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Ugly moire in A720IS video

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Offline wap4

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Re: Ugly moire in A720IS video
« Reply #10 on: 17 / April / 2008, 09:24:02 »
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Quote from: vit40 on 17 / April / 2008, 07:54:38
but on the other side, it's rarely a problem, unless you shooting mostly trains, fences and similar things with many horizontal or vertical lines

Unfortunately,a railfan like me will always try to shoot trains escpecially those alcos which are being rare in India. :(

I have the following words to say-

You can notice moire clearly when there are things with horizontal/vertical lines but its ill effects are always present,you may only
wrongly think it as sensor noise,in fact I came to this conclusion after observing many indoor video clips. Now I also have
one doubt,in still shots,the 'noise' always present even in good light & iso 80 is actually a moire pattern,which canon decides to keep
(not to sacrifice the resolution & thereby applying only a weak AA filter). Look once again at the tracks(2nd pic),there are not
many horizontal/vertical line here & blurred orange-violet moire pattern has appeared which may seem like noise at the first sight.   
« Last Edit: 17 / April / 2008, 09:32:29 by wap4 »
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Offline vit40

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Re: Ugly moire in A720IS video
« Reply #11 on: 17 / April / 2008, 09:38:41 »
Well, moire can happen when pattern of the object is aproximatelly aligned with pixels. Horizontal and vertical lines (or near) are the most common source of problem, although it can be anything, so if you watch closely, you can see some traces of moire on various parts of the picture

There is also a "low frequency" color noise from the sensor, that is more visible when ISO (or gain in video terminology) is high, and looks somewhat similar to moire - a group of several pixels slightly toned green, next groupu slightly red etc .... If you take a movie of some stationary object, you can se what is moire - it looks the same on succesive frames, and what is LF color noise - that is different on every frame
« Last Edit: 17 / April / 2008, 09:40:29 by vit40 »
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Offline Barney Fife

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Re: Ugly moire in A720IS video
« Reply #12 on: 17 / April / 2008, 16:11:55 »
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« Last Edit: 22 / April / 2008, 18:02:08 by Barney Fife »
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[acseven/admin commented out: please refrain from more direct offensive language to any user. FW complaints to me] I felt it imperative to withdraw my TOTAL participation. Nobody has my permission, nor the right, to reinstate MY posts. Make-do with my quoted text in others' replies only. Bye
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Offline vit40

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Re: Ugly moire in A720IS video
« Reply #13 on: 17 / April / 2008, 17:10:49 »
Well, you are partially right. Modern camcorders with several megapixel sensors behave just as you wrote - all pixels are used (in case it has optical IS, otherwise only a part - that's how electronic stabilization works), they are binned to 720x576 (PAL) or 720x480 (NTSC) resolution and there's almost no color moire (unless it's a HD camcorde, where resolution of video is higher). As amount of pixel on the sensor grow, it's harder and harder to solve a problem with high data rate from CCD sensor, so manufacturers are switching to CMOS, where binnig is done in sensor and data rate remains low

But ... these cameras are using different CCD sensors. In video mode, only a part of pixels are in use and part of pixels not. For example, sensor in S3IS has so called 4/8 readout mode, meaning that 4 pixel rows from every group of 8 rows are used for video (= 1/2 of whole sensor surface - we don't have unwanted "digital zoom"). Pixels are binned in vertical direction in sensor, and then horizontally outside of the sensor (see pdf-s from Sony for more details), but because part of pixels are not in use, we still end with big RGBG pattern, where color moire is visible - but mostly on horizontal patterns, like horizontal elements on the train above

Because of pixel binning, we also have higher sensitivity in video than in photo mode and we can use 1/30 video exposure in relatively dark places. For A620, ISO 400 is maximum in photo mode, but I measured ISO up to about 1100 in video mode, while up to 287 is (wrongly) reported by CHDK, which obviously take amplifier gain into account, but not pixel binning in video mode
« Last Edit: 17 / April / 2008, 17:37:07 by vit40 »
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Offline wap4

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Re: Ugly moire in A720IS video
« Reply #14 on: 18 / April / 2008, 00:31:19 »
Thanks a lot vit40,for throwing some light on how the video mode works. Also,you rightly pointed out that moire seems to be
a problem only on horizontal direction,I have not observed any moire in vertical straight lines, although I'll verify this later
more carefully.

I've just found a pdf file regarding ICX-636/646 series which I believe A720 or S5 uses. (Anyone interested may download
from attachment below.) More such documents can be found at http://www.sony.net/Products/SC-HP/cx_news/new_pro.html  . 
As you see from it, 4/10 line readout is used for 30fps vga.

And moire patterns do appear in dimly lit indoors also(I've tested with bar codes :D) but that is much less pronunced.
Also,moire patterns are least visible if you're shooting green objects (not surprising,as half of the bayer pattern is green,we 've
much more information on this channel) So if we're shooting purely 'natural' objects,we don't have much problem.   
« Last Edit: 18 / April / 2008, 03:03:38 by wap4 »
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Offline vit40

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Re: Ugly moire in A720IS video
« Reply #15 on: 18 / April / 2008, 03:47:48 »
You can also visit   Riddle HQ - Digital Camera CCD development history
There are many pdf-s for various sensors. Some older ones cover video mode with more details

It's to be expected that there is less moire in low light, because noise actually helps reducing interference. Similar technique is used on audio CD-s. Did you ever notice "SBM" (super bit mapping) on audio CD-s ? A special shaped low intensity noise is added to audio signal when downsampling from "studio quality" to "CD quality", to kill interference in high frequencies and improve reproduction quality and dynamic range (these techniques are generally called dithering - frequently used in digital signal processing).
« Last Edit: 18 / April / 2008, 03:54:00 by vit40 »
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Offline Barney Fife

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Re: Ugly moire in A720IS video
« Reply #16 on: 18 / April / 2008, 10:22:24 »
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« Last Edit: 22 / April / 2008, 18:02:38 by Barney Fife »
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[acseven/admin commented out: please refrain from more direct offensive language to any user. FW complaints to me] I felt it imperative to withdraw my TOTAL participation. Nobody has my permission, nor the right, to reinstate MY posts. Make-do with my quoted text in others' replies only. Bye
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Offline intrnst

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Re: Ugly moire in A720IS video
« Reply #17 on: 18 / April / 2008, 14:36:27 »
Quote from: vit40 on 17 / April / 2008, 17:10:49
...
Modern camcorders with several megapixel sensors behave
...

Better late than never:
Yes you were right about light.
Yes you were right in your statement about lines in video, that I misunderstood with final video format.

Again, thanks for the free class, professor.
« Last Edit: 18 / April / 2008, 14:39:08 by intrnst »
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-- funny english, be aware -- CHDK for Dummies - The Very First Steps
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