Provide a global tolerance when comparing for equality

[cburgmer]

This PR tries to solve #18.

As proposed it implements an optional parameter set along the following example:

{
  tolerance : 0.05,
  totalTolerance : true
}

Following those changes imagediff.equal(a, b, 128) would be expressed through imagediff.equal(a, b, {tolerance: 0.5}) in the future (however the old API remains intact).
A global tolerance can be given through adding totalTolerance: true to the option object.

[cburgmer]

My thoughts: I did want to follow your suggestion and see how things pan out. However, I am not happy with the API. There are a few things:

• Providing a by-pixel-tolerance through a rate from 0 to 1 seems pretty pointless and counter-intuitive. And on top of that, do we divide by 255 or by 256? Technically only values up to 255 exist, but 256 feels natural and would give "exact" floats (as far as I understand floating point arithmetic with powers of two) making comparison easy.
• Providing globalTolerance: true feels cumbersome.
• It is unclear/not well communicated how the globalTolerance: false implementation works.
• Providing new ways of providing tolerance values would in the future collide with the rather fixed API above.

Will try to come up with an alternative idea.

[cburgmer]

Here's my current proposal:

imagediff.equal(a, b, {toleranceValue: 5, toleranceMethod: 'relativePerPixel'});

and

imagediff.equal(a, b, {toleranceValue: 0.25, toleranceMethod: 'totalRatio'});

[cesutherland]

Can you give me an example of relativePerPixel? Would that be a total difference of 5 across the rgba values for a single pixel?

[cburgmer]

Sorry for the late response.

relativePerPixel is the current implementation, where any pixel maybe different as long as it each absolute difference stays below the threshold.

totalRation is what I'd like to add, where an accumulated difference is used.

I'm not sure about the proposed API anymore though. I think it would be easier if we'd provide a method on it's own for the different tolerances?