ruby draw color specific pixels from source image to new image

Question

how can i take all the whitish pixels from an image and draw them on to a new image with a new color. the code below is how i would do it with python but the majority of the project has been written in ruby so i am trying to stick with it.

from PIL import Image

im = Image.open("image_in.png")
im2 = Image.new("P",im.size,255)
im = im.convert("P")

temp = {}

for x in range(im.size[1]):
  for y in range(im.size[0]):
    pix = im.getpixel((y,x))
    temp[pix] = pix
    if pix == 225:
      im2.putpixel((y,x),0)

im2.save("image_out.png")

this is what i got so far:

require 'rubygems'
require 'RMagick'
include Magick

image = Magick::Image.read('image_in.png').first
image2 = Image.new(170,40) { self.background_color = "black" }

pixels = []

(0..image.columns).each do |x|
    (0..image.rows).each do |y|
        pixel = image.pixel_color(x, y)
        if pixel == 54227 >> pixels #color value
        image2.store_pixels(pixels)
        end
    end
end

image2.write('image_out.png')

Answer 1

You don't need your pixels array at all, you can use pixel_color to set a pixel's color as well as read it. If you say pixel_color(x, y) then it behaves like getpixel in your Python code, if you say pixel_color(x, y, color) then it behaves like putpixel. So that gets ride of pixels and store_pixels.

Then the problem is figuring out if a pixel is white. The pixel_color method returns you a Pixel instance. Pixel has two methods of particular interest here:

1. The Pixel.from_color class method for converting a color name to a Pixel.
2. The fcmp instance method for comparing Pixels with an optional fuzziness in the comparison.

You can get a white Pixel with white = Pixel.from_color('white'). Then you can copy white pixels with:

pixel = image.pixel_color(x, y)
if pixel.fcmp(white)
    image2.pixel_color(x, y, pixel)
end

If you want to make the comparison fuzzy, then supply the second argument to fcmp:

if pixel.fcmp(white, 10000)
    image2.pixel_color(x, y, pixel)
end

You'll probably have to play around with the fuzz argument to fcmp to get something that works for you.

Answer 2

require 'RMagick'
include Magick

image = Magick::Image.read('image_in.png').first
image2 = Image.new(170,40) # { self.background_color = "black" }

color = Pixel.from_color('#D3D3D3') #color to extract

(0..image.columns).each do |x|
    (0..image.rows).each do |y|
        pixel = image.pixel_color(x, y)
        if pixel.fcmp(color)
        image2.pixel_color(x, y, "#000000") #change color to black
        end
    end
end

image2.write('image_out.png')