Home > Archive > Compression > October 2006 > Re: "Real WMV", 148.50 mhz sample-rate, 1920 X 1080 progressive scan image,
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Re: "Real WMV", 148.50 mhz sample-rate, 1920 X 1080 progressive scan image,
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| Bob Myers 2006-10-30, 6:56 pm |
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"Radium" <glucegen1@excite.com> wrote in message
news:1162144640.877277.191230@m73g2000cwd.googlegroups.com...
> I hate pixelation and aliasing with a passion. Pixelation and aliasing
> make me sick. I don't mind the artifacts -- that I think -- are
> associated with a WMV whose color-depth has been compressed even to
> extremes while the sample-rate and pixel resolution are left alone. It
> looks similar to what a WMA file with a 44.1 khz and 20 kbps sounds
> like -- I think.
>
> What would be to the human eye what 44.1 khz, 20kbps is to the eye?
Think about what you're asking - it basically depends on the ability
of the eye to resolve detail, right? The term for this is "spatial acuity,"
which is typically given in terms of the number of cycles (or line pairs)
of white and black the eye can detect PER VISUAL DEGREE. That
last part means that it depends on how "wide" each line pair appears
within the visual field. So it's never as simple as just "how many pixels
do I need before I can't see the pixels any more?" The answer depends
on the image size, and how far away the viewer will be - in other
words, how much of the visual field will be filled by the image.
Google for "spatial acuity" and you will no doubt find all the
information you need to solve this one.
> The human ear needs at least 20 hz to hear the sound. The human eye
> needs at least 60 hz for the light to appear solid. E.g. a hummingbird
> wing flap is too high of a video-frequency for the human eye to see,
> much like the sound of a dog-whistle is too high an audio-frequency for
> the human ear to hear.
Actually, 60 Hz isn't always enough for the light to appear "solid"
("flicker-free" is the more common term). Again, try Googling
for "flicker fusion frequency" and you will find all you need to
know.
> WMA is my preferred type of perceptual encoding. Both WMAs and MP3s
> will produce artifacts with a too-low bit rate. However, WMA's
> artifacts are rather pleasant, while MP3's are disgusting.
Better than either, of course, is uncompressed data; second
best is data run through a completely lossless compression
algorithm. Lossy compression is used when you have no other
choice, due to constraints of storage space, data transmission
capacity, etc.. So before you pick one system as your "preferred,"
you need to better define the problem you're really trying to
solve at any given time.
The blocky-looking artifacts that result from some compression
schemes are not the result of "pixelization" per se; they come about
because these schemes are based on transforming a block of
pixels (such as an 8 x 8 or 16 x 16 pixel square) into a more
compact form, all across the image. But you can push the
compression algorithm too far, to the point where there is no longer
sufficient information to accurately recover that block of pixels
- and so the block itself becomes visible.
Bob M.
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