Converting Quartz Paths - MoveTo, LineTo, CurveTo

Cello has it's own support for curves - internally it has support for cubic and quadratic splines, as well as lines - the equivalent to what exists in CoreGraphics (Quartz) and generally in vector graphics systems. Within Cello there are classes that represent multi contoured splines and the attendant trickery to draw and manipulate them. They are of significance to me now as the existing Cello code requires the geometry of glyphs in its own internal format. So I needed to write conversion functions from a CGPaths to Cello's internal format.

Cello's curve format is unusual so writing the conversion has taken a little time. Most graphic systems that I have come across describe things in the form of, moveto, lineto and curveto. The idea being that "from where the pen is do this". Cello seems to be the other way round. Instead the curve verbs appear to be of the form linefrom, curvefrom, and movefrom.

My approach to writing the conversion has been first lift some existing code that converts the output of the D-Type font scalar (which is similar to the way that CoreGraphics thinks of curves) and to write a converter from Cello's internal format back to a CGPath. Again for this I could lift some code from Cello's output - again in the output the format paths are similar to CoreGraphics. Using the to and from converters it was possible to write Unit Tests to make sure that an original path is the same as a path converted, and then converted back. CFEqual will let you know if two paths are equal or not.

CPTAssert(::CFEqual(origPath, shape.AsPath()));

Converting and then back converting to, demonstrably, the same thing gives some idea of correctness but it does not guarantee that what you have done is correct, it just asserts that these two components are consistent. When I can look at the glyph shapes in the final output format I will be able to check this.