Make line loops and strips continuous

This patch implements joining the outer sides of the lines making up
strips and loops, with the exception of the connections between the
first and the last loop lines, which are left as they were before.

src/core/StelPainter.cpp

@@ -2464,6 +2464,8 @@ void StelPainter::drawFixedColorWideLinesAsQuads(const ArrayDesc& vertexArray, i
 	Q_ASSERT(vertexArray.type == GL_FLOAT);
 	const auto lineVertData = static_cast<const float*>(vertexArray.pointer) + vertexArray.size*offset;
 	const int step = mode==Lines ? 2 : 1;
+	const bool connected = mode!=Lines;
+	Vec2f prevV1aNDC, prevV1bNDC, prevLineDirNDC;
 	for(int n = 0; n < count-1; n += step)
 	{
 		Vec4f in0;
@@ -2498,16 +2500,67 @@ void StelPainter::drawFixedColorWideLinesAsQuads(const ArrayDesc& vertexArray, i
 		const Vec3f ndc0 = Vec3f(clip0.v) / clip0[3];
 		const Vec3f ndc1 = Vec3f(clip1.v) / clip1[3];
 
-		const Vec2f lineDir2d = normalize((Vec2f(ndc1.v) - Vec2f(ndc0.v)) * viewportSize);
+		const Vec2f lineDirNDC = normalize(Vec2f(ndc1.v) - Vec2f(ndc0.v));
+		const Vec2f lineDir2d = normalize(lineDirNDC * viewportSize);
 		const Vec2f perpendicularDir2d = Vec2f(-lineDir2d[1], lineDir2d[0]);
 
 		const Vec2f offset2d = (Vec2f(glState.lineWidth) / viewportSize) * perpendicularDir2d;
 
-		// 2D screen coordinates of the new pairs (a,b) of vertices for each input vertex (0,1)
-		const Vec2f v0a_xy = Vec2f(clip0.v) + offset2d*clip0[3];
-		const Vec2f v0b_xy = Vec2f(clip0.v) - offset2d*clip0[3];
-		const Vec2f v1a_xy = Vec2f(clip1.v) + offset2d*clip1[3];
-		const Vec2f v1b_xy = Vec2f(clip1.v) - offset2d*clip1[3];
+		// 2D NDC of the new pairs (a,b) of vertices for each input vertex (0,1)
+		      Vec2f v0aNDC = Vec2f(ndc0.v) + offset2d;
+		      Vec2f v0bNDC = Vec2f(ndc0.v) - offset2d;
+		const Vec2f v1aNDC = Vec2f(ndc1.v) + offset2d;
+		const Vec2f v1bNDC = Vec2f(ndc1.v) - offset2d;
+
+		if(connected && n > 0) // at n==0 we initialize prevV1{a,b}XY
+		{
+			const auto processLineSide = [lineDirNDC,prevLineDirNDC](const Vec2f& prevV1, Vec2f& currV0) -> bool
+			{
+				// Find prevT and currT for which prevV1+prevLineDirNDC*prevT==currV0+lineDirNDC*currT.
+				// This will define the intersection point of the line side.
+				const float prevT = (lineDirNDC[1]*(prevV1[0]-currV0[0]) + lineDirNDC[0]*(currV0[1]-prevV1[1]))
+				                                                               /
+				                               (lineDirNDC[0]*prevLineDirNDC[1] - lineDirNDC[1]*prevLineDirNDC[0]);
+				const float currT = (prevLineDirNDC[1]*(prevV1[0]-currV0[0]) + prevLineDirNDC[0]*(currV0[1]-prevV1[1]))
+				                                                               /
+				                               (lineDirNDC[0]*prevLineDirNDC[1] - lineDirNDC[1]*prevLineDirNDC[0]);
+				if(prevT > 0 && currT < 0)
+				{
+					// This is the outer corner of a joint. Extrapolate beginning
+					// of the current line back to the intersection.
+					currV0 += lineDirNDC*currT;
+					return true;
+				}
+				return false;
+			};
+
+			// Process one side of the line (we call it A)
+			if(processLineSide(prevV1aNDC, v0aNDC))
+			{
+				// Extrapolate the previous line end forward to the intersection
+				newVertices[6*(n-1)+2][0] = v0aNDC[0]*clip0[3];
+				newVertices[6*(n-1)+2][1] = v0aNDC[1]*clip0[3];
+				newVertices[6*(n-1)+3][0] = v0aNDC[0]*clip0[3];
+				newVertices[6*(n-1)+3][1] = v0aNDC[1]*clip0[3];
+			}
+
+			// Process the second side of the line (we call it B)
+			if(processLineSide(prevV1bNDC, v0bNDC))
+			{
+				// Extrapolate the previous line end forward to the intersection
+				newVertices[6*(n-1)+5][0] = v0bNDC[0]*clip0[3];
+				newVertices[6*(n-1)+5][1] = v0bNDC[1]*clip0[3];
+			}
+		}
+		prevV1aNDC = v1aNDC;
+		prevV1bNDC = v1bNDC;
+		prevLineDirNDC = lineDirNDC;
+
+		// 2D clip space coordinates of the new pairs (a,b) of vertices for each input vertex (0,1)
+		const Vec2f v0a_xy = v0aNDC*clip0[3];
+		const Vec2f v0b_xy = v0bNDC*clip0[3];
+		const Vec2f v1a_xy = v1aNDC*clip1[3];
+		const Vec2f v1b_xy = v1bNDC*clip1[3];
 
 		// Final 4D coordinates of the new vertices
 		const auto v0a = Vec4f(v0a_xy[0], v0a_xy[1], clip0[2], clip0[3]);