How to count the amount of white in an image

[markmarques]

Hi...
After some tryout from the "usage" docs I still can not figure it out how to count the amount of white existing in a grayscale image....
I have tryed several variations ( including -fuzz, -unique-colors, -opaque , etc) with no sucess

my original idea is to send a percentage value to a variable inside a script ( Windows ) something like :
set a = convert yyyy.jpg -colorspace gray -unique-colors -print "%fx[:white/(w*h)]" null:

No matter what "convert" arguments I try I simply get the value "1" ...

According to the docs I should be careful with the black color ...

Any ideas ?

[fmw42]

use -fuzz if desired if you want near-white as well as pure white, negate the image, threshold at 0, negate again, then compute the mean and multiply by 100.


percentwhite=`convert logo: -fuzz 0% -negate -threshold 0 -negate -format "%[fx:100*mean]" info:`
echo "percent white = $percentwhite"
percent white = 83.4128


If on windows, then the syntax for variables and other things is different. So see http://www.imagemagick.org/Usage/windows/

[anthony]

Another way to get the actual count is to get a color histogram.

See Extracting Image Colors
http://www.imagemagick.org/Usage/quantize/#extract

[fmw42]

Per Anthony's suggestions, this gives the total count of white pixels


convert logo: -format %c histogram:info: | grep "white" | sed -n 's/^ *\(.*\):.*$/\1/p'
256244



So if you want the percent

count=`convert logo: -format %c histogram:info: | grep "white" | sed -n 's/^ *\(.*\):.*$/\1/p'`
percent=`convert logo: -format "%[fx:100*$count/(w*h)]" info:`
echo $percent
83.4128

But my earlier method only requires one line of processing rather than the two here.

[markmarques]

Just a quick reply:
After trying out the previous ideas I got what I needed by using something like :

"convert logo: -colorspace gray -fuzz 5% +opaque white -format "%[fx:mean*100]" info: "
83.996

No need to double negate anything ...

Thanks ...

[xpt]

Q1: Why my calculation is different than all of yours?

Code: Select all

$ convert logo: -fuzz 0% -negate -threshold 0 -negate -format "%[fx:100*mean]" info:
82.4707

$ convert logo: -colorspace gray -fuzz 5% +opaque white -format "%[fx:mean*100]" info: 
99.9733

I.e., instead of advertised 83.4128 or 83.996, I get 82.4707 and 99.9733. My imagemagick version is 6.6.2.6-1, BTW.

Q2: to get white count from color histogram, if I want near-white as well as pure white to be counted, it is possible?

Thanks

[fmw42]

Q1: Why my calculation is different than all of yours?

First, your two calculations are quite different.

Second, at one point, I am not sure when, the meaning of [fx:mean] was changed from red channel only to global (all channel) mean. Look over changelog or upgrade your IM

Q2: to get white count from color histogram, if I want near-white as well as pure white to be counted, it is possible?

Thanks

That is the point of the -fuzz value to get values near your color with some color distance expressed as XX%

[xpt]

use -fuzz if desired if you want near-white as well as pure white, negate the image, threshold at 0, negate again, then compute the mean and multiply by 100.

Will such algorithm confuse pure black as white?

case 1 (55.3489):


As the reference, here is
case 2 (55.3376):


which calculates *less* white space than case 1!

[fmw42]

what command did you run on these two images?

if -fuzz XX% is small, then it won't confuse white and black.

The reason is that when you threshold the image a lot of near white is becoming black.

[xpt]

More examples to test:

[xpt]

what command did you run on these two images?

Exactly using

Code: Select all

convert file -fuzz 0% -negate -threshold 0 -negate -format "%[fx:100*mean]" info:

what do you get from this?

[fmw42]

The reason is that when you threshold the image a lot of near white is becoming black.

As you have some gray that is very close to white from antialiasing, you can see this by changing the fuzz value. The larger the fuzz value the more gray that is considered to be white.


convert 69bugcoloringpage03.jpg -fuzz 0% -fill white -opaque white -fill black +opaque white -format "%[fx:100*mean]" info:
55.3376

convert 69bugcoloringpage03.jpg -fuzz 10% -fill white -opaque white -fill black +opaque white -format "%[fx:100*mean]" info:
77.8961

You can see the effects by doing

convert 69bugcoloringpage03.jpg -fuzz 0% -fill white -opaque white -fill black +opaque white tmp1.png

convert 69bugcoloringpage03.jpg -fuzz 10% -fill white -opaque white -fill black +opaque white tmp2.png

[xpt]

Thanks Fred,

Your

Code: Select all

convert file.jpg -fuzz 10% -fill white -opaque white -fill black +opaque white -format "%[fx:100*mean]" info:

algorithm works better for me, according to my visual inspection, between this one and the previous one.

Thank you.

[fmw42]

Thanks Fred,

Your

Code: Select all

convert file.jpg -fuzz 10% -fill white -opaque white -fill black +opaque white -format "%[fx:100*mean]" info:

algorithm works better for me, according to my visual inspection, between this one and the previous one.

Thank you.

You should adjust the fuzz XX% to suit your situation as to how close/far from pure white you want to consider as still "white".