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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Moire Issues

I just looked at the wheels of some cars going by... if I blink, I can see a much-less-blurred version of the hubcap appear just briefly (persistence of vision effect?). If I blink rapidly, I can get a strobe-like effect. But I don't need to blink; if I watch a wheel and then suddenly look away, there is a similar brief freeze-frame effect at the moment I look away.

If you are driving and the road is slightly bumpy or the engine shakes, etc. I wonder if that vibration when combined with the persistence of vision effect could give you the backwards rotating wheels.
 
Well, didn't notice it on the drive home ... definitely made me weave a bit more than usual. :)

I wouldn't be surprised if it depended on a number of factors: speed, number of spokes, pattern of spokes (?)

Y'all drive safe now...
 
You can see the same effect on aircraft propellers. At certain speeds they appear to the eye to slow down and even reverse. The ability of the human eye to discern relative circular motion at certain speeds must be limited...
 
Yeah, come to think of it, I have seen it on propellers as well.

At the risk of going completely over the rails and down into Offtopic Ditch, here's a cool site with mind-bending optical illusions that illustrate our mind's wacky machinations:

http://www.michaelbach.de/ot/
 
Checked it out quickly...very cool.

I don't think it's off topic since as shooters, editors, etc. we all deal with images and the eyes ability to discern certain shapes, colors, and shades...
 
"We do see aliasing on stuff that moves " Can't say I've seen that myself - care to elucidate? Is that spatial (which is what I'm on about) or temporal aliassing?
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I was talking about spatial aliasing there. In real world situations, we generally don't see spatial aliasing on moving subjects.

To see for yourself, you can try printing out the following zone plate test pattern. You likely need to resize the image with "nearest neighbour" sampling in photoshop (or its equivalent in another program) so that it will print well.
http://www.worldserver.com/turk/opensource/z128.gif
Hold the printed version in your hand and warp the paper, or move the paper around.

A variation on the zone plate is as below.
http://glennchan.info/Proofs/forums/blue-zone.png
If you flick your eyes (or just move em around) between two spots on the zone plate, you'll see spurious sets of circles appear.

I believe the reason why this occurs is because our visual system can scan scenes through eye movements ("vernier scanning" is the phrase I'd use). Assuming that the target is stationary, this allows our visual system to get higher resolution and not be very prone to aliasing. This gels with experimental data, which shows that we can see alignment detail that is much smaller than our cones/photosites (hyperacuity on michael bach's site). From my own experiments with printing out a zone plate, it's also consistent since we see virtually no aliasing. This is especially so for the blue zone plate. Our eyes have three types of cones, one of which is very sensitive to blue (we can think of them as 'blue cones', which is somewhat accurate). These blue cones are only 2% of the total cones in the eye, and must be spaced with huge gaps in between them. These huge gaps will be very prone to aliasing- any detail that falls in between photosites can alias. The HVS avoids this by moving the eye, which means the photosites can scan the stuff that falls in between gaps.
Contrast this to camera systems, which don't do vernier scanning and are bounded by Nyquist/Shannon. They can't see detail smaller than a photosite, and they are prone to aliasing (unless heavy filtering is employed, which reduces resolution).

Vernier scanning should break down if the target isn't stationary. Our eyes can compensate for this by tracking the motion. However, for motion it cannot track, aliasing should occur. This (to my eyes) happens with the zone plate tests above.

2- Frame rate of the HVS:
Perhaps the frame rate of the HVS is flexible. In low light conditions, lowering the 'frame rate' will increase exposure time, which will increase sensitivity. This sort of gels with experimental data on flicker, since flicker sensitivity goes down as luminance decreases.

This might also explain why our eyes have less aliasing in dim conditions (when looking at a zone plate). Lower FPS = more motion blur = less fine detail that will alias.
 
chuck colburn said:
I was wondering also how Jeff was, and now you, are seeing car wheels that appear to be rotating at a different speed then in real time.
Click to expand...

I haven't seen it with car wheels.. But as I did say, I've seen it with very fast spinning motors and power tools. It's easy to replicate indoors under flickering fluorescent lights, but I have seen it out in the mid-day sun too. Trying to keep your eye on a label of a cutting blade as it does 10Krpm or more can play tricks on your eyes. I suppose it could happen with car wheels if the conditions are right -- I have not seen this myself.
 
AppliedVisual said:
I'm waiting for the day I can dump my thoughts/dreams directly to physical media and edit them in real-time. :D
Click to expand...

I had a dream once in which I had a firewire port installed in my neck, matrix style. I could go to sleep (in my dream), and when i woke up, I could dump my dreams as a quicktime file and edit away. It was awesome, experimental-freaky footage.

When i woke up, without thinking i went to my computer (to dump the footage). Groggy, i got frustrated when i realized i had no footage to dump, and no hole in my neck.

::sigh:: One day....
 
Glenn, I have seen the effect you mentioned, and it nearly made me ill the first time. I'd got a new Epson printer, and there was a diagram in the manual with lots of diagonal lines on it. I'd moved the manual for a closer look and "wow" weird things happened. If you zoom in and out (manually by moving the manual) you get all sorts of weirdness.

So yes, we do get these effects, but it's rare.

All cameras really need the correct amount of low pass filtering - no escaping that one here.

Graeme
 
Graeme,

I think you'll get a kick out of some of those optical illusions on the link Pixelchef put up. Explanations are interesting too - they talk about aliasing, LPF, and things that are the kind of things that interest you. When you get a break between the whisky/coffee cycle, check 'em out.

Cheers!
 
Gibby said:
....When you get a break between the whisky/coffee cycle, check 'em out.
Click to expand...

LOL Gibby. Graeme, your results may vary wildly depending on which part of the cycle you decide to test these visual abstractions.:eek:

Cheers Guys.
 
Moire/Spatial Aliasing

Moire/Spatial Aliasing

Aliasing only happens when a regularly patterned sensor encounters a regularly patterned surface whose detail is finer grained than the sensor. In randomly detailed surfaces such as on living things alaising is never a problem.

There is one method of avoiding moire patterns that is seldom mentioned but my favorite. It's called stochastic oversampling and its the method used by Pixars renderman to avoid moire patterns on surface textures in CG. Basically if we were to downsample a 4K image to 2K we could, for example, use four pixels to compose one downsampled pixel, by say averaging all of their values. (the actual method is a bit more complicated) In stochastic sampling we would use only say three out of four pixels selected randomly. This additional randomness adds a tiny bit of noise to high detail areas but effectively eliminates Moire by removing any regular pattern in the sensor. Since red is a cinema camera there are also effective algorithms for temporarily ignoring pixels in 4K when interpolating the Bayer pattern. Since the eye is seeing many frames per second, there will be no perceived loss of detail in the moving image, while each individual frame might show some loss. In fact this is exactly what is happening when we look at projected film, of which each still frame does not appear nearly as detailed as the moving image.

You should be able to take advantage of these techniques on the RAW image but I would love to see such algorithms built into Red's firmware as I believe it leads to a more filmic image.

Cheers,
Ian Bloom
http://www.ianbloom.com
Red #657
 
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