Allow custom ratio/width/height in fromDataURL

[halo]

Dear @szimek,

I know you're as busy as I am :) I'd still kindly ask you consider this PR as a small change that potentially solves a number of issues and PRs in one strike.

It simply allows a custom ratio, width, and height to be passed in when loading an image onto the canvas.

It should solve these Issues:

• The last comment in fromDataUrl fails on tablet #89 suggests a monkey patch that is solved by this PR
• FromDataURL reducing image size #105 appears to be solvable by overriding width/height via fromDataURL as implemented in this PR
• Bug in fromDataURL for high DPI screen devices #153 complains about guessing the ratio automagically, should be solved by this PR
• Image size on mobile device #200 describes just the problem that this PR solves

And you should be able to close these PRs:

• don't stretch image in fromDataURL #241 suggests to shrink any larger image to the canvas. While this makes sense at first sight, it's probably magic that may not fit all. This PR allows the developer to do the calculation outside of the SignaturePad library and simply pass in the resulting width and height.
• Fix issue with smaller image on mobile device #165 butchers away any ratio guessing. That only solves one specific problem. This PR solves it by passing in ratio: 1 as an argument.
• Fixed "fromDataURL" function making weird resize #164 removes the ratio altogeher. That also works for that person's use case but is not desireable for others. This PR solves it by passing in ratio: 1 as an argument.

If there is anything more I could do to assist, let me know.

[mymattcarroll]

👍

[halo]

Yes, no, maybe? It would be sad if I had to uphold a fork. It's not a complex change. But if you don't have time in the next weeks, just let me know. So I won't have to follow up. Thank you!

(I'm a just little bit suspicious when I see issues and pull requests that are several years old :)

[szimek]

Hey, huge thanks for this PR! In general I think it's a good idea, but I simply haven't got time yet to check it out. Hopefully, I'll find some time during this weekend.

[halo]

Sure, just wanted to ask. No stress. I'm grateful you created this library in the first place! :D

[szimek]

After rewrite to ES6, you can probably set the default values for function parameters, so you could write it as

function (dataUrl, options = {}) {...}

and get rid of opts variable.

[halo]

Indeed. I wasn't sure so I followed the pattern i saw in other functions.

I changed it now, this is what it compiles to:

SignaturePad.prototype.fromDataURL = function (dataUrl) {
  var _this = this;

  var options = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : {};

  var image = new Image();
  var ratio = options.ratio || window.devicePixelRatio || 1;
  var width = options.width || this._canvas.width / ratio;
  var height = options.height || this._canvas.height / ratio;
  // ...

[halo]