Re: [PATCH v2.11.1] High-resolution PS plotting

[Ilya Zakharevich <nospam-abuse@ilyaz.org>]

On Mon, Nov 11, 2019 at 11:35:42PM -0800, Ilya Zakharevich wrote:
> It looks like the low-precision problem is avoided with SVG, due to (≈l.2088)
> 
>     gen_draw(&pl, w, x, y, SVG_SCALE, SVG_SCALE);
> 
> (Only 1/SVG_SCALE = 2⁻¹⁰ is later rounded by "%.2f"; I see no reason
> for such a strange code…)

A bit more clarification on the strangeness of the code:
  • The coordinates are initially stored (inside the “rectangles”) as
    floating point;
  • Then SVG code in rect2svg() asks to scale it up 1024 times (still
    to floats);
  • Then gen_draw() converts it to integers to pass to the low-level
    (format-specific) engine;
  • Then every low-level entry point (such as svg_lines()) scales
    these coordinates DOWN 1024 times (to floats);
  • Then these coordinates are str_printf()ed with "%.2f").

Seems like whoever wrote SVG engine realized the problem with
low-level entry points taking integer values — but instead of
converting the API to behave sensibly they put in this mess with
scaling up/down (with the coefficient incompatible with the
printf-format!).

    (This mess with integer coefficients is most probably MY fault!
     Myself, I wanted to convert all the low-level API to “double”s
     for more than a decade! — These functions are statics anyway!)

Hope this helps,
Ilya