Image Gradient
"The Music Lesson" by Chelsea Porcelain Manufactory from the collection of the Metropolitan Museum of Art is licensed under CC0 1.0.
Computing the gradient:
require "imgrb"
file_name = "MET_music_lesson_cropped"
img = Imgrb::Image.new("#{file_name}.png")
img_gray = img.to_gray.get_channel(0) #Grayscale without alpha channel.
sigma = 1
grad_x, grad_y = img_gray.gradient(sigma)
grad_mag = ((grad_x**2 + grad_y**2)**0.5).rescale
theta = (-grad_y).atan2(grad_x)
#Colormap
hsv_lambda = ->(angle){
val = ((angle / (Math::PI)) + 1) * 3
rgb = [0,0,0]
comp = 255 * (1 - ((val % 2) - 1).abs)
if 0 <= val && val <= 1
rgb = [255, comp, 0]
elsif 1 < val && val <= 2
rgb = [comp, 255, 0]
elsif 2 < val && val <= 3
rgb = [0, 255, comp]
elsif 3 < val && val <= 4
rgb = [0, comp, 255]
elsif 4 < val && val <= 5
rgb = [comp, 0, 255]
elsif 5 < val && val <= 6
rgb = [255, 0, comp]
end
rgb
}
#Illustrate gradient by coloring based on direction and magnitude.
angle_rgb = theta.apply_lambda(hsv_lambda)*Imgrb::Image.new(grad_mag, grad_mag, grad_mag)
(grad_x.rescale*255).round.save("#{file_name}_grad_x.png")
(grad_y.rescale*255).round.save("#{file_name}_grad_y.png")
(grad_mag*255).round.save("#{file_name}_grad_mag.png")
angle_rgb.round.save("#{file_name}_angles.png")
