What is important is that both articles place denoise phase at the same place in the workflow. In some workflows 3 and 4 can swap places. Let's quickly "run thru" stages of data reduction and processing: Here it advises you to do denoise at linear stage.
My personal noise reduction routine of choice for the images we are about to post-process was using MultiscaleLinearTransform (as described in the tutorial on noise reduction) with stretched clone copies of the images themselves acting as masks. Please note that this is beyond the scope of this tutorial and is covered amply by another tutorial specially written on the subject of noise reduction. The choice of whether or not to do this, or how aggressively to do it, depends on the level of noise in your images. "Though your images may differ, it is common to apply some noise reduction to images in their linear state. Here is quote from LV article that you just linked (I just did quick search on term noise and this is pretty much where it appears in text): I think you are confused with ambiguous usage of term "combine". The LV tutorial ( ) advises the opposite - perform denoising before combining. Combined images must be equally exposed, have the same pixel resolution, and be registered by projective transformation with no distortion correction." Signal-to-noise ratio (SNR) will be enhanced by combining noisy images and denoising the result.
The PI forum article (the link I've sent) says in Warning (Note) 2 : " Do not combine denoised images. Non linear transform skews relationship between signal strength and associated noise in Poisson distribution. It also says that during registering and combining you need to be careful not to do any sort of projective corrections (like fixing lens distortion in wide field images and such).īecause this algorithm utilizes noise distribution statistics - it needs linear data, so you should apply it after stacking and before any sort of non linear transform (histogram stretch). Refers to exactly what I mentioned - stacking of denoised images vs denoising stack and not channel combine. It assumes image is combination of Poisson and Gaussian noise distributions and does some clever math to remove associated noise. From what I can see in first post of that topic you linked to, MURE denoise works as first type of algorithm that I described.