[cairo] Observations from a newb

Dirk Schönberger dirk.schoenberger at sz-online.de
Fri Nov 9 00:01:42 PST 2007

> Sorry if I ask silly questions, but I do not savvy the "source" concept
> yet. I
> can see that it's a place where data comes from, but not how the flavours,
> of
> what a source can be, work.
>  I sort of picture a source as being an array of rgba values that have
> been
> preset in memory from some other place (the source of the source). So it's
> hard to picture how a source can said to be SVG or PDF, given my oddball
> angle on it.

You have to separate between image and meta surfaces. An image surface
works as you described, it is an array of RGBA values (and some other
colour models)
A meta surface stores basically the "service level primitives" and can
play these back. So if you use such a surface as a source, you can e.g.
fill areas with scalable patterns.
Problem is that these meta surfaces don't exist as a public API, but are
"hidden" behind other surfaces (PS and PDF I think). So in order to use
them, you have to create a PDF surface and use that as source in your
"real" context. This is a little non-obvious.

>  For example, I assume one would use librsvg to open an SVG file and then
> some cairo footwork would turn that into a source; after that it can be
> stamped around as per the clever cairo idiom. But how does this source
> from a bitmap? I mean, what are it's properties that differentiate it?
Can it be
> smoothly scaled for example (i.e. redrawn as a vector and then updated
> across the cairo surface wherever it has been used).

I am not sure, but I have not yet seen a usebale SVG (or PDF) renderer
which targets cairo directly. All I have seen are modules which render to
image buffers, which in turn can be used as a image surface. In using this
approach, all vector possibilities are lost.

What I would like to see would be a "real" metafile system for cairo.
Basically a way to store a sequence of cairo calls (high level, not
service level) and be able to replay this later. I envision some kind of
string format using reverse polish notation operands for storing the
commands. Basically this is how the Adobe family of graphics formats work

M is moveto, C is curveto, L is lineto, Z is closepath, f is fill, s is
stroke, r is set_source_rgb etc

a metadata file would look something like

10 10 M 100 0 L 100 100 L 0 0 L 1 0 0 r f

Maybe I will give this idea a try.


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