Finding the (x,y) indexes of specific (R,G,B) color values from images stored in NumPy ndarrays

Question

I have an image stored in a numpy array, as yielded by `imread()`: ``` >>> ndim array([[[ 0, 0, 0], [ 4, 0, 0], [ 8, 0, 0], ..., [247, 0, 28], [251, 0, 28], [255, 0, 28]], [[ 0, 255, 227], [ 4, 255, 227], [ 8, 255, 227], ..., [247, 255, 255], [251, 255, 255], [255, 255, 255]]], dtype=uint8) >>> ndim.shape (512, 512, 3) ``` I want to efficiently find the (x, y) coordinate (or coordinates) of pixels with a specific color value, e.g. ``` >>> c array([ 32, 32, 109], dtype=uint8) >>> ndim[200,200] array([ 32, 32, 109], dtype=uint8) >>> ndim.T[0, 200, 200] 32 >>> ndim.T[1, 200, 200] 32 >>> ndim.T[2, 200, 200] 109 ``` ... in this case, I know the pixel at (200, 200) has the RGB value (32, 32, 109) -- I can test for this. What I want to do is query the ndarray for a pixel value and get back the coordinates. In the above case, the putative function `find_pixel(c)` would return (200, 200). Ideally this `find_pixel()` function would return a list of coordinate tuples and not just the first value…

Accepted Answer

For some array colour array `a` and a colour tuple `c`:

```
indices = numpy.where(numpy.all(a == c, axis=-1))
```

`indices` should now be a 2-tuple of arrays, the first of which contains the indices in the first dimensions and the second of which contains the indices in the second dimension corresponding to pixel values of `c`.

If you need this as a list of coordinate tuples, use zip:

```
coords = zip(indices[0], indices[1])
```

For example:

```
import numpy
a = numpy.zeros((4, 4, 3), 'int')

for n in range(4):
    for m in range(4):
        a[n, m, :] = n + m
        if (n + m) == 4:
            print n, m

c = (4, 4, 4)
indices = numpy.where(numpy.all(a == c, axis=-1))
print indices
print zip(indices[0], indices[1])
```

will output:

```
1 3
2 2
3 1
(array([1, 2, 3]), array([3, 2, 1]))
[(1, 3), (2, 2), (3, 1)]
```

which corresponds to all the pixels of value (4, 4, 4).

Finding the (x,y) indexes of specific (R,G,B) color values from images stored in NumPy ndarrays

Solution

Problem