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Add a big comment for the heuristic to detect ligatures.

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This commit is contained in:
Jacob Sandlund
2026-01-07 10:13:33 -05:00
parent 48dd6314dc
commit 13e125a057
1 changed files with 26 additions and 12 deletions
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38
src/font/shaper/coretext.zig
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@@ -389,8 +389,8 @@ pub const Shaper = struct {
var cell_offset: CellOffset = .{};
// For debugging positions, turn this on:
var run_offset_y: f64 = 0.0;
var cell_offset_y: f64 = 0.0;
//var run_offset_y: f64 = 0.0;
//var cell_offset_y: f64 = 0.0;
// Clear our cell buf and make sure we have enough room for the whole
// line of glyphs, so that we can just assume capacity when appending
@@ -437,13 +437,13 @@ pub const Shaper = struct {
if (cell_offset.cluster != cluster) {
// We previously asserted that the new cluster is greater
// than cell_offset.cluster, but for rtl text this is not
// true. We then used to break out of this block if cluster
// was less than cell_offset.cluster, but now this would
// fail to reset cell_offset.x and cell_offset.cluster and
// lead to incorrect shape.Cell `x` and `x_offset`. We
// don't have a test case for RTL, yet.
// true. We then used to break out of this block if the new
// cluster was less than cell_offset.cluster, but now this
// would fail to reset cell_offset.x and
// cell_offset.cluster and lead to incorrect shape.Cell `x`
// and `x_offset`. We don't have a test case for RTL, yet.
const is_codepoint_first_in_cluster = blk: {
const is_first_codepoint_in_cluster = blk: {
var i = index;
while (i > 0) {
i -= 1;
@@ -455,19 +455,33 @@ pub const Shaper = struct {
} else break :blk true;
};
if (is_codepoint_first_in_cluster) {
// We need to reset the `cell_offset` at the start of a new
// cluster, but we do that conditionally if the codepoint
// `is_first_codepoint_in_cluster`, which is a heuristic to
// detect ligatures and avoid positioning glyphs that mark
// ligatures incorrectly. The idea is that if the first
// codepoint in a cluster doesn't appear in the stream,
// it's very likely that it combined with codepoints from a
// previous cluster into a ligature. Then, the subsequent
// codepoints are very likely marking glyphs that are
// placed relative to that ligature, so if we were to reset
// the `cell_offset` to align it with the grid, the
// positions would be off. (TBD if there are exceptions to
// this heuristic, but using the logging below seems to
// show it works well.)
if (is_first_codepoint_in_cluster) {
cell_offset = .{
.cluster = cluster,
.x = run_offset_x,
};
// For debugging positions, turn this on:
cell_offset_y = run_offset_y;
//cell_offset_y = run_offset_y;
}
}
// For debugging positions, turn this on:
try self.debugPositions(alloc, run_offset_x, run_offset_y, cell_offset, cell_offset_y, position, index);
//try self.debugPositions(alloc, run_offset_x, run_offset_y, cell_offset, cell_offset_y, position, index);
const x_offset = position.x - cell_offset.x;
@@ -483,7 +497,7 @@ pub const Shaper = struct {
run_offset_x += advance.width;
// For debugging positions, turn this on:
run_offset_y += advance.height;
//run_offset_y += advance.height;
}
}