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| paulus |
| Posted: Aug 11 2012, 11:16 PM |
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Newbie
Group: Members
Posts: 2
Member No.: 35303
Joined: 11-August 12
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Hello,
I have several videos shot in low-light conditions which show severe chroma noise. The phenomenon is typically in the blue-yellow 'channel' of the picture. It exhibits itself as large irregular spots of differing color, whereas the luminance info is more or less OK. I've found a page where someone experiences the same problem in a photograph, and shows the effect and a method to significantly reduce it at http://www.myfinepix.com/article/428/473144 .
The method there used is to transform to Lab colour space, then low-pass filter the 'b' channel, and transform back to the original colour space.
Do any of you know of any existing VirtualDub filter that can do this, or alternatively, a set of filters that can be used to compose this behaviour? In the latter case, one filter could transform to Lab space, one filter does a low-pass on the 'b' channel only, and a final filter transforms from Lab space back to some generic colour space.
If this does not exist, do you have any hints on how to implement this with the filter SDK? |
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| phaeron |
| Posted: Aug 18 2012, 08:43 PM |
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Virtualdub Developer
Group: Administrator
Posts: 7773
Member No.: 61
Joined: 30-July 02
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Lab color space may be overkill for this, as based on the way that video recording works it may be appropriate just to use YCbCr and filter in Cb (chroma blue). I wonder what phenomenon in the camera is responsible for this noise.
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| levicki |
| Posted: Oct 6 2012, 06:14 PM |
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Advanced Member
Group: Members
Posts: 167
Member No.: 22605
Joined: 13-December 07
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You can try this plugin that I wrote:
http://levicki.net/tmp/ChromaEnhancer.rar
It is an experimental alpha, so please do not distribute further, and do not rely on its output too much.
Current version is 32-bit only, and requires minimum SSE2 to run, but it will also use AVX and multiple cores if present.
To install, unpack files from ChromaEnhancer.rar to VirtualDub folder. If you did it right, you will end up with ChromaEnhancer.vdf in plugins folder and libiomp5md.dll in VirtualDub folder.
Once you load it, you will have the option to select search radius for chroma (U/V) reconstruction. Note that larger radius is more CPU intensive and also leads to smearing of colors on color transitions.
Try it out and let me know what you think, you can also post some screenshots of chroma channels before and after if you have time.
@phaeron:
Cameras have the worst sensitivity in the blue channel. To compensate for this, sensitivity is boosted by applying bias voltage to the sensor. Unfortunately, this introduces more noise because increase in voltage also increases the sensor temperature which creates thermal noise its diodes. Since this is well known problem with all but the most expensive cameras producers often make a blue background or add a lot of blue light to the stage thus adding natural bias and dragging the sensor out of "dark noise + blotches" area of operation.
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| paulus |
| Posted: Dec 3 2012, 06:17 PM |
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Newbie
Group: Members
Posts: 2
Member No.: 35303
Joined: 11-August 12
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Hi Levicki,
I tried to download the filter, but unfortunately the link is broken. It sounds promising though!
About the blue channel noise; didn't know that extra blue light is normally used to offset the noise. About the offset voltage and thermal noise; could be that it works that way, but in general, any amplification of a low strength signal, will also amply the noise, hence worsen the SNR. Some extra info on noise in the blue channel of a CMOS sensor can be found here : http://photo.stackexchange.com/questions/1...el-the-noisiest |
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| levicki |
| Posted: Dec 12 2012, 07:46 PM |
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Advanced Member
Group: Members
Posts: 167
Member No.: 22605
Joined: 13-December 07
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| QUOTE (paulus @ Dec 3 2012, 07:17 PM) | Hi Levicki,
I tried to download the filter, but unfortunately the link is broken. It sounds promising though!
About the blue channel noise; didn't know that extra blue light is normally used to offset the noise. About the offset voltage and thermal noise; could be that it works that way, but in general, any amplification of a low strength signal, will also amply the noise, hence worsen the SNR. Some extra info on noise in the blue channel of a CMOS sensor can be found here : http://photo.stackexchange.com/questions/1...el-the-noisiest |
Hi.
Since there was no feedback for a long time I have removed the file, it is an alpha version and I don't want it mirrored.
Wikipedia has this to say:
| QUOTE | | The standard model of amplifier noise is additive, Gaussian, independent at each pixel and independent of the signal intensity, caused primarily by Johnson–Nyquist noise (thermal noise), including that which comes from the reset noise of capacitors ("kTC noise"). Amplifier noise is a major part of the "read noise" of an image sensor, that is, of the constant noise level in dark areas of the image. In color cameras where more amplification is used in the blue color channel than in the green or red channel, there can be more noise in the blue channel. |
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