Tag Archives: artefact

Temporal Aliasing – Beware!

As camera resolutions increase and the amount of detail and texture that we can record increases we need to be mindful more and more of temporal aliasing. 

Temporal aliasing occurs when the differences between the frames in a video sequence create undesirable sequences of patterns that move from one frame to the next, often appearing to travel in the opposite direction to any camera movement. The classic example of this is the wagon wheels going backwards effect often seen in old cowboy movies. The cameras shutter captures the spokes of the wheels in a different position in each frame but the timing of the shutter relative to the position of the spokes means that the wheels appear to go backwards rather than forwards. This was almost impossible to prevent with film cameras that were stuck with a 180 degree shutter as there was no way to blur the motion of the spokes so that they were contiguous from one frame to the next. A 360 degree shutter would have prevented this problem in most cases. But it’s also reasonable to note that at 24fps a 360 degree shutter would have introduced an excessive amount of motion blur elsewhere.

Another form of temporal aliasing that often occurs is when you have rapidly moving grass, crops, reeds or fine branches. Let me try to explain:

You are shooting a field of wheat, the stalks are very small in the frame, almost too small to discern individually. As the stalks of wheat move left, perhaps blown by the wind, each stalk will be captured in each frame a little more to the left, perhaps by just a few pixels. But in the video they appear to be going the other way. This is  because every stalk looks the same as all the others and in the following captured frame,  the original stalk may have moved  say 6 pixels to the left. But now there is also a different stalk just 2 pixels to the right of where the original was. Because both stalks look the same it appears that the stalk has moved right instead of left. As the wind speed and the movement of the stalks changes they may appear to move randomly left or right or a combination of both. The image looks very odd, often a jumbled mess, as perhaps the tops of the stalks appear to move one way while lower parts appear to go the other.

There is a great example of temporal aliasing here in this clip on Pond5 https://www.pond5.com/stock-footage/item/58471251-wagon-wheel-effect-train-tracks-optical-illusion-perception

Notice in the pond 5 clip how it’s not only the railway sleepers that appear to move in the wrong direction or at the wrong speed but notice how the stones between the sleepers appear to look like some kind of boiling noise.

Like the old movie wagon wheels one thing that makes this worse is the use of too fast a shutter speed. The more you freeze the motion of the offending objects or textures in each frame the higher the risk of temporal aliasing with moving textures or patterns. Often a slower shutter speed will introduce enough motion blur that the motion looks normal again. You may need to experiment with different shutter speeds to find the sweet spot where the temporal aliasing goes away or is minimised.  If shooting at 50fps or faster try a 360 degree 1/50th shutter as by the time you get to a 1/50th shutter motion is already starting to be as crisp as it needs to be for most types of shots unless you are intending to do some for of frame by frame motion analysis.

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More info on CMOS sensor grid artefacts.

Cameras with bayer CMOS sensors can in certain circumstances suffer from an image artefact that appears as a grid pattern across the image. The actual artefact is normally the result of red and blue pixels that are brighter than they should be which gives a magenta type flare effect. However sometimes re-scaling an image containing this artefact can result in what looks like a grid type pattern as some pixels may be dropped or added together during the re scaling and this makes the artefact show up as a grip superimposed over the image.

grid-pattern-1024x576 More info on CMOS sensor grid artefacts.
Grid type artefact.

The cause of this artefact is most likely off-axis light somehow falling on the sensor. This off axis light could come from an internal reflection within the camera or the lens. It’s known that with the F5/F55 and FS7 cameras that a very strong light source that is just out of shot, just above or below the image frame can in some circumstances with some lenses result in this artefact. But this problem can occur with almost any CMOS Bayer camera, it’s not just a Sony problem.

The cure is actually very simple, use a flag or lens hood to prevent off axis light from entering the lens. This is best practice anyway.

So what’s going on, why does it happen?sony-grid-artefact-explained More info on CMOS sensor grid artefacts.

When white light falls on a bayer sensor it passes through color filters before hitting the pixel that measures the light level. The color filters are slightly above the pixels. For white light the amount of light that passes through each color filter is different.  I don’t know the actual ratios of the different colors, it will vary from sensor to sensor, but green is the predominant color with red and blue being considerably lower, I’ve used some made up values to illustrate what is going on, these are not the true values, but should illustrate the point.

In the illustration above when the blue pixel see’s 10%, green see 70% and red 20%, after processing the output would be white. If the light falling on the sensor is on axis, ie coming directly, straight through the lens then everything is fine.

But if somehow the light falls on the sensor off axis at an oblique angle then it is possible that the light that passes through the blue filter may fall on the green pixel, or the light from the green filter may fall on the red pixel etc. So instead of nice white light the sensor pixels would think they are seeing light with an unusually high red and blue component. If you viewed the image pixel for pixel it would have very bright red pixels, bright blue pixels and dark green pixels. When combined together instead of white you would get Pink or Blue. This is the kind of pattern that can result in the grid type artefact seen on many CMOS bayer sensors when there are problems with off axis light.

This is a very rare problem and only occurs in certain circumstances. But when it does occur it can spoil an otherwise good shot. It happens more with full frame lenses than with lenses designed for super 35mm or APSC and wide angles tend to be the biggest offenders as their wide Field of View  (FoV) allows light to enter the optical path at acute angles. It’s a problem with DSLR lenses designed for large 4:3 shaped sensors rather than the various wide screen format that we shoot video in today. All that extra light above and below the desired widescreen frame, if it isn’t prevented from entering the lens has to go somewhere. Unfortunately once it enters the cameras optical path it can be reflected off things like the very edge of the optical low pass filter, the ND filters or the face of the sensor itself.

The cure is very simple and should be standard practice anyway. Use a sun shade, matte box or other flag to prevent light from out of the frame entering the lens. This will prevent this problem from happening and it will also reduce flare and maximise contrast. Those expensive matte boxes that we all like to dress up our cameras with really can help when used and adjusted correctly.

I have found that adding a simple mask in front of the lens or using a matte box such as any of the Vocas matte boxes with eyebrows will eliminate the issue. Many matte boxes will have the ability to be fitted with a 16:9 or 2.40:1 mask ( also know as Mattes hence the name Matte Box) ahead of the filter trays. It’s one of the key reason why Matte Boxes were developed.

IMG_1022 More info on CMOS sensor grid artefacts.
Note the clamp inside the hood for holding a mask in front of the filters on this Vocas MB216 Matte Box. Not also how the Matte Box’s aperture is 16:9 rather than square to help cut out of frame light.

SMB-1_mpa_04-1024x576 More info on CMOS sensor grid artefacts.
Arri Matte Box with Matte selection.

You should also try to make sure the size of the matte box you use is appropriate to the FOV of the lenses that you are using. An excessively large Matte Box isn’t going to cut as much light as a correctly sized one.  I made a number of screw on masks for my lenses by taking a clear glass or UV filter and adding a couple of strips of black electrical tape to the rear of the filter to produce a mask for the top and bottom of the lens. With zoom lenses if you make this mask such that it can’t be seen in the shot at the wide end the mask is effective throughout the entire zoom range.

f5-f55-mask More info on CMOS sensor grid artefacts.

Many cinema lenses include a mask for 17:9 or a similar wide screen aperture inside the lens.