What's new
Photoshop Gurus Forum

Welcome to Photoshop Gurus forum. Register a free account today to become a member! It's completely free. Once signed in, you'll enjoy an ad-free experience and be able to participate on this site by adding your own topics and posts, as well as connect with other members through your own private inbox!

How do I select a specific value range in an image?


seppy93

New Member
Messages
1
Likes
0
I have a microscope image below, and I would like to select all the grey long blobs on the image, and get how many pixels they are. I was thinking about selecting a specific value range, but I do not know how to accomplish that. Could someone give me some suggestions to make a value range selection that can be set to any value? Or if there's any other way to select those areas, that would be appreciated too.
58d5319d1a.jpg
 

Tom Mann

Guru
Messages
7,223
Likes
4,343
The easiest way to select a certain range of brightness is to use the "Blend IF" sliders. Here's a short intro, but many more are available:

http://blogs.adobe.com/jkost/2013/02/blend-if-sliders-in-photoshop.html .

One puts your image on a layer above a solid color layer. Usually, this background layer is either white or black, but even having no layer, just transparency is also an option. Then one adjusts the blendIF sliders to select the upper and lower brightness bounds that you want to see, e.g.,

Screen_capture-using_BlendIF_sliders_to_select_tonal_range2.jpg

Here is the result of the above:

57257d1437412309-how-do-i-select-specific-value-range-image-58d5319d1a-jpg-ps01a-01.jpg

Unfortunately, as you can see, such a simple approach rarely, if ever, yields good, clean results. One reason is that even the best photomicrographs have variations in illumination level over the field of view. In addition, in no photomicrograph will particles ever have truly sharp edges. Consider a very dark, uninteresting particle on a white background. In the transition region between the background and this particle, there will always be a thin ring of gray pixels that encircle the particle, and this ring will always show up when using the BlendIF approach.

The way it's usually done is using a full blown image analysis program such as ImageJ from NIH or the image processing toolbox for Matlab:
http://imagej.nih.gov/ij/
http://www.mathworks.com/help/images/index.html

Many, many algorithms, scripts and plugins have been developed for these programs to count single bacteria, inorganic particles, fibers, stars, colonies of bacteria, etc.. Many of these incorporate a priori knowledge of the shape / morphology of the particle to aid in the selection process. If you have other versions of your images that contain color information, that can also usually be incorporated into the selection criteria to improve the results.

For more info, Google search terms like {imagej count bacteria}, or for a more general search, omit "imagej" from the search criteria and search for other types of particles.

HTH,

Tom M
 

Top