fix markers on lines with variable aesthetics #2094
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…el semantics of markers: - markerStart matches the start of the line - markerMid matches the points which are not at the start or the end - markerEnd matches the end of the line Since these lines are implemented as multiple paths, we have change the low-level implementation of markers: - markerStart only applies to the first segment of a line - markerMid applies to all the segments, complemented by the start of all but the first segments - markerEnd only applies to the last segment of a line closes #2093
mbostock
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Functionally this seems correct, but I expect we could more efficiently implement it by augmenting applyMarker, rather than re-grouping the data and re-selecting the elements. The paths are already grouped, so it feels funny to redo all the work to group the data again.
src/marker.js
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| const end = markerEnd && applyMarker(markerEnd); | ||
| if (Z) { | ||
| for (const g of group( | ||
| path.filter((i) => i.length >= 2), |
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I wonder if segments of length 1 are not a bug already—independently of the grouping?
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Could be. The only case I noticed was at the end of a line, which seems unavoidable. Are there others?
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No, I think that's the only case. It makes "broken arrows" (if we apply a marker-end: arrow), which is why we have to filter it out here. I'll investigate.
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Here’s a patch which “fixes” it. But I think it’s valid for the last segment to have a single index if it has different aesthetics. It might be visible in the case where the stroke-linecap is round or square.
diff --git a/src/style.js b/src/style.js
index a1af100f..420d7bd3 100644
--- a/src/style.js
+++ b/src/style.js
@@ -261,6 +261,7 @@ export function* groupIndex(I, position, mark, channels) {
for (const G of Z ? groupZ(I, Z, z) : [I]) {
let Ag; // the A-values (aesthetics) of the current group, if any
let Gg; // the current group index (a subset of G, and I), if any
+ let singleton = false; // does this series only have one group?
out: for (const i of G) {
// If any channel has an undefined value for this index, skip it.
for (const c of C) {
@@ -273,8 +274,8 @@ export function* groupIndex(I, position, mark, channels) {
// Otherwise, if this is a new group, record the aesthetics for this
// group. Yield the current group and start a new one.
if (Ag === undefined) {
- if (Gg) yield Gg;
(Ag = A.map((c) => keyof(c[i]))), (Gg = [i]);
+ singleton = true;
continue;
}
@@ -287,13 +288,15 @@ export function* groupIndex(I, position, mark, channels) {
if (k !== Ag[j]) {
yield Gg;
(Ag = A.map((c) => keyof(c[i]))), (Gg = [i]);
+ singleton = false;
continue out;
}
}
}
- // Yield the current group, if any.
- if (Gg) yield Gg;
+ // Yield the current group, if any. For non-singleton series, the last group
+ // might contain only a single index; this is ignored.
+ if (Gg?.length > 1 || singleton) yield Gg;
}
}
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When the last segment has only one point the linecap has the default ("horizontal") orientation, which makes it feel broken (here, shown with transparency so we can see better):
With your patch it shows as I expect:
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I think I want to apply this patch! Not strictly necessary for this PR, but it seems reasonable to do it, and it will clean up the awkward tests on I.length > 1
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I’m not convinced we want the patch… Let’s land this without it and revisit if needed.
for lines with variable aesthetics we want to maintain the higher-level semantics of markers:
Since these lines are implemented as multiple paths, we need to change the low-level implementation of markers:
closes #2093