How can I sample colored areas of a certain size or greater?

Forum|Forum|6 years ago
February 26, 2020
3 replies
1011 views

I am working with the Next Generation Blue Marble artwork from NASA and trying to create a mask of all of the areas covered by water. The problem I have been facing is two-fold: Using the Select Range feature tends to also select lushly green land areas. Pre-posterizing or applying threshold to a copy of the image for selection ends up with thousands of little tiny isolated pixels or pixel groups.

What I want to do is make the water look metallic. I'm rendering the image in Cinema 4D and then using Photoshop to add some finishing touches. I am also using Photoshop to generate my textures using my modified version of the NASA images as sources.

Every method I have tried so far selects all of the lakes and rivers in every landmass--I just want the largest lakes and the oceans but I want it to be tight.

Is there any way to select all of the water areas that are larger than 15 pixels in area? Any other way to minimize all of the little fragments that I don't need. The source image is 21600px wide and I have been trying to do it manually, but there has to be a better way. Any suggestions?

How to

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mglush

Community Expert

Hi! It would be great to see a sample of the image -- could you post a screen shot? One idea is to use the Magic Wand tool especially because you can make a selection by color AND you can choose whether you want the area to be Contiguous or Non-Contiguous. If you select Contiguous, it will only select the area that is touching what you have selected and not the other stray and smaller bodies of water. You can then refine the edges by running Levels on the mask to clean up and adjust your selection.

The Magic Wand tool may be one of the original tools but it still has a lot of tricks up its sleeve!

Michelle

Conrad_C

Community Expert

Have you tried using Channel Mixer to alter the color-to-tone relationships, so that the water areas are easier to isolate via thresholds? In the example below, Channel Mixer is used to drop out the foliage and return a grayscale result, and then two Curves layers are used to force apart the resulting land and water tones. It’s a quick and dirty example, and I’m not sure I downloaded the same image you’re using, but hopefully it gives you enough ideas to reduce the amount of work you have to do.

Chuck Uebele

Community Expert

That's going to require hand work. I would try to make a rough selections of the areas you want, then use color selection to refine that area. Using LAB color might help you select just the blues easier, as it's not luminosity dependent.

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