SVG and Dashed Lines

I’ve moved on from SVG to the Canvas element, and am still suffering whiplash at the downward steepness of the step. Leaving aside that one can defeat canvas easily just by turning off script, about the best documentation I found on the element is Mozilla’s implementation. The specification is buried within the HTML5 work, which if you think on it, defeats every last bit of argument about not including SVG because it’s “presentational”.

About the only negative associated with SVG is how to include it in the pages–the specification, itself, is really nice. Once all of the features are supported in the main browsers, you’re going to see some amazing stuff.

You don’t realize how nice SVG is, until you look at the Canvas element. For instance, I created an SVG example on how quadratic bèzier curves work in SVG. The canvas element also supports the same type of curves, so I went to re-create the example. I hit a wall immediately. Why? Two words:

“dashed lines”

No, there is no way of adding either dashes or dots or combination to lines in canvas. You can use a custom function and create tiny little lines using lineTo and/or one of the curve functions, but there’s no way to actually just specify the dash-dot pattern to use, and forget about it. My canvas demonstration of quadratic bèzier curves ended up looking a tad bit different.

I went searching on HTML5, the canvas element, and dashed line support and found a couple of discussions about supporting dashed lines in canvas. The official word is: not trivial to implement, no one really wants them.

Considering that one of the first implementations on the canvas element I created uses dashed lines, I beg to differ on the ‘no one really wants’ assumption. Sure, I can use a custom function, such as the one I’m using to emulate a rounded corner rectangle, but I can’t help thinking this isn’t the most efficient approach. The user agent should be able to optimize for dashes in line drawing if these are part of the specification, but there is no real optimization for functions that create hundreds of little dashes based on rotate, curve, line, move.

While researching the missing ‘dashes’, I ended up being more caught up in the discussions associated with the canvas element. It wasn’t until looking for dashed line that it hit me: why was there such deeply presentational discussions associated with a version of HTML that’s specific to semantics, only? Especially when you consider that in the recent discussions about support for SVG in HTML5 there was pushback for supporting SVG within HTML5 because SVG is ‘presentational’. In fact, Jacques just wrote, the following on this topic:

The subject came up in a discussion over at Sam’s blog about SVG-in-HTML5. Some people were arguing that, even if it were technically possible, it would be undesirable to sully HTML5 with the inclusion of foreign dialects, like MathML or SVG. The latter, in particular, was in some quarters considered too “presentational.”

This annoyed me sufficiently that I gave the following example:

Say I wish to discuss irreducible representations of SU(3). I think it should be perfectly acceptable to point out that: Rank-2 Symmetric Tensor Representation⊗Fundamental Representation=Adjoint Representation⊕Rank-3 Symmetric Tensor Representation

No, I don’t understand what Jacques wrote, either, but the SVG shows properly.

Of course, the issues associated with SVG (and MathML and RDF/XML, if it comes to that) go beyond just the presentation. SVG is a well-defined markup based on XML, which requires precise handling on the part of the user agent and the agent generating or creating the SVG. HTML5, on the other hand, is meant to be a less rigorous environment, where the lack of precise application of markup is handled with greater indulgence.

Lack of precise application of semantic markup, I should say, as there are new semantic attributes, values, and elements being added to the spec. One assumes this is so that search engines, such as Google, can better process the web page ‘semantics’. In fact, much of the semantics associated with HTML5 seems to be specific to search engine optimization.

I can understand some of the ‘foreign markup’ implementation concerns about SVG. What I can’t understand is why the HTML5 group hasn’t split the canvas element off into a separate specification–not if presentational purity is a goal of the specification. Just because it’s easier to include the canvas element in HTML5, doesn’t mean it should be included, if the underlying purpose of HTML5 is to provide an interim, better behaved, but more forgiving, semantic web page markup. According to the HTML5 spec:

This specification is limited to providing a semantic-level markup language and associated semantic-level scripting APIs for authoring accessible pages on the Web ranging from static documents to dynamic applications.

The scope of this specification does not include addressing presentation concerns (although default rendering rules for Web browsers are included at the end of this specification).

The scope of this specification does not include documenting every HTML or DOM feature supported by Web browsers. Browsers support many features that are considered to be very bad for accessibility or that are otherwise inappropriate. For example, the blink element is clearly presentational and authors wishing to cause text to blink should instead use CSS.

Blink is presentational, while dashed lines are not? I suppose one could also push line patterns off on CSS, but then why would one not also push stroke style and fill pattern, as well as such esoteric items as maximum miter height?

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