Reading that document on Macromedia's site gives me light.
It says, swf format only supports quadratic beziers, what less accurate than cubic beziers, but easy to render fast. I think, in swf file's beziers are quadratic interpolation of better looking quadratic beziers. Threshold value drives this accuracy (more control nodes, bigger files)
(PHP 4 >= 4.0.5, PHP 5, PECL ming SVN)
ming_setcubicthreshold — Set cubic threshold
Sets the threshold error for drawing cubic beziers.
The Threshold. Lower is more accurate, hence larger file size.
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