C API functions in libcairo or librsvg, to convert lengths to other units?

Question

I'm trying to develop a C program based on https://github.com/songulabuzar/librsvg-minimal-example, which reads and renders and SVG file, using both librsvg and cairo libraries.

I have noticed, that both libraries contain enums that define length units.

In (lib)cairo, in https://github.com/freedesktop/cairo/blob/7bf3a78/src/cairo-svg.h there is:

typedef enum _cairo_svg_unit {
    CAIRO_SVG_UNIT_USER = 0,
    CAIRO_SVG_UNIT_EM,
    CAIRO_SVG_UNIT_EX,
    CAIRO_SVG_UNIT_PX,
    CAIRO_SVG_UNIT_IN,
    CAIRO_SVG_UNIT_CM,
    CAIRO_SVG_UNIT_MM,
    CAIRO_SVG_UNIT_PT,
    CAIRO_SVG_UNIT_PC,
    CAIRO_SVG_UNIT_PERCENT
} cairo_svg_unit_t;

... which is most obviously used with cairo_svg_surface_set_document_unit function (declared in the same header file).

On the other hand, (lib)rsvg in https://github.com/GNOME/librsvg/blob/d158d11/include/librsvg/rsvg.h defines:

typedef enum {
    RSVG_UNIT_PERCENT,
    RSVG_UNIT_PX,
    RSVG_UNIT_EM,
    RSVG_UNIT_EX,
    RSVG_UNIT_IN,
    RSVG_UNIT_CM,
    RSVG_UNIT_MM,
    RSVG_UNIT_PT,
    RSVG_UNIT_PC
} RsvgUnit;

... and specifically, it also defines a length with a unit:

typedef struct {
    double   length;
    RsvgUnit unit;
} RsvgLength;

So, what I'd like to know, is: is there a function in either of these APIs, that converts one length with units to another unit?

Say, I have a defined length of 1.5 inches (in librsvg, that would be a RsvgLength with { .length = 1.5, .unit = RSVG_UNIT_IN }) - is there a built-in function (in either of these APIs) to convert this kind of length to centimeters?

( as a reminder, for this example, 1.5 in = 3.81 cm ... I am aware that not all conversions would be a-priori possible, that is, converting cm to px (pixels) would also require the document DPI (dots-per-inch) ).

score 1 · Answer 1 · answered May 02 '22 at 19:55

Well, I tried looking through the code again, and I think I'm reasonably certain, that there are no unit conversion functions in neither (lib)cairo, nor (lib)rsvg (as of writing of this answer).

The commit in cairo that introduces the enum unit, does not contain any conversion functions https://github.com/freedesktop/cairo/commit/15559b54af473d720da9e03b0e769c54a53505a9 and its message "svg: add a new function to specify the SVG document unit" also implies limited scope
librsvg mentions in https://github.com/GNOME/librsvg/blob/d158d11/include/librsvg/rsvg.h :

 * It is up to the calling application to convert lengths in non-pixel units
 * (i.e. those where the @unit field is not `RSVG_UNIT_PX`) into something
 * meaningful to the application.  For example, if your application knows the
 * dots-per-inch (DPI) it is using, it can convert lengths with @unit in
 * `RSVG_UNIT_IN` or other physical units.

So, I guess, no SVG unit conversion functions here (in these two libraries) as of writing of this answer.

There is also a similar enum CRNumType in https://gitlab.gnome.org/Archive/libcroco/-/blob/master/src/cr-num.h - but again, I cannot find any unit conversion functions there.

However, I found SVG unit conversion functions here:

nsvg__convertToPixels() from https://github.com/memononen/nanosvg/blob/a5486fa/src/nanosvg.h#L786
length_value() from https://github.com/sammycage/plutosvg/blob/b8475a7/source/plutosvg.c#L973

Interestingly, both of these convert from "any" unit to pixels - but I couldn't find a reverse function. For reference, I'll paste nsvg__convertToPixels, as it looks like it can be more easily translated to librsvg:

static float nsvg__convertToPixels(NSVGparser* p, NSVGcoordinate c, float orig, float length)
{
    NSVGattrib* attr = nsvg__getAttr(p);
    switch (c.units) {
        case NSVG_UNITS_USER:       return c.value;
        case NSVG_UNITS_PX:         return c.value;
        case NSVG_UNITS_PT:         return c.value / 72.0f * p->dpi;
        case NSVG_UNITS_PC:         return c.value / 6.0f * p->dpi;
        case NSVG_UNITS_MM:         return c.value / 25.4f * p->dpi;
        case NSVG_UNITS_CM:         return c.value / 2.54f * p->dpi;
        case NSVG_UNITS_IN:         return c.value * p->dpi;
        case NSVG_UNITS_EM:         return c.value * attr->fontSize;
        case NSVG_UNITS_EX:         return c.value * attr->fontSize * 0.52f; // x-height of Helvetica.
        case NSVG_UNITS_PERCENT:    return orig + c.value / 100.0f * length;
        default:                    return c.value;
    }
    return c.value;
}

So I guess, to implement an svg unit conversion from "any" unit to "any" unit, one has to implement the reverse function (so, something like nsvg__convertFromPixels), then call them both, with the pixel value of the length as "intermediary".

C API functions in libcairo or librsvg, to convert lengths to other units?

1 Answers1