Python 3: I am trying to find find all green pixels in an image by traversing all pixels using an np.array, but can't get around index error

Question

My code currently consists of loading the image, which is successful and I don't believe has any connection to the problem.

Then I go on to transform the color image into a np.array named rgb

    # convert image into array
    rgb = np.array(img)
    red = rgb[:,:,0]
    green = rgb[:,:,1]
    blue = rgb[:,:,2]

To double check my understanding of this array, in case that may be the root of the issue, it is an array such that rgb[x-coordinate, y-coordinate, color band] which holds the value between 0-255 of either red, green or blue.

Then, my idea was to make a nested for loop to traverse all pixels of my image (620px,400px) and sort them based on the ratio of green to blue and red in an attempt to single out the greener pixels and set all others to black or 0.

for i in range(xsize):
for j in range(ysize):
    color = rgb[i,j]  <-- Index error occurs here
    if(color[0] > 128):
        if(color[1] < 128):
            if(color[2] > 128):
                rgb[i,j] = [0,0,0]

The error I am receiving when trying to run this is as follows:

IndexError: index 400 is out of bounds for axis 0 with size 400

I thought it may have something to do with the bounds I was giving i and j so I tried only sorting through a small inner portion of the image but still got the same error. At this point I am lost as to what is even the root of the error let alone even the solution.

Answer 1

In direct answer to your question, the y axis is given first in numpy arrays, followed by the x axis, so interchange your indices.

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Less directly, you will find that for loops are very slow in Python and you are generally better off using numpy vectorised operations instead. Also, you will often find it easier to find shades of green in HSV colourspace.

Let's start with an HSL colour wheel:

and assume you want to make all the greens into black. So, from that Wikipedia page, the Hue corresponding to Green is 120 degrees, which means you could do this:

#!/usr/local/bin/python3
import numpy as np
from PIL import Image

# Open image and make RGB and HSV versions
RGBim = Image.open("image.png").convert('RGB')
HSVim = RGBim.convert('HSV')

# Make numpy versions
RGBna = np.array(RGBim)
HSVna = np.array(HSVim)

# Extract Hue
H = HSVna[:,:,0]

# Find all green pixels, i.e. where 100 < Hue < 140
lo,hi = 100,140
# Rescale to 0-255, rather than 0-360 because we are using uint8
lo = int((lo * 255) / 360)
hi = int((hi * 255) / 360)
green = np.where((H>lo) & (H<hi))

# Make all green pixels black in original image
RGBna[green] = [0,0,0]

count = green[0].size
print("Pixels matched: {}".format(count))
Image.fromarray(RGBna).save('result.png')

Which gives:

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Here is a slightly improved version that retains the alpha/transparency, and matches red pixels for extra fun:

#!/usr/local/bin/python3
import numpy as np
from PIL import Image

# Open image and make RGB and HSV versions
im = Image.open("image.png")

# Save Alpha if present, then remove
if 'A' in im.getbands():
    savedAlpha = im.getchannel('A')
    im = im.convert('RGB')

# Make HSV version
HSVim = im.convert('HSV')

# Make numpy versions
RGBna = np.array(im)
HSVna = np.array(HSVim)

# Extract Hue
H = HSVna[:,:,0]

# Find all red pixels, i.e. where 340 < Hue < 20
lo,hi =  340,20
# Rescale to 0-255, rather than 0-360 because we are using uint8
lo = int((lo * 255) / 360)
hi = int((hi * 255) / 360)
red = np.where((H>lo) | (H<hi))

# Make all red pixels black in original image
RGBna[red] = [0,0,0]

count = red[0].size
print("Pixels matched: {}".format(count))

result=Image.fromarray(RGBna)

# Replace Alpha if originally present
if savedAlpha is not None:
    result.putalpha(savedAlpha)

result.save('result.png')

Keywords: Image processing, PIL, Pillow, Hue Saturation Value, HSV, HSL, color ranges, colour ranges, range, prime.