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simplify cairo pixel alignment in Canvas::LineSet

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1. do the math when we add a new coord, not at every render call
2. correctly decide whether to force an odd-width line on an integer
   coordinate to a half pixel, or an even-width line on a non-integer
   coordinate to a whole pixel.
This commit is contained in:
Paul Davis 2025-06-02 19:19:30 -06:00
parent 705fe2b52d
commit ae913acacf
1 changed files with 31 additions and 20 deletions
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51
libs/canvas/line_set.cc
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@ -101,21 +101,18 @@ LineSet::render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const
for (auto const & l : _lines) { for (auto const & l : _lines) {
Rect self; Rect self;
const double shift = 0.5;
if (_orientation == Horizontal) { if (_orientation == Horizontal) {
self = Rect (0, l.pos - (l.width/2.0), _extent, l.pos + (l.width/2.0)); self = Rect (0, l.pos - (l.width/2.0), _extent, l.pos + (l.width/2.0));
if (fmod (l.width, 2.)) {
self.y0 += shift;
}
} else { } else {
self = Rect (l.pos - (l.width/2.0), 0, l.pos + (l.width/2.0), _extent); self = Rect (l.pos - (l.width/2.0), 0, l.pos + (l.width/2.0), _extent);
if (fmod (l.width, 2.)) {
self.x0 += shift;
}
} }
self = item_to_window (self); /* Note the 2nd argument, to avoid rounding after we may have
* just shifted to a half-pixel grid (see
* doc/cairo-single-pixel-lines)
*/
self = item_to_window (self, false);
Rect intersection = self.intersection (area); Rect intersection = self.intersection (area);
if (!intersection) { if (!intersection) {
@ -125,20 +122,12 @@ LineSet::render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) const
Gtkmm2ext::set_source_rgba (context, l.color); Gtkmm2ext::set_source_rgba (context, l.color);
context->set_line_width (l.width); context->set_line_width (l.width);
/* Not 100% sure that the computation of the invariant
* positions (y and x) below work correctly if the line width
* is not 1.0, but visual inspection suggests it is OK.
* See doc/cairo-single-pixel-lines lines to understand why we add 0.5
*/
if (_orientation == Horizontal) { if (_orientation == Horizontal) {
const double y = item_to_window (Duple (0, l.pos)).y; context->move_to (intersection.x0, self.y0);
context->move_to (intersection.x0, y + shift); context->line_to (intersection.x1, self.y0);
context->line_to (intersection.x1, y + shift);
} else { } else {
const double x = item_to_window (Duple (l.pos, 0)).x; context->move_to (self.x0, intersection.y0);
context->move_to (x + shift, intersection.y0); context->line_to (self.x0, intersection.y1);
context->line_to (x + shift, intersection.y1);
} }
context->stroke (); context->stroke ();
@ -149,6 +138,28 @@ void
LineSet::add_coord (Coord pos, Distance width, Gtkmm2ext::Color color) LineSet::add_coord (Coord pos, Distance width, Gtkmm2ext::Color color)
{ {
_lines.push_back (Line (pos, width, color)); _lines.push_back (Line (pos, width, color));
Line& l (_lines.back());
if (_orientation == Horizontal) {
/* If width is odd (width % 2 != 0) and position is on
a whole pixel, shift it to a half-pixel position. Otherwise
force it back to an integer position. See
doc/cairo-single-pixel-lines for more details.
*/
if (fmod (l.width, 2.) && fmod (l.pos, 1.0)) {
l.pos += 0.5;
} else {
l.pos = floor (l.pos);
}
} else {
if (fmod (l.width, 2.) && fmod (l.pos, 1.0)) {
l.pos += 0.5;
} else {
l.pos = floor (l.pos);
}
}
} }
void void