Iain’s Noise Reduction for G’MIC and GIMP

With noise reduction software it is a case of picking your poison. Well I hope this poison at least offers a pleasant taste!

Iain’s Noise Reduction is a filter for the G’MIC plug-in for GIMP 2.8. It is currently found in the Testing->Iain Fergusson folder.

Image

Control descriptions

Luma NR

This adjusts the amount of luma noise reduction. If the control is set to 0 this step is skipped and processing will be faster.

Chroma NR

This adjusts the amount of colour noise reduction. If the control is set to 0 this step is skipped and processing will be faster.

Despeckle

This reduces the amount ‘salt-and-pepper’ noise. It is performed just before the luma noise reduction, so adjustments made here will affect the luma noise reduction.

If the luma noise reduction is set to 0, the despeckle step is also skipped.

When set to level 4, the main luma noise reduction is adjusted to skip removal of fine noise because it should have been taken care of by the despeckle algorithm. This should speed up the denoising.

Adjust noise reduction by tone

This allows you to adjust the amount of noise reduction in the shadows and highlights.

Recover Details

The detail recovery step works with the information removed by the luma noise reduction and recovers details that are stronger than a certain threshold. There are three options recovering details:

  • Do not Recover Details
  • Recover Details
  • Recover Details as separate layer – this will output the recovered details to a separate layer that can be combined with other noise reduction layer using ‘grain merge’ blending mode in GIMP will produce the final image. This option allows the user to apply masks and further processing to the details in GIMP.

Recovering details is the slowest part of this filter, you will have to have patience.

The recovery amount slider sets the overall recovery amount. At low values only the strongest details are recovered. As the slider is raised more subtle details are revealed, but there is a risk that the recovery algorithm will see false details in noisy areas. Setting the slider to 10 will recover all details including noise.

Detail recovery before

Noisy image before processing.

Detail recovery good and bad

Details on the red feather are recovered well in the lower right corner, but some false details appear in the shadow area of the feather.

Adjust detail recovery by size

These controls allow the user to adjust the detail recovery based on the size of the details. This is useful to control the appearance of false details

Detail Emphasis

This allows the user to increase the strength of the details so that they are more obvious. This often allows the user to use a lower recovery threshold and reduce false details.

Other considerations

Although the detail recovery is quite good, it can produce artefacts. False details has already been mentioned, another artefacts is detail bleed. Sometimes you will see that recovered pattern actually extend past the boundaries of the original pattern and ‘bleeds’ into other areas. This is because the detail recovery looks for patterns in blocks, and if a pattern is found, with certain settings, the whole block is filled with the pattern, even if the pattern did not fill the entire block originally.

Another artefact is ‘ringing’. If the original luma noise reduction softens strong edges too much the detail recovery algorithm can produce parallel lines running along side the edge.

Image

Noisy image.

Ringing is visible as parallel lines running beside the edges.

Ringing is visible as parallel lines running beside the edges.

Sharpen edges

This sharpens the luma channel, but does not sharpen the details.

Preview Shows

  • Normal Output
  • Luma Noise – shows an amplified view of what is being remove from the luma channel
  • Chroma Noise – shows an amplified view of what is being removed from the chroma channel
  • Details – shows an amplified view of what is being recovered in the detail recovery step.

Notes on using the filter

Successful detail recovery is a balance of the luma noise reduction setting (including despeckle) and the detail recovery settings. The detail recovery section works with the information removed in the luma noise reduction section, so adjusting the luma noise reduction will affect the amount of details recovered. Generally the luma noise reduction should be set to slightly over-smooth the image so that the detail recovery section has enough information to detect the details. Small adjustments of the luma noise reduction amount can improve the detail recovery, so a bit of fine tuning is required for the best results.

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