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Denoising with darktable 2.6

First part deals with easy cases and uses image shot with ISO 6400 as an example: 00:00 - Introduction 01:04 - Zoom to 100% when working on denoising your images. 01:57 - Using new presets for denoise (profiled) in Darktable 2.6, example image ISO 6400. 02:40 - Bug/hack (?) - disable highlight reconstruction module to help denoise module work on colour channels based on red/green/blue multipliers in white balance. Using color reconstruction is fine and won't interfere here. This behaviour is likely to get fixed in future versions. 05:07 - Manipulating new wavelet decompose curve in denoise (profiled) module. For more information see also here. 07:06 - Use equalizer to boost local contrast a little bit. 07:41 - Changing opacity to control level of denoising. Second example photo shot at ISO 25600: 08:14 - Second example image: ISO 25600. Disable color reconstruction, then use two instances of denoise (profiled) and equalizer to boost local contrast. 09:12 - Use equalizer to further reduce chroma noise. Third example photo shot at ISO 51200 and a stop of underexposure, more advanced techniques: 09:43 - Third example image: ISO 51200 plus one stop underexposure. More advanced techniques will be described. 10:13 - Using channel mixer with destination gray and setting red, green, and blue multipliers one by one to visualise noise in each colour channel. 10:59 - Denoising channel by channel: Set channel mixer with destination gray and the most noisy channel. Then adjust denoise (profiled) in wavelets mode for that colour channel. 12:52 - Adding second instance of denoise (profiled) to further reduce chrominance noise. Alternatively use equalizer to do the same. Optionally also dull edges in equalizer. 13:40 - Another method: Keeping just the first instance of denoise (profiled) (which was setup channel by channel). Then set input color profile to linear Rec709 RGB, compensate by increasing saturation in contrast brightness saturation module. 14:41 - Optionally use RGB curves in raw denoise module. 14:45 - Use hot pixels module, set threshold to 0 and strength to 1, could also use detect by 3 neighbors. 15:25 - Explanation of slider search radius in non-local means mode of denoise (profiled) module. Good for coarse grain noise, the higher the radius the better but it increases computation complexity quickly (it is function of radius square). Consider this an emergency use only. Using denoise (bilateral filter) module and more advanced techniques: 17:38 - One last method with denoise (bilateral filter) module. This is demonstrated from the starting point of the third image file, with only exposure increase applied. Using the same trick of looking at separate colour channels with help of channel mixer. Starting point for channel multipliers in denoise (bilateral filter): 0.5 for the most noisy channel and 0.3 for the other channels. 18:33 - Not working too well on this image. How to fix it: Remove one stop exposure compensation, remove base curve, and use tone curve to achieve similar tonality. That comes after denoising in pixel pipeline so the denoising, which is a useful trick. 19:20 - Re-enable denoise (bilateral filter), increase its radius to deal with coarse grain noise. Still on first (most noisy, here blue) channel. 19:52 - Do the same process for green and red channels. 20:33 - Deal with remaining colour noise using equalizer. 20:55 - Fine-tuning radius in denoise (bilateral filter) for separate channels by using a second instance and blend mode RGB blue channel. 22:27 - Why we should use equalizer to remove remaining chroma noise and not denoise (profiled). This has to do with the latter introducing interaction among colour channels and preventing denoise (bilateral filter) from working on isolated channels. 23:19 - Closing note on denoise (bilateral filter) use, especially suitable for high noise images, and when one colour channel is much more noisy than the other two. 23:40 - Last two tips: Use colour smoothing in demosaic module. 24:03 - Use lowpass module to deal with bright pixels in dark areas. Blend mode darken, radius 1, brightness 0.50. 25:05 - Similarly use lowpass module to deal with dark pixels in light areas. Blend mode lighten, radius 2, brightness -0.20. 25:37 - Optionally remove sharpening or lower its opacity. Note that lowpass comes after sharpening in pixel pipeline. 26:12 - Things to remember: Use 100% zoom level, use channel mixer to look at each colour channel separately. Use provided presets for denoise (profiled) if your photos are not too noisy. These timings and descriptions were kindly provided by Tomas Sobek (see https://tomassobekphotography.co.nz/a...)