No, seriously, I believe in video terms an artifact would be "stuff" left over from the compression process. If the compression process didn't give a smooth image with no noise or little blips here and there, then the video is said to have artifacting in it. It can occur in the picture or sound and and especially with analog video and compression schemes like MJPEG or even MPEG. Some artifacting is probably unavoidable. Too much is unacceptable.

Artifact is an interesting word in the NLE world. Outside of NLE it refers to something left behind as a result of past human activity. Tools, pottery, etc. In computer video, I would think the term was chosen because "left behind" is an apt description. Something that didn't get cleaned up by the processes involved or that was left behind by a flawed process would be an artifact.

My bit of philosophy for the day.

(Hi Doc, I was typing too long on this response and didn't see your post. I'll go ahead and leave my 2 cents worth anyway)

[This message has been edited by dchip (edited 04 September 2000).]

[Brian Ellis]

This discussion inspired me to refer to my trusty Oxford dicky on CD-ROM. It tells me:

artefact // n. (US artifact)
1 a product of human art and workmanship.
2 Archaeol. a product of prehistoric or aboriginal workmanship as distinguished from a similar object naturally produced.
3 Biol. etc. a feature not naturally present, introduced during preparation or investigation (e.g. in the preparation of a microscope specimen).
artefactual adj. (in senses 1 and 2).
[Latin arte (ablative of ars ‘art’) + factum (neut. past part. of facere ‘make’)]

It would therefore seem, in this context, to fall into definition 3 which is eloquent in a few words.

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Brian (the terrible)

[dchip]

Fascinating stuff, these artifacts. They can save lives, too. I am thinking about the scene in The Matrix when Neo notices the black cat twice go by the doorway. Were it not for that little artifact (and Neo spotting it) that got introduced when the agents attempted to edit that portion of the Matrix into a deathtrap, it could have all ended right there. So let's show some love for the artifact.

[Asterix]

Many thanks for your explanations.
Even for the black cat... ;-)

-Asterix.

[Sillyname]

Has anybody ever intentionally used artifacting as a digital effect? I could imagine severe artifacting could be used on "weak sub-space transmission" episodes of Star Trek or something...

Robocop V: Robocop is nearly completely dismantled, except for head which resides in bad guy's kitchen as a conscious juicer. The video through its eyes is extremely distorted with artifacting from the last big battle that put him in this predicament.

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Deep is not the root word of depression.

[Sillyname]

Or how about in The Matrix 2, there's a compression gun! That compresses your digital persona to the point of being corrupted and full over errors; past what PKZIPFIX.EXE can help!