Legacy ImageMagick Discussions Archive

Is there an easy way to calculate the brightness of an entire image? I want to compare brightness values of thousands of images.

Also, if I have a black & white image, is there a way to calculate how clustered the black pixels are? I'm subtracting the background out of camera images and thresholding it to generate a binary image. Because the image background jiggles over time, the differenced-and-thresholded image isn't very clean -- there are lots of streaks and dots in addition to the dark ovals (it's the dark ovals that I'm interested in). I need a way to distinguish the images containing only the streaks and dots from the ones with the dark ovals. I thought there might be a way to calculate how clustered the black pixels are. Is there?

Thanks.....

First it is not clear what you mean by brightness. You can get IM to tell you what the average gray level value is for a grayscale image (in IM 6.3.9.1+) using:

identify -format "%[mean]" <image>

or

convert <image> -format "%[mean]" info:

see
http://www.imagemagick.org/script/escape.php

If you are on an earlier IM version, you can extract it from the -verbose info: as follows:

data=`convert <image> -verbose info:`
echo "$data" | sed -n '/^.*Mean:.*[(]\([0-9.]*\).*$/{ s//\1/; p; q; }'

or in one line as

echo "$(convert <image> -verbose info:)" | sed -n '/^.*Mean:.*[(]\([0-9.]*\).*$/{ s//\1/; p; q; }'

With respect to your second issue, can you post some links to pictures to explain your problem?

fmw42 wrote:With respect to your second issue, can you post some links to pictures to explain your problem?

I'd like to calculate a metric so that I can tell "clustered" from "scattered."

Clustered:


Scattered:


One way that I've seen this done (in GIS) is to calculate the average "patch" size where a "patch" is a continuous blob of a single color. But, for one, I don't know how to do that in ImageMagick, and it seems like a very slow operation.

Thanks.

I will try to look at this more over the weekend. But my first thought is to do a fairly large median filter (or a number of iterations of a binary morphologic filter) to remove small specks and fill in holes. What is left are the blobs.

How do you define a patch size? How big is it? What is the minimum number of pixels in a patch? Are patches nearly the same size? There are many factors.

See my answer to another recent question at

viewtopic.php?f=1&t=10889

He has now written something custom. Perhaps if appropriate to your problem he will share with you.

IM does not have blob analysis functions to my knowledge.