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Film grain or noise?
Before the digital era, one the main source of image noise was from film grain. It is a random texture on film due to small metallic silver grains after exposure. During the digital transition, CRT had certain characteristics that could reduce digital image noise. With the advent of new digital displays, image noise is now becoming a problem again, for the following reasons:
1) Brighter displays enhancing pixel fluctuation.
2) Increased contrast ratio of image.
3) Box distribution of light output makes pixel structure more noticeable.
4) Poor digital source is surprisingly too common.
5) Poor signal processing in HDTVs.
Image noise on digital displays is usually manifested as fluctuation of pixel in an image. Image noise is most apparent in image regions with low light, such as shadow regions or underexposed images.There are many sources of digital noise, listed below.
- image sensor noise
- poor analogue reception
- analog to digital conversion, or vice versa
- digital processing, editing
- pulse width modulation artifacts in plasma TVs - `shimmering pixels'
- dithering to reduce posterization
Some of the noise reduction strategies are briefly listed below. Remember all noise reduction causes loss of detail from the original image; it is an unavoidable trade-off.
- Spatial whole frame filter - A blur filter that removes fine detail throughout the frame. Not useful if applied excessively.
- Temporal filter - A better filter. It studies image noise over several frames and applies the filter accordingly. Good for static scenes, but causes motion smearing with moving objects.
- Region based, motion adaptive temporal filter - Images are broken up into segments. Less filtering is applied on moving pixels so a greater amount of detail is maintained.
- Per-pixel motion adaptive temporal filter - Ultimate processing in noise reduction. Generally available in high end external video processors only.
Let's hope that as technology becomes cheaper, we will get some of the latest noise processing incorporated into HDTV.
|Lyris says:||03/12/2007 - 14:27|
See this article: http://www.lyris-lite.net/2007/03/film_grain_is_not_your_enemy.html
|Colin Tang says:||03/12/2007 - 21:18|
Noise is inevitable. Even when some films were shot digitally (film grain is traditionally analog), the post-processing and encoding of DVDs created noise similar to film grain. Some DVD authors have used digital processing to imitate `film grain' in DVDs to give a theatrical effect.
While I never switch on noise reduction as a general rule, I have been tempted occasionally to do this with a few SD broadcast programmes. The `box' point spread function of digital displays accentuates pixel fluctuation and make this noise more visible than CRT; adding a touch of blur can help with this.
|Tony Koorlander says:||03/18/2007 - 03:42|
|Jeff Allen says:||04/05/2007 - 19:18|
The bigger public issue on digital broadcast is not always picture quality but sound with increasing processing of the picture lip-sync error is back again and a variable problem and one the TV manufacturers and MPEG need to resolve.