Randomly choosing a pixel with a desired value

[Rasaa84]

Hi,

How can I randomly select a pixel with a particular value from a single-band image? I need to have its coordinates.

Your help is much appreciated.

Best,

[jcupitt]

Hello @Rasaa84,

You could maybe do:

image == 127

Now the image will have 255 in every position which had the value 127 and 0 elsewhere, so you just need to find the 255s.

If you take the average you can find out how many 255s there are:

$ python
Python 3.11.2 (main, May 30 2023, 17:45:26) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pyvips
>>> x = pyvips.Image.new_from_file("mono.jpg")
>>> (x == 127).avg() * x.width * x.height / 255.0
20066.0

So 20066 pixels have the value 127 in this case.

You can ask max to make an array of the positions of the N maximum values:

>>> max_value, optional_results = (x == 127).max(x_array=True, y_array=True, size=20066)
>>> len(optional_results['x_array'])
20066
>>> optional_results['x_array'][42], optional_results['y_array'][42]
(637, 88)
>>> 

So it found 20066 255s, and position 43 is at (637, 88).

This will be pretty slow for large images, but it might be fast enough if all your images are small.