[cairo] New query functions: cairo_current_path, cairo_current_path_flat
keithp at keithp.com
Wed Feb 18 10:35:44 PST 2004
Around 10 o'clock on Feb 18, Bill Spitzak wrote:
> It seems to me that only the "flattened" version is really needed:
It is possible that most application uses will want the flattened version,
however, flattening the path discards information; as Carl said,
reconstructing the path from the flattened version may result in
rasterization which exceeds the specified error bounds, while
reconstructing the path from the canonical version will not.
> 1. It's the only one that returns information that the calling program does
> not already have. Looking through my postscript examples it seems that is the
> only one used by interesting programs, such as ones drawing text along a path.
This API also provides a way to get at the path constructed from glyphs
> 2. I'm worried that we will need to support the SVG conic functions. If we
> decide that these are approximated by curve-to, it means the callback is an
> approximation anyway. Or you will have to make even more callback functions
> with different sets of callbacks.
To some extent, cairo is not just an API specification, but also a
specification for how those APIs should map to a primitive graphics
system. The PostScript specification makes it abundantly clear that the
only 'primitive' objects in Postscript are lines and cubic splines; I
suggest the same is true for cairo. Yes, this 'limits' the
implementation, but I believe we have sound mathematical reasons for this
limitation in terms of guaranteeing rasterization precision.
> 3. I would think a possible Cairo implementation is to immediatly flatten the
> curve or to otherwise cook the data so that recovering the original curveto
> is impractical. It would be nice if Cairo allowed as much leeway in
> implementation as possible.
No; flattening the path generates wildly inaccurate results. PostScript
actually specifies that objects are drawn this way and we've seen the
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