[core] Fix issue #9843 (overzoomed line label rendering glitch).

Ports fix for GL JS issue #5112.
Line label projection can't be based on tile geometry that's behind the plane of the camera.
The relevant tests are still ignored because the overzoomed collision behavior is different between native and JS.

1 files changed, 53 insertions, 19 deletions

diff --git a/src/mbgl/layout/symbol_projection.cpp b/src/mbgl/layout/symbol_projection.cpp
index 8ccd6a0410..279d251f8f 100644
--- a/src/mbgl/layout/symbol_projection.cpp
+++ b/src/mbgl/layout/symbol_projection.cpp
@@ -94,10 +94,12 @@ namespace mbgl {
     }
 
 
-    Point<float> project(const Point<float>& point, const mat4& matrix) {
+    typedef std::pair<Point<float>,float> PointAndCameraDistance;
+
+    PointAndCameraDistance project(const Point<float>& point, const mat4& matrix) {
         vec4 pos = {{ point.x, point.y, 0, 1 }};
         matrix::transformMat4(pos, pos, matrix);
-        return { static_cast<float>(pos[0] / pos[3]), static_cast<float>(pos[1] / pos[3]) };
+        return {{ static_cast<float>(pos[0] / pos[3]), static_cast<float>(pos[1] / pos[3]) }, pos[3] };
     }
 
     float evaluateSizeForFeature(const ZoomEvaluatedSize& zoomEvaluatedSize, const PlacedSymbol& placedSymbol) {
@@ -150,9 +152,20 @@ namespace mbgl {
         NotEnoughRoom,
         NeedsFlipping
     };
+    
+    Point<float> projectTruncatedLineSegment(const Point<float>& previousTilePoint, const Point<float>& currentTilePoint, const Point<float>& previousProjectedPoint, const float minimumLength, const mat4& projectionMatrix) {
+        // We are assuming "previousTilePoint" won't project to a point within one unit of the camera plane
+        // If it did, that would mean our label extended all the way out from within the viewport to a (very distant)
+        // point near the plane of the camera. We wouldn't be able to render the label anyway once it crossed the
+        // plane of the camera.
+        const Point<float> projectedUnitVertex = project(previousTilePoint + util::unit<float>(previousTilePoint - currentTilePoint), projectionMatrix).first;
+        const Point<float> projectedUnitSegment = previousProjectedPoint - projectedUnitVertex;
+
+        return previousProjectedPoint + (projectedUnitSegment * (minimumLength / util::mag<float>(projectedUnitSegment)));
+    }
 
 	optional<PlacedGlyph> placeGlyphAlongLine(const float offsetX, const float lineOffsetX, const float lineOffsetY, const bool flip,
-            Point<float> anchorPoint, const uint16_t anchorSegment, const GeometryCoordinates& line, const mat4& labelPlaneMatrix) {
+            const Point<float>& projectedAnchorPoint, const Point<float>& tileAnchorPoint, const uint16_t anchorSegment, const GeometryCoordinates& line, const mat4& labelPlaneMatrix) {
 
         const float combinedOffsetX = flip ?
             offsetX - lineOffsetX :
@@ -172,8 +185,8 @@ namespace mbgl {
 
         int32_t currentIndex = dir > 0 ? anchorSegment : anchorSegment + 1;
 
-        Point<float> current = anchorPoint;
-        Point<float> prev = anchorPoint;
+        Point<float> current = projectedAnchorPoint;
+        Point<float> prev = projectedAnchorPoint;
         float distanceToPrev = 0.0;
         float currentSegmentDistance = 0.0;
         const float absOffsetX = std::abs(combinedOffsetX);
@@ -185,7 +198,18 @@ namespace mbgl {
             if (currentIndex < 0 || currentIndex >= static_cast<int32_t>(line.size())) return {};
 
             prev = current;
-            current = project(convertPoint<float>(line.at(currentIndex)), labelPlaneMatrix);
+            PointAndCameraDistance projection = project(convertPoint<float>(line.at(currentIndex)), labelPlaneMatrix);
+            if (projection.second > 0) {
+                current = projection.first;
+            } else {
+                // The vertex is behind the plane of the camera, so we can't project it
+                // Instead, we'll create a vertex along the line that's far enough to include the glyph
+                const Point<float> previousTilePoint = distanceToPrev == 0 ?
+                    tileAnchorPoint :
+                    convertPoint<float>(line.at(currentIndex - dir));
+                const Point<float> currentTilePoint = convertPoint<float>(line.at(currentIndex));
+                current = projectTruncatedLineSegment(previousTilePoint, currentTilePoint, prev, absOffsetX - distanceToPrev + 1, labelPlaneMatrix);
+            }
 
