How to select high/low contrast areas?

Forum|Forum|1 year ago
January 25, 2025
2 replies
429 views

I would like to apply a blur effect to low contrast areas of the image, but not to high contrast areas (edges) of the image? How to go about doing this?

I have been playing around with "Find Edges" but don't know how to go about this.

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Rick Gerard

Community Expert

Without seeing a screenshot of the footage you want to modify, Warren Heaton's suggestion is the best, fairly easy solution to the design problem you are trying to solve.

Your footage may require a pre-composed and severely color-corrected copy of the footage to give you more control over the edges.

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paul_7484Author

Known Participant

Okay, yeah. This is not my footage but I'm posting a black and white to simplify: I would like to keep the hard edges sharp, while softening low contrast areas (skin tones, fabrics, material surfaces etc.)

I see what you're getting at in the first reply, but I am not trying to soften a particular range of luminence. When I try that approach, it softens the edges where the edge is a white against a black, for example. That low-contrast area that I want to blur (let's say, the coat, the face) has an edge against the background that I don't want to blur. Maybe I was not clear on the approach.

I think I need an expression that selects pixels by proximate high contrast areas. I can sort of see Find Edges doing that, but I don't know how to translate that into a usable matte layer that I can keep sharp over top of a blurred layer. With Find Edges with a "Multiply" blend on top and the blurred one on bottom I am sort of getting there, but there is no control over how it defines the edges.. so in color it's not quite usable.

From researching it seems this is not what IT guys call a "solved problem" although maybe some plugin does it.

Warren Heaton

Community Expert

You should be able to do this with Compound Blur. This uses the luminance of a Blur Layer to determine where the blur happens. White gets the Maximum Blur value and that falls off as the luminance changes from white to black. There's an Invert Blur option to reverse this.

Applying Compound Blur directly may not be enough. Sometimes, I will duplicate an image, apply a combination of Black & White, Gaussian Blur, and Levels to better define the areas to be blurred (white) and not blurred (black) and then use that as the Blur Layer with Compound Blur applied to the original.

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paul_7484Author

Known Participant

Thank you for the suggestion. I'm struggling to get the right effect. I should say it's a normal photographic image, not a text block or something. I want to keep the edges sharp, while softening the bits inside the lines.

I'm seeking a way to select pixels by contrast with some threshold, so that I can build a matte and choose which to apply the blur to. Like, if difference between adjacent pixels in luminence is >X, then select both of them.

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