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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
  • encoding
  • compression
  • 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.


Questions/Comments

Lyris says: 03/12/2007 - 14:27

Often the best thing to do with film grain is leave it alone - why remove it if it's part of the director's intention?

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

The artistic license to manipulate film grain or noise lies entirely in the director's hands. No one should alter production values to suit their individual taste on this aspect, otherwise there is no freedom in expression.

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

As an early HD video shooting nutter.. I recognise that one of the most significant contributions to the noticiability of noise is the slower response time of the present displays compared to CRT. This essentially lowers the frequency rate of displaying noise and makes it more objectionable for viewing. WORST on TFT screens with less than a 6mSec grey to grey response. Put the same HD video onto something faster and the noise is less noticeable. The issue of film grain is completely different... however, you should be aware that the newer HD video cameras suffer from a lot more source noise than older SD video cameras... this is due to the tiny size of the sensor area that is receiving light for conversion to signal. With more than 4 x the number of pixels on the same size area.. the light input is a fraction of what it would be on a standard definition camera... this passes through the edit and post process into the TV domain .. where it is further exacerbated by the artifacts of mpg compression. AKA .. garbage in - garbage plus out.
Jeff Allen says: 04/05/2007 - 19:18

The BBC recently announced it will ban 16mm film from 2010 for all production due to film grain for HD broadcast. The tests they conducted used MPEG 4 AVC / H.264 which supposedly gives a better picture from greater compression to anable less bandwidth for HD broadcaston scarce spectrum (well we did all want endless channels of dross Tv!)The fact is and contary to some of the articles or past solutions Kodak and others have worked on tools to reduce grain whilst not degrading edge sharpness to allow larger compression. The BBC and others like the emperor new cloths of MPEG 4 but are blind to its arefacts and concantination issues in public.
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.