             distanceToPrev += currentSegmentDistance;
             currentSegmentDistance = util::dist<float>(prev, current);
@@ -211,29 +235,28 @@ namespace mbgl {
                               const mat4& posMatrix,
                               const mat4& labelPlaneMatrix,
                               const mat4& glCoordMatrix,
-                              gl::VertexVector<SymbolDynamicLayoutAttributes::Vertex>& dynamicVertexArray) {
+                              gl::VertexVector<SymbolDynamicLayoutAttributes::Vertex>& dynamicVertexArray,
+                              const Point<float>& projectedAnchorPoint) {
         const float fontScale = fontSize / 24.0;
         const float lineOffsetX = symbol.lineOffset[0] * fontSize;
         const float lineOffsetY = symbol.lineOffset[1] * fontSize;
 
-        const Point<float> anchorPoint = project(symbol.anchorPoint, labelPlaneMatrix);
-
         std::vector<PlacedGlyph> placedGlyphs;
         if (symbol.glyphOffsets.size() > 1) {
 
             const float firstGlyphOffset = symbol.glyphOffsets.front();
             const float lastGlyphOffset = symbol.glyphOffsets.back();
             
-            optional<PlacedGlyph> firstPlacedGlyph = placeGlyphAlongLine(fontScale * firstGlyphOffset, lineOffsetX, lineOffsetY, flip, anchorPoint, symbol.segment, symbol.line, labelPlaneMatrix);
+            optional<PlacedGlyph> firstPlacedGlyph = placeGlyphAlongLine(fontScale * firstGlyphOffset, lineOffsetX, lineOffsetY, flip, projectedAnchorPoint, symbol.anchorPoint, symbol.segment, symbol.line, labelPlaneMatrix);
             if (!firstPlacedGlyph)
                 return PlacementResult::NotEnoughRoom;
 
-            optional<PlacedGlyph> lastPlacedGlyph = placeGlyphAlongLine(fontScale * lastGlyphOffset, lineOffsetX, lineOffsetY, flip, anchorPoint, symbol.segment, symbol.line, labelPlaneMatrix);
+            optional<PlacedGlyph> lastPlacedGlyph = placeGlyphAlongLine(fontScale * lastGlyphOffset, lineOffsetX, lineOffsetY, flip, projectedAnchorPoint, symbol.anchorPoint, symbol.segment, symbol.line, labelPlaneMatrix);
             if (!lastPlacedGlyph)
                 return PlacementResult::NotEnoughRoom;
 
-            const Point<float> firstPoint = project(firstPlacedGlyph->point, glCoordMatrix);
-            const Point<float> lastPoint = project(lastPlacedGlyph->point, glCoordMatrix);
+            const Point<float> firstPoint = project(firstPlacedGlyph->point, glCoordMatrix).first;
+            const Point<float> lastPoint = project(lastPlacedGlyph->point, glCoordMatrix).first;
 
