what is a band pass mask really doing?

[jimh12345]

I want to understand band pass masking.  I know it somehow selects edges but that doesn't explain what's going on - I want to understand the concepts, not just play with sliders. 

The AP Help has text like "Low Band—sets the lower limit of the frequency band".  Um, thanks...

Yes, I watched Affinity's tutorial video.  It was an example of using it, but not an explanation. Unfortunately it left me confused. 

Can anyone point me to an explanation of the concept of band pass masking?

[debraspicher]

2 minutes ago, jimh12345 said:

I want to understand band pass masking.  I know it somehow selects edges but that doesn't explain what's going on - I want to understand the concepts, not just play with sliders. 

The AP Help has text like "Low Band—sets the lower limit of the frequency band".  Um, thanks for that...

Yes, I watched Affinity's tutorial video.  It was just an example of using it, not an explanation but a "just do this..." walkthrough that left me confused. 

Can anyone point me to an explanation of the concept of band pass masking. 

A topic for your consideration is Frequency Separation, which is a technique where we break an image into "high" and "low" and touch them up separately. That helps us to separate fine details from "larger area of details" such as shadows, tone and color. That would give you a better idea of what the frequencies mean in practical terms. So think separating the texture of a blemish (the highs) versus the color (the lows). It gives us the ability to maintain detail while working with different types of problems. There's controversy around the technique because it can easily be overdone, but band pass mask allows us to work within a specific range, for instance, applying a filter. (as you saw in the official video)

Fstoppers: https://fstoppers.com/post-production/ultimate-guide-frequency-separation-technique-8699

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High frequencies in an image will contain information about fine details, such as skin pores, hair, fine lines, skin imperfections (acne, scars, fine lines, etc.).

Low frequencies are the image data that contains information about volume, tone and color transitions. In other words: shadows and light areas, colors and tones. If you look at only the low frequency information of an image, you might be able to recognize the image, but it will not hold any precise detail.

AILearner: https://theailearner.com/2019/05/05/understanding-frequency-in-images/
 

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There are many videos on the topic, so I am just going to attach a shorter one. It's just for introduction:

 

 

[jimh12345]

debraspicher,  thanks, some good stuff there.

I think the problem for me is that I want a way to separate edges from dust specks.  Maybe that's not really possible since they're both high frequency.

[debraspicher]

2 hours ago, jimh12345 said:

debraspicher,  thanks, some good stuff there.

I think the problem for me is that I want a way to separate edges from dust specks.  Maybe that's not really possible since they're both high frequency.

I remember that thread. I couldn't find a "best" image to test with, but it does seem like it requires manually masking the mask itself and retouching out those areas so that other areas can still be managed. Fun times.