python - numpy: unique list of colors in the image

Question

I have an image img:

>>> img.shape
(200, 200, 3)

On pixel (100, 100) I have a nice color:

>>> img[100,100]
array([ 0.90980393,  0.27450982,  0.27450982], dtype=float32)

Now my question is: How many different colors are there in this image, and how do I enumerate them?

My first idea was numpy.unique(), but somehow I am using this wrong.

Answer 1

Your initial idea to use numpy.unique() actually can do the job perfectly with the best performance:

numpy.unique(img.reshape(-1, img.shape[2]), axis=0)

At first, we flatten rows and columns of matrix. Now the matrix has as much rows as there're pixels in the image. Columns are color components of each pixels.

Then we count unique rows of flattened matrix.