             if (keepUpright && !flip &&
                 (symbol.useVerticalMode ? firstPoint.y < lastPoint.y : firstPoint.x > lastPoint.x)) {
@@ -244,7 +267,7 @@ namespace mbgl {
             for (size_t glyphIndex = 1; glyphIndex < symbol.glyphOffsets.size() - 1; glyphIndex++) {
                 const float glyphOffsetX = symbol.glyphOffsets[glyphIndex];
                 // Since first and last glyph fit on the line, we're sure that the rest of the glyphs can be placed
-                auto placedGlyph = placeGlyphAlongLine(glyphOffsetX * fontScale, lineOffsetX, lineOffsetY, flip, anchorPoint, symbol.segment, symbol.line, labelPlaneMatrix);
+                auto placedGlyph = placeGlyphAlongLine(glyphOffsetX * fontScale, lineOffsetX, lineOffsetY, flip, projectedAnchorPoint, symbol.anchorPoint, symbol.segment, symbol.line, labelPlaneMatrix);
                 placedGlyphs.push_back(*placedGlyph);
             }
             placedGlyphs.push_back(*lastPlacedGlyph);
@@ -252,15 +275,23 @@ namespace mbgl {
             // Only a single glyph to place
             // So, determine whether to flip based on projected angle of the line segment it's on
             if (keepUpright && !flip) {
-                const Point<float> a = project(convertPoint<float>(symbol.line.at(symbol.segment)), posMatrix);
-                const Point<float> b = project(convertPoint<float>(symbol.line.at(symbol.segment + 1)), posMatrix);
+                const Point<float> a = project(symbol.anchorPoint, posMatrix).first;
+                const Point<float> tileSegmentEnd = convertPoint<float>(symbol.line.at(symbol.segment + 1));
+                const PointAndCameraDistance projectedVertex = project(tileSegmentEnd, posMatrix);
+                // We know the anchor will be in the viewport, but the end of the line segment may be
+                // behind the plane of the camera, in which case we can use a point at any arbitrary (closer)
+                // point on the segment.
+                const Point<float> b = (projectedVertex.second > 0) ?
+                    projectedVertex.first :
+                    projectTruncatedLineSegment(symbol.anchorPoint,tileSegmentEnd, a, 1, posMatrix);
+                
                 if (symbol.useVerticalMode ? b.y > a.y : b.x < a.x) {
                     return PlacementResult::NeedsFlipping;
                 }
             }
             assert(symbol.glyphOffsets.size() == 1); // We are relying on SymbolInstance.hasText filtering out symbols without any glyphs at all
             const float glyphOffsetX = symbol.glyphOffsets.front();
-            optional<PlacedGlyph> singleGlyph = placeGlyphAlongLine(fontScale * glyphOffsetX, lineOffsetX, lineOffsetY, flip, anchorPoint, symbol.segment,
+            optional<PlacedGlyph> singleGlyph = placeGlyphAlongLine(fontScale * glyphOffsetX, lineOffsetX, lineOffsetY, flip, projectedAnchorPoint, symbol.anchorPoint, symbol.segment,
                 symbol.line, labelPlaneMatrix);
             if (!singleGlyph)
                 return PlacementResult::NotEnoughRoom;
@@ -275,6 +306,7 @@ namespace mbgl {
         return PlacementResult::OK;
     }
 
+
     void reprojectLineLabels(gl::VertexVector<SymbolDynamicLayoutAttributes::Vertex>& dynamicVertexArray, const std::vector<PlacedSymbol>& placedSymbols,
 			const mat4& posMatrix, const style::SymbolPropertyValues& values,
             const RenderTile& tile, const SymbolSizeBinder& sizeBinder, const TransformState& state, const FrameHistory& frameHistory) {
@@ -311,12 +343,14 @@ namespace mbgl {
             const float pitchScaledFontSize = values.pitchAlignment == style::AlignmentType::Map ?
                 fontSize * perspectiveRatio :
                 fontSize / perspectiveRatio;
+            
+            const Point<float> anchorPoint = project(placedSymbol.anchorPoint, labelPlaneMatrix).first;
 
-            PlacementResult placeUnflipped = placeGlyphsAlongLine(placedSymbol, pitchScaledFontSize, false /*unflipped*/, values.keepUpright, posMatrix, labelPlaneMatrix, glCoordMatrix, dynamicVertexArray);
+            PlacementResult placeUnflipped = placeGlyphsAlongLine(placedSymbol, pitchScaledFontSize, false /*unflipped*/, values.keepUpright, posMatrix, labelPlaneMatrix, glCoordMatrix, dynamicVertexArray, anchorPoint);
 
             if (placeUnflipped == PlacementResult::NotEnoughRoom ||
                 (placeUnflipped == PlacementResult::NeedsFlipping &&
-                 placeGlyphsAlongLine(placedSymbol, pitchScaledFontSize, true /*flipped*/, values.keepUpright, posMatrix, labelPlaneMatrix, glCoordMatrix, dynamicVertexArray) == PlacementResult::NotEnoughRoom)) {
+                 placeGlyphsAlongLine(placedSymbol, pitchScaledFontSize, true /*flipped*/, values.keepUpright, posMatrix, labelPlaneMatrix, glCoordMatrix, dynamicVertexArray, anchorPoint) == PlacementResult::NotEnoughRoom)) {
                 hideGlyphs(placedSymbol.glyphOffsets.size(), dynamicVertexArray);
             }
         }