Ha ha, funny you should mention circles! It's so just much fun filling and stroking arcs and circles correctly with X11. From the horse's mouth:
https://archive.org/details/xlibprogrammingm01adri/page/144/...
Xlib Programming Manual and Xlib Reference Manual, Section 6.1.4, pp 144:
>To be more precise, the filling and drawing versions of the rectangle routines don't draw even the same outline if given the same arguments.
>The routine that fills a rectangle draws an outline one pixel shorter in width and height than the routine that just draws the outline, as shown in Figure 6-2. It is easy to adjust the arguments for the rectangle calls so that one draws the outline and another fills a completely different set of interior pixels. Simply add 1 to x and y and subtract 1 from width and height. In the case of arcs, however, this is a much more difficult proposition (probably impossible in a portable fashion).
https://news.ycombinator.com/item?id=11484148
DonHopkins on April 12, 2016 | parent | context | favorite | on: NeWS – Network Extensible Window System
>There's no way X can do anti-aliasing, without a ground-up redesign. The rendering rules are very strictly defined in terms of which pixels get touched and how.
>There is a deep-down irreconcilable philosophical and mathematical difference between X11's discrete half-open pixel-oriented rendering model, and PostScript's continuous stencil/paint Porter/Duff imaging model.
>X11 graphics round differently when filling and stroking, define strokes in terms of square pixels instead of fills with arbitrary coordinate transformations, and is all about "half open" pixels with gravity to the right and down, not the pixel coverage of geometric region, which is how anti-aliasing is defined.
>X11 is rasterops on wheels. It turned out that not many application developers enjoyed thinking about pixels and coordinates the X11 way, displays don't always have square pixels, the hardware (cough Microvax framebuffer) that supports rasterops efficiently is long obsolete, rendering was precisely defined in a way that didn't allow any wiggle room for hardware optimizations, and developers would rather use higher level stencil/paint and scalable graphics, now that computers are fast enough to support it.
>I tried describing the problem in the Unix-Haters X-Windows Disaster chapter [1]:
>A task as simple as filing and stroking shapes is quite complicated because of X's bizarre pixel-oriented imaging rules. When you fill a 10x10 square with XFillRectangle, it fills the 100 pixels you expect. But you get extra "bonus pixels" when you pass the same arguments to XDrawRectangle, because it actually draws an 11x11 square, hanging out one pixel below and to the right!!! If you find this hard to believe, look it up in the X manual yourself: Volume 1, Section 6.1.4. The manual patronizingly explains how easy it is to add 1 to the x and y position of the filled rectangle, while subtracting 1 from the width and height to compensate, so it fits neatly inside the outline. Then it points out that "in the case of arcs, however, this is a much more difficult proposition (probably impossible in a portable fashion)." This means that portably filling and stroking an arbitrarily scaled arc without overlapping or leaving gaps is an intractable problem when using the X Window System. Think about that. You can't even draw a proper rectangle with a thick outline, since the line width is specified in unscaled pixel units, so if your display has rectangular pixels, the vertical and horizontal lines will have different thicknesses even though you scaled the rectangle corner coordinates to compensate for the aspect ratio.
[1] The X-Windows Disaster: http://www.art.net/~hopkins/Don/unix-haters/x-windows/disast...