Expert: Neal Ziring - 4/18/2004

Question
Hello!

Is there a way to get exact information about the percentage of space one of the base-colours of an image covers?

To make myself clearer:
Sometimes you read something like "standardized documents that have 5% of the page covered with each of the 4 colours C M Y and K".
Or printer-builders themself state something like "you can print x sheets of paper with a coverage of 7.5% of each colour wih our colour-tanks"

Now I have a given image and want to know how many % are covered with each C M Y and K.

To get for example the ammount of yellow of an image...
- I would have to add the Y-values of every single pixel in the image
- set this number in relation to the maximum of colour the image could have overall

Do you know if this is possible with PhotoPaint?
Or do you know other Software (Freeware would be best) that shows the exact colour-coverage?

Thank you very much in advance!
And greetings fromm Erlangen / Germany!

Harald Dehner

Answer
Harald,

>
> Is there a way to get exact information about the
> percentage of space one of the base-colours of an
> image covers?

Actually, no, there isn't.  With substantial difficulty
you can get a very precise figure for it, but not easily.

You can get a fairly good approximation of it quickly.
Here's how:

  - Convert the image to CMYK color model
    (Image->Color mode->CMYK Color)

  - View the image histogram
    (Image->Histogram)

  - In the Histogram window, select the individual color
    channels: Cyan, Magenta, Yellow, Black.
    For each channel, you'll see a Mean value of the pixels
    for that color.  Call that value M, it will be between
    0 (none of that color) and 255 (all of that color).
    The percentage coverage for that color is then
              P = (M/255) * 100.

So, when I do this for a beach picture, I see that the mean
value for Cyan is 58.  So, P = (58/255)*100 = 22.7%.
You can calculate this for each of the four colors.

Of course, it also matters how big the image is.  If
you have a 20x15 image on a 30x21 piece of paper, then
you're only covering 47% of the sheet of paper.

Sorry this answer is so late, I was away on a business
trip and could not get my e-mail.

...nz