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The goal of an edge detection algorithm is identifying pixels that belong to an edge of an object in an image ...

The rest of the sentence should say something along the lines of "and point them out by ..." - but "point out" seems too informal.

Pointing out is also the wrong phrase because the pixels don't need to actually be "displayed" to some user at all, and most likely will just be used as information for some next step.

So the word/phrase I am looking for should not imply something visual, but informative.

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You could also consider the term "marked" (or graded). First, the pixels are graded as to their edginess, then a heuristic selects candidates to be part of a possible boundary. For you see, you really don't know if a specific pixel is an "edge" (which flagging does, since it is binary: the flag is on or off), until it is successfully part of a boundary--and determining boundaries usually comes after the pixels have been given a relative weight as to their being an edge (gradient and directionality for that area). There are canned Fourier transforms that do this, but you're probably in school. – F.E. Apr 12 '14 at 17:36
@F.E. yeah "marked" would probably be fine as well, but I like "flagged" more. FYI I'm using the Sobel method for edge detection (blog post). – jcora Apr 12 '14 at 22:50
up vote 2 down vote accepted

Flag might be useful. It's a bit of an 'old skool' programming term but seems appropriate.

The goal of an edge detection algorithm is identifying pixels that belong to an edge of an object in an image and flagging them with ...

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I don't know how I managed to miss that. Thanks! – jcora Apr 12 '14 at 16:26

How about 'and identifying them by...'?

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Well it wouldn't really be correct, semantically speaking, because the algorithm does not identify the pixels by coloring them black or white, it colors them black or white after they have been identified by some method (this is what the actual algorithm is for, the coloring part is just visualization.) – jcora Apr 12 '14 at 22:47
@akled 'Identify' has different meanings. I could 'identify' you as a mad scientist. I would do that by listening and speaking to you. It would be the result of a discovery on my part. That is one meaning of identify. However, if I were a clerk in Orwell's Ministry of Peace, I could 'identify' all those members of the armed forces whose political views were already deemed suspect by placing a marker on their files. In this case I am not discovering something but recording something. This latter is the 'identify' I think you need. – WS2 Apr 13 '14 at 8:52

What about emphasise or underline meaning:

Give special importance or prominence to (something) in speaking or writing:

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