Face with "NoMethodError: undefined method `-' for nil:NilClass" error if compare images with NO DIFFERENCES

[kay4444]

When I try to compare images with no differences I face with "NoMethodError: undefined method `-' for nil:NilClass" error. I Use Windows 7 OS. Please see screenshot from my project.
But if I compare images with differences - no errors are shown.

[wvanbergen]

The error is coming from your own code (ukr_net_helper.rb), not ChunkyPNG.

If you can provide a snippet of code that reproduces it, I can look into it

[wvanbergen]

Closing this due to missing information. Feel free to reopen if the problem persists and you can provide more information.

[shoesCodeFor]

@wvanbergen I ran into this too. It was caused by trying to compare identical images. @kay4444's implementation does not have a catch for diff.length == 0.

Thanks for this awesome gem 👍 chunky_png is great!

[wvanbergen]

Can you share the full stack trace of the error?

[shoesCodeFor]

Hi @wvanbergen, here is an A/B test(using identical images). The first is the code with a conditional to catch identical images and the 2nd example is the way that @kay4444 implemented the code above.

require("chunky_png")

images = [
    ChunkyPNG::Image.from_file("1.png"),
    ChunkyPNG::Image.from_file("1copy.png")
]
diff = []
images.first.height.times do |y|
  images.first.row(y).each_with_index do |pixel, x|
    diff << [x,y] unless pixel == images.last[x,y]
  end
end

# Catch identical images
if diff.length > 0
  percentage_change = (diff.length.to_f / images.first.pixels.length) * 100
  x, y = diff.map{ |xy| xy[0] }, diff.map{ |xy| xy[1] }
  images.last.rect(x.min, y.min, x.max, y.max, ChunkyPNG::Color.rgb(0,255,0))
  images.last.save("diff.png")
else
  percentage_change = diff.length
end
puts "pixels (total):     #{images.first.pixels.length}"
puts "pixels changed:     #{diff.length}"
puts "pixels changed (%): #{percentage_change}%"

Outputs:

pixels (total):     3686400
pixels changed:     0
pixels changed (%): 0%

Process finished with exit code 0

Here is the intial implementation:

require("chunky_png")

images = [
    ChunkyPNG::Image.from_file("1.png"),
    ChunkyPNG::Image.from_file("1copy.png")
]
diff = []
images.first.height.times do |y|
  images.first.row(y).each_with_index do |pixel, x|
    diff << [x,y] unless pixel == images.last[x,y]
  end
end

percentage_change = (diff.length.to_f / images.first.pixels.length) * 100
x, y = diff.map{ |xy| xy[0] }, diff.map{ |xy| xy[1] }
images.last.rect(x.min, y.min, x.max, y.max, ChunkyPNG::Color.rgb(0,255,0))
images.last.save("diff.png")

puts "pixels (total):     #{images.first.pixels.length}"
puts "pixels changed:     #{diff.length}"
puts "pixels changed (%): #{percentage_change}%"

Returns the following error:

Traceback (most recent call last):
	2: from /Users/someuser/Library/Preferences/RubyMine2018.2/scratches/img_module/image_diff.rb:16:in `<main>'
	1: from /Users/someuser/.rvm/gems/ruby-2.5.1/gems/chunky_png-1.3.11/lib/chunky_png/canvas/drawing.rb:235:in `rect'
/Users/someuser/.rvm/gems/ruby-2.5.1/gems/chunky_png-1.3.11/lib/chunky_png/canvas/drawing.rb:97:in `line_xiaolin_wu': undefined method `-' for nil:NilClass (NoMethodError)

Process finished with exit code 1

I think this is caused by the attempt to draw a rectangle with no data.

Thanks again for this module, it has helped me immensely!
Schuyler

[wvanbergen]

The problem is that you are sending a nil value as parameter to the rect call. It expects that all parameters are integers. ChunkyPNG should probably give you a better error message in this case, but the issue is in your sample application.