Hi Dave, Thank you for your message. It's good to see that you are still interested in what's happening in WebKit MathML. I believe we agree with the goals you state, however most of your claims on flexbox VS a separate renderer class seemed to be assumptions you made 3 years ago. People (including me) have tried to maintain the MathML code after you left and these assumptions turned out to be at least inaccurate. With Alejandro's approach, I believe that in the upcoming months we can at the same time get a good MathML implementation and address all of reviewer's concerns on your original code. At the moment, I do not think there is an alternative proposal with these features, and even less someone willing to work on any other approach...
More replies inline. Le 23/12/2015 08:11, dbar...@mathscribe.com a écrit : > 1. Ideally, we want most webkit developers to not even have to think about > MathML. If you make some change to the rendering and layout code and it > doesn't break non-MathML layout features, including flexbox, then ideally > that should almost always mean that MathML layout is still ok. However when a legitimate change elsewhere does break something in MathML (Alejandro mentioned one bug but I have seen several of them) it becomes a burden for the non-MathML developers. This is especially true because it's hard to understand the code given the complexity of the current flexbox-based implementation and the vagueness of the MathML spec (which we are trying to solve by using http://www.mathml-association.org/MathMLinHTML5 and http://tests.mathml-association.org/). Using an independent renderer class allows to isolate the MathML implementation and reduces the risk of breaking the MathML code while changing another part of the code. This has been confirmed with the experience in Gecko (actually, such an issue only happened twice in Gecko in the past decade and that was due to changes in graphics and font modules, not layout). > 2. Conversely, it would be ideal if the MathML layout code didn't have to > worry about a lot of non-math things - floats, columns, margin collapsing, > internal RenderBlock voodoo, etc. We just want good fast MathML layout, that > integrates well with HTML, CSS, and JavaScript. By using our own renderer class we can actually ignore CSS properties that we consider irrelevant (see https://github.com/MathML/MathMLinHTML5/issues/9). As Alejandro said in his initial message, we now only care about math features and not about the whole flexbox rules. Hence that's less code executed and we hope it would actually improve performance. > 3. MathML must introduce few or no bugs, and especially no security bugs. With a separate renderer class, the layout code essentially becomes arithmetic computations (of box coordinates and size). This is more reliable than manipulation of anonymous renderers and style that need to be kept updated with respect to dynamic tree/style changes. See for example the current implementation of mmultiscripts which is just crazy (http://trac.webkit.org/browser/trunk/Source/WebCore/rendering/mathml/RenderMathMLScripts.cpp). If I understand correctly, one of the improvements you obtained by using flexboxes was that you removed some of the (potentially unsafe) manipulations of margin / border style that used to be necessary for box alignments. With the new approach, we go even farther and expect to remove all the anonymous renderers and style, so that these are no longer a problem. > At the end of 2012, the webkit MathML code was around 2,500 lines (admittedly > it didn't implement all of MathML). In comparison, the MathPlayer plugin for > Internet Explorer, MathML implementation inside Gecko, and the MathJax > javascript library, were each about 20,000-40,000+ lines of code. Another > comparison is that the webkit rendering/svg source code folder then contained > about 17,700 lines. If we try to implement MathML from scratch, sort of like > SVG, then I think it will take many developer-years to do, which frankly may > never happen. I'm not sure the comparison is really relevant, given the limited MathML implementations in WebKit and the fact that the coding style are very different (e.g. MathJax is written in Javascript and has various hacks to workaround CSS bugs from old browsers, Gecko has a 80 characters limit for lines and has fallback code when a MATH font is not available). Instead, let's compare the two approaches in WebKit. The flexbox-based implementation is really naive and just aligns the edges of base and scripts (see attachment integrals.png) ; implementing mmultiscripts that way was overly complex as I said above. As a comparison, in only a single day, Alejandro and I were able to get an implementation of msub/msup/msubsup/mmultiscripts using the TeX rules and a decade of parameters from the OpenType MATH table (see http://www.mathml-association.org/MathMLinHTML5/S3.html#SS4.SSS1). Moreover, RenderMathMLScripts.[h|cpp] went down from 631 lines to 454 lines i.e. a ratio of 72%. > MathML may not absolutely need all these, but several (like changing the > order for mroot) are very useful, and outlawing floats and columns and margin > collapsing is great for avoiding bugs or unintended interactions with other > code. So again by using flexbox, I think MathML can be thousands of lines > instead of tens of thousands of lines, and will need a lot less new tricky > lines to avoid buggy interactions with existing layout features and voodoo. AFAIK, neither Gecko nor MathJax rely on flexboxes but just absolutely place the math boxes (as we want to do in the new approach) and they are able to get good math rendering. With hundred of lines, you can do a pure CSS implementation (cf https://github.com/fred-wang/mathml.css/blob/master/mathml.css) and that's fine as long as you only expect low quality and limited support. However, users want high-quality rendering similar to what is done in TeX or Microsoft Word and so we need more flexibility to position math boxes. Like for mmultiscripts, you can probably always add more and more nested anonymous boxes but then the code becomes even more complex and we spend too much time maintaining the integrity of the render tree hierarchy instead of just focusing on arithmetic computations and simple rules. > Finally, I'd like to say that I think it's great that you guys are asking for > feedback on this. There are obviously webkit layout and rendering experts > that know a lot more than I do, and I urge you very strongly to try to get > one of them to help you with this design decision. Otherwise someone else in > a few years may delete your code, as has happened to me. :) As Alejandro said, we are going to submit patches for review soon. Additionally, we are really interested in hearing any feedback from WebKit experts on the following implementation note and tests: http://www.mathml-association.org/MathMLinHTML5/ http://tests.mathml-association.org/ Finally, I believe you are seeing the glass half empty if you reduce our work to "deleting your code". I would rather say that the flexbox implementation was an opportunity for developers/users to experiment MathML in WebKit, improve our test suite, get feedback and discuss the pros and cons of that approach. In particular, thanks to your work we have a good idea of what would be acceptable by the Google reviewers in the future. For now, we are just rewriting the MathML layout to address the feedback from the WebKit community and get something better! Such a refactoring happens everytime in software development, so there are no reasons to worry... -- Frédéric Wang
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