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Pixel Burnout


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#1 David Regan

David Regan
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Posted 13 May 2012 - 10:40 AM

So the other day noticed this strange issue while shooting straight into the sun with the Alexa. Looks like the sensor overloaded and introduced some strange electronic noise/color, right in the hot spot of the sun. Shooting with Super Baltars, we were at a 4/5.6, with some ND in front, but this doesn't look optical at all. The aberration would move as the camera did, depending on the angle into the sun, this small digital noise would move around, but always right within the hot spot. Haven't noticed it since or been able to reproduce it. Any thoughts about what could cause this?

Thanks!

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#2 Phil Rhodes

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Posted 13 May 2012 - 11:26 AM

I suppose you could call this a form of solarization. A few digital cameras have been shown to do similar things, including Viper and, most notoriously, Red.

Sometimes it may be the result of a digital count overflowing, where most microelectronics will reset the count to zero on an attempt to count over the maximum, which tends to produce black or very nearly black dots. This is a bug and should be fixed in a professional-use product like Alexa, even if it required a hardware mod.

More critically, some sensors intrinsically do this, either in the digital electronics, glitches in the output amplifiers, or direct photovoltaic effects. Similar things happen to long-wave infra-red cameras such as those used on military aircraft targeting systems - when the bomb hits the target, the (very hot) explosion often reverses out, hot reading as cold, although the modern ones seem to be less prone to this. This can be more difficult to fix, even if you were willing to replace all the sensors.

An anti-ringing application was produced for Viper footage which would analyse a DPX stack and look for high contrast excursions in the image data, assisted by the knowledge that Viper did this in the pixel immediately to the right of the overloaded pixel. It wasn't perfect but it was good enough and largely made the problem an irrelevance. Bayer-patterned devices like Alexa complicate the issue and make an automated fix difficult because the demosaic process will intrinsically smudge the artefact across several output pixels and may create odd colouration depending on the assumptions made by its demosaic engine; it would be easier to fix in a raw file.

That said the actual damage here is minimal as the depicted problem is trivially easy to fix, much as I know that's not really the answer you wanted.

As to whether the camera really should do this or whether it's a bug, well, give it to Arri and see what they say.
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