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(This article isn't about clipping paths or graphics or Vecto, it's about CLOS, protocols, and change-class.)

Clipping paths in in Vecto have these semantics:

  • the initial clipping path covers the whole canvas, so it has no effect on drawing
  • the clipping path can never get bigger (updates always intersect with the current clipping path), but
  • the clipping path is saved and restored by WITH-GRAPHICS-STATE, so you can localize changes

Vecto implements clipping with a grayscale mask channel. But I wanted a couple optimizations:

  • no clipping path creation or drawing overhead for the initial, empty clipping path
  • no data copying overhead in WITH-GRAPHICS-STATE unless the clipping path is actually modified (i.e. copy-on-write)

The protocol to support this has the generic functions EMPTYP, COPY, CLIPPING-DATA, and WRITABLE-CLIPPING-DATA. A more efficient drawing function is used if the canvas's clipping path object is EMPTYP, otherwise a slower function that references CLIPPING-DATA is used. WITH-GRAPHICS-STATE copies the clipping path with COPY. Clipping path updates are written to the channel returned by WRITABLE-CLIPPING-DATA.

I initially implemented this with three classes: CLIPPING-PATH, EMPTY-CLIPPING-PATH, and PROXY-CLIPPING-PATH. Here's how each class handles each generic function:

EMPTYP returns true returns false
data slot is EQ to original
CLIPPING-DATA no method returns data slot value
initializes and returns data
returns data slot change-class to CLIPPING-PATH,
set data slot to copy-seq of data slot

Then I read Joe Marshall's take on CHANGE-CLASS, which he calls — with some caveats — horrendous. I don't want to use horrendous things! So I implemented a different approach using only one CLIPPING-PATH class:

  • add a COPY-ON-WRITE-P slot, initially false
  • a CLIPPING-PATH is EMPTYP if its data slot is unbound
  • COPY creates a new CLIPPING-PATH instance
    • if the original is EMPTYP, the new instance is also empty
    • otherwise it has the same (EQ) data as the original and a copy-on-write flag of t.
  • CLIPPING-DATA returns the data slot
  • WRITABLE-CLIPPING-DATA checks if the clipping path is empty:
    • if so, initializes and returns the data slot
    • otherwise, checks the copy-on-write flag:
      • if set, COPY-SEQs the data slot to itself, returns the new data
      • otherwise it returns the data directly

The second approach seems ok to me (and I don't need a table to explain it), but the first approach doesn't seem that horrendous either. I didn't choose either technique for any special reasons; they're just the first things that occurred to me.

What do you think? Are they about the same, or does one seem better to you? Is there some other approach that would be even nicer? Let me know.



Change Class

I recently had a similar dilemma parsing elf files.

Once I'd read all the sections of the file, I had an array of elf-section objects. Some of the sections have quite complicated semantics (eg relocation entries referencing symbols, symbol names, and some other code/data section) that couldn't really be accounted for while the sections were being read.

So I had two options - do a second pass over the sections, chaging them to a more specialised type than elf-section and have specific methods specialise on these for symbol lookup, or string lookup or whatever, or just have a number of different methods for retreiving different things specialised only to the base section type.

I went with this but I'm beginning to think it was the wrong decision in terms of long-term extensibility - it would be better to have the sections specialise themselves as there are many possible types: ultimately it's a means of reducing the combinatorial explosion becaue the section-entry acessors specialise only on what they have to and not on the general case and validate they are used correctly.