Blending stacked layers

[jrickards]

In this video (https://www.youtube.com/watch?v=E-GbuXU8YMQ), Tony uses image stacking to improve the final image as a result of the combining. Although he uses Photoshop, a similar technique can be used in GIMP.

I have two questions as a result of this video.

(1) In GIMP, to "blend" images, I use G'MIC-Qt. Amongst other options, there are Median and Mean. Mathematically, I know the difference between the two but in terms of the use of those terms in this filter, what are the differences between the two and when might it be better to use one over the other?

(2) When stacking the images in layers in the video referenced above, prior to applying the blending,Tony uses the following procedure (that I will simplify). The opacity of each layer is different and are set to 1/(layer position from the bottom). This means that the bottom layer is 1/1=100%, the second layer from the bottom is 1/2=50%, the third layer from the bottom is 1/3=33%, the fourth layer from the bottom is 1/4=25%, etc. Is this a valid approach? Are there other approaches?

[Ofnuts]

1) The "mean" filter is more a way to find the most frequent value. If you have 5 layers, on a pixel position where two layers are black and three are white, the result is white, while the average would be some gray. It is typically used to find common pixels among layers. For example, take several shots of the same monument with tourists walking by, the median filter will produce an unmasked monument, while the average filter will just blur the tourists. On the other hand the average filter is appropriate to remove noise, if you consider than noise is a random difference with the "true" value, centered around that "true" value.

2) That rule is mathematically sound and there is no other approach (except an average filter). In practice with Gimp 2.8 you get round-off errors and you cannot apply that rule to more than 5-6 layers (above that dark values are rounded off to 0 and don't appear in the result), and the average filter is a better solution.

[Espermaschine]

Where in G'MIC is the Mean Filter ? I can only find Blend [median].

[Ofnuts]

(04-03-2018, 05:03 AM)Espermaschine Wrote: Where in G'MIC is the Mean Filter ? I can only find Blend [median].

Layers>Blend [average all]

[jrickards]

Thank you, this is great!!