Degrain Noise Reduction

vfxwiki  >  Post Techniques  >  Image Processing  >  Correcting Fottage  >  Degrain Noise Reduction
Page's that link from here:

Intro

  • If noise in the plate is creating a chattering key, degrain can help. This should be done by subtly adjusting each channel individually.
  • This is usually done to the plate before keying, although can also be done on=top mattes.
  • A de-noised plate is only used to pull a key, and not for the RGB final comp.
  • The key here is to examine the images different colour channels in different spaces to see where the grain is residing.

Methods

Averaging frames

  • Plate has to be stabilized.
  • Cannot be used on objects animating in frame.
  • Averaging frames is a very efficient method
  • In shake in the FileIn - in the Timing tab, set reTiming to Speed, Speed to 1, Method to Blend, and turn up the blendRange to average over more frames. You can't do it in the middle of a tree, you'll have to render out what you want to average and bring it back in.

Rank Order or Median Filter

Examines neighboring pixes and averages them, good for removing extreme dark or light grain. Expensive.
Edge Filter Blur
  • Take image and apply a Sobel edge detection filter. Then apply output of filter as a selective luminace matte with a blur on the clean image. Then sharpen if necessary.
  • For more control apply a Sobel on each individual RGB channel (although some people say you only need the red and green) and then have separate blurs using each sobel output selection using that colour channel rather than just the luminance to do the selective blur.
  • In addition to this method you can try or combine the outputs of the above method but using a colour space that separates luma & chroma i.e. YUV. and doing it in log with a colour correction. This method is very good at retaining detail.
  • The limitation of this is that the edges don't get degrained, but this can also be used as an advantage.

simple example

Image
  • the above example uses a side mask but keymixing each channel is advisable.
  • Use a blur, but limit the area via a piped in mask
  • Create this mask with an inverted edge detect/sobel filter. This finds the areas of detail and then limits the blur to not effect theses areas.
  • To increase the control separate blurs to serperate channels in different colour spaces. Commonly different channels (often blue) have more grain, also by separating the channels into lumminance and chroma detail can be maitained by not bluring the luminance.
  • Complexity can then be further increased by RGB edges in the edgeDetect and using separate channels for the mask.

expanded example

  • Doing this in a colour corrected log & YUV colour-space is very effective.
Image

Adaptive spacial filtration

This is a often a FFT (wavelet transform) high noise pass.
  • 'Furnace plugin Degrain'' - Furnace plugin unstable, costly and limited licences so its recommended to render out as pre-comp.
  • 'Neat Video' is one of the best, but has a limited tool and file formats

Temporal filtration

  • Furnace plugin Denoise (Combining a Denoise and then a Furnace Degrain can be effective) - Furnace unstable, costly so its recommended to render it out.
  • A simple temporal median filter can be done using IF a < min(a,c) THEN b = min(a,c) IF b > max(a,c) THEN b = max(a,c). This would need the threshold of subtraction reduced to only reduce grain.

This is a very simple example and not production ready. Image

Chroma only Blur

Perform the blur to the Hue and Saturation only (in HLS colour space). good for ringing edges and digital footage. see Keys and Compression

Notes and Tips

Sharpening A sharpen can be added after a blur but care must be taken to avoid ringing artefacts Matte noise suppression Use sparingly as it destroys detail. If a keying method that produces a low-contrast matte is used. (i.e. like colour difference) suppress the noise (usually a blur or median) before the matte is scaled so it has clean black and white values.

Links

External Links


Created by ome. Last Modification: Thursday 22 of April, 2010 11:20:55 GMT by ome.
List of attached files
ID Name desc uploaded Size Downloads Actions
79 default degrain.flw FUSION4 Thu 26 of Jan., 2006 16:22 GMT by ome 7.77 Kb 1244 View Download