About comparison two large size png

[ghost]

I use iPad to take a screenshot and save as png image.
Its size is about 1.3M.
When I run the code below:
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

There will be an error "out of bound " or sth.
Sorry I cannot remember the detail error message.
How can I solve this issue.

[wvanbergen]

Can you run this again, and paste the error message plus the stack trace here?

[ghost]

I ran it again and paste the error message here:

/Library/Ruby/Gems/2.0.0/gems/chunky_png-1.3.4/lib/chunky_png/canvas.rb:334:in assert_xy!': Coordinates (768,0) out of bounds! (ChunkyPNG::OutOfBounds) from /Library/Ruby/Gems/2.0.0/gems/chunky_png-1.3.4/lib/chunky_png/canvas.rb:174:in[]'
from diff.rb:49:in block (2 levels) in <main>' from diff.rb:48:ineach'
from diff.rb:48:in each_with_index' from diff.rb:48:inblock in

'
from diff.rb:47:in times' from diff.rb:47:in'

[wvanbergen]

Most likely, the two images you're using do not have the same dimensions. So the x coordinate you get from the first image is too high for the last image. Mind sharing the two images?

[ghost]

Thank you for your help.
You are right the dimensions are not same.
I think QA team take the screenshot by different ipads.
I think it can be closed now.