Monday, 30 January 2012

xhtml, css, boxes n shit

For some stupid reason I delved into CSS and XHTML and rendering thereof.

CSS is so deceptively simple: a few boxes, layouts in lines, and a few properties to set. Saying the devil is in the details here isn't doing the term justice. It's all in the details. And they're ugly.

From the fairly complex cascading rules, to the number of properties. The layout merging. The badly written documentation: filled with "x inline y box" "a block b box c" to such specificity, and no with definitions it is quite difficult to decipher what it's even talking about. It's also quite hard to debug, since it needs a fair bit of data structure to represent it.


Anyway, after some mucking about, I have a relatively complete CSS lexer and parser, a fairly incomplete cascade resolver, a fairly incomplete layout engine, and a very incomplete style system. It's just enough to show paragraphs of text with some basic formatting. For a book reader I don't want the document to control the text too much anyway.

I'm attempting to do it while streaming the input, and (obviously thus) in a single pass using the pull parser from XMLStreamReader. Therefore initial parsing is quite quick, but it's still taking a relatively long time to lay out the boxes ...


And the problem here is TextLayout. It's just quite slow. I tried my own version of layout using FontMetrics.getCharsWidth(), but inside that just creates a TextLayout anyway, so it's even slower (or maybe not, now it's a bit faster?).

I know why it's so complicated; for laying out complex scripts and handling all the special cases. Anyway, that is the primary factor of constraint on performance at the moment, although as the implementation is so far from finished, i'm sure it wont be the last one.

(I played some more, and the font used plays a big part in the speed taken, so there's hope yet).

I suppose I should try it on the kobo to see how it goes there.

Dead end?

It's taken a lot of effort to get this far, and i'm not really happy with the result. So i'm not sure if i'll keep plugging away at it or throw it away (and if i need such a functionality, use cssbox). There's a lot left to get it to be useful for anything.

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