From e24a872fad485783729cc2a7f08cc0f494367bc9 Mon Sep 17 00:00:00 2001
From: martinfouilleul <martinfouilleul@gmail.com>
Date: Mon, 3 Jul 2023 15:16:27 +0200
Subject: [PATCH] [wip, win32, canvas] encode strokes

---
 src/gl_canvas.c | 656 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 655 insertions(+), 1 deletion(-)

diff --git a/src/gl_canvas.c b/src/gl_canvas.c
index c792647..48745a2 100644
--- a/src/gl_canvas.c
+++ b/src/gl_canvas.c
@@ -209,6 +209,660 @@ void mg_gl_canvas_encode_element(mg_gl_encoding_context* context, mg_path_elt_ty
 	}
 }
 
+bool mg_intersect_hull_legs(vec2 p0, vec2 p1, vec2 p2, vec2 p3, vec2* intersection)
+{
+	/*NOTE: check intersection of lines (p0-p1) and (p2-p3)
+
+		P = p0 + u(p1-p0)
+		P = p2 + w(p3-p2)
+	*/
+	bool found = false;
+
+	f32 den = (p0.x - p1.x)*(p2.y - p3.y) - (p0.y - p1.y)*(p2.x - p3.x);
+	if(fabs(den) > 0.0001)
+	{
+		f32 u = ((p0.x - p2.x)*(p2.y - p3.y) - (p0.y - p2.y)*(p2.x - p3.x))/den;
+		f32 w = ((p0.x - p2.x)*(p0.y - p1.y) - (p0.y - p2.y)*(p0.x - p1.x))/den;
+
+		intersection->x = p0.x + u*(p1.x - p0.x);
+		intersection->y = p0.y + u*(p1.y - p0.y);
+		found = true;
+	}
+	return(found);
+}
+
+bool mg_offset_hull(int count, vec2* p, vec2* result, f32 offset)
+{
+	//NOTE: we should have no more than two coincident points here. This means the leg between
+	//      those two points can't be offset, but we can set a double point at the start of first leg,
+	//      end of first leg, or we can join the first and last leg to create a missing middle one
+
+	vec2 legs[3][2] = {0};
+	bool valid[3] = {0};
+
+	for(int i=0; i<count-1; i++)
+	{
+		vec2 n = {p[i].y - p[i+1].y,
+	              p[i+1].x - p[i].x};
+
+		f32 norm = sqrt(n.x*n.x + n.y*n.y);
+		if(norm >= 1e-6)
+		{
+			n = vec2_mul(offset/norm, n);
+			legs[i][0] = vec2_add(p[i], n);
+			legs[i][1] = vec2_add(p[i+1], n);
+			valid[i] = true;
+		}
+	}
+
+	//NOTE: now we find intersections
+
+	// first point is either the start of the first or second leg
+	if(valid[0])
+	{
+		result[0] = legs[0][0];
+	}
+	else
+	{
+		ASSERT(valid[1]);
+		result[0] = legs[1][0];
+	}
+
+	for(int i=1; i<count-1; i++)
+	{
+		//NOTE: we're computing the control point i, at the end of leg (i-1)
+
+		if(!valid[i-1])
+		{
+			ASSERT(valid[i]);
+			result[i] = legs[i][0];
+		}
+		else if(!valid[i])
+		{
+			ASSERT(valid[i-1]);
+			result[i] = legs[i-1][0];
+		}
+		else
+		{
+			if(!mg_intersect_hull_legs(legs[i-1][0], legs[i-1][1], legs[i][0], legs[i][1], &result[i]))
+			{
+				// legs don't intersect.
+				return(false);
+			}
+		}
+	}
+
+	if(valid[count-2])
+	{
+		result[count-1] = legs[count-2][1];
+	}
+	else
+	{
+		ASSERT(valid[count-3]);
+		result[count-1] = legs[count-3][1];
+	}
+
+	return(true);
+}
+
+vec2 mg_quadratic_get_point(vec2 p[3], f32 t)
+{
+	vec2 r;
+
+	f32 oneMt = 1-t;
+	f32 oneMt2 = Square(oneMt);
+	f32 t2 = Square(t);
+
+	r.x = oneMt2*p[0].x + 2*oneMt*t*p[1].x + t2*p[2].x;
+	r.y = oneMt2*p[0].y + 2*oneMt*t*p[1].y + t2*p[2].y;
+
+	return(r);
+}
+
+void mg_quadratic_split(vec2 p[3], f32 t, vec2 outLeft[3], vec2 outRight[3])
+{
+	//NOTE(martin): split bezier curve p at parameter t, using De Casteljau's algorithm
+	//              the q_n are the points along the hull's segments at parameter t
+	//              s is the split point.
+
+	f32 oneMt = 1-t;
+
+	vec2 q0 = {oneMt*p[0].x + t*p[1].x,
+		   oneMt*p[0].y + t*p[1].y};
+
+	vec2 q1 = {oneMt*p[1].x + t*p[2].x,
+		   oneMt*p[1].y + t*p[2].y};
+
+	vec2 s = {oneMt*q0.x + t*q1.x,
+		   oneMt*q0.y + t*q1.y};
+
+	outLeft[0] = p[0];
+	outLeft[1] = q0;
+	outLeft[2] = s;
+
+	outRight[0] = s;
+	outRight[1] = q1;
+	outRight[2] = p[2];
+}
+
+vec2 mg_cubic_get_point(vec2 p[4], f32 t)
+{
+	vec2 r;
+
+	f32 oneMt = 1-t;
+	f32 oneMt2 = Square(oneMt);
+	f32 oneMt3 = oneMt2*oneMt;
+	f32 t2 = Square(t);
+	f32 t3 = t2*t;
+
+	r.x = oneMt3*p[0].x + 3*oneMt2*t*p[1].x + 3*oneMt*t2*p[2].x + t3*p[3].x;
+	r.y = oneMt3*p[0].y + 3*oneMt2*t*p[1].y + 3*oneMt*t2*p[2].y + t3*p[3].y;
+
+	return(r);
+}
+
+void mg_cubic_split(vec2 p[4], f32 t, vec2 outLeft[4], vec2 outRight[4])
+{
+	//NOTE(martin): split bezier curve p at parameter t, using De Casteljau's algorithm
+	//              the q_n are the points along the hull's segments at parameter t
+	//              the r_n are the points along the (q_n, q_n+1) segments at parameter t
+	//              s is the split point.
+
+	vec2 q0 = {(1-t)*p[0].x + t*p[1].x,
+	           (1-t)*p[0].y + t*p[1].y};
+
+	vec2 q1 = {(1-t)*p[1].x + t*p[2].x,
+	           (1-t)*p[1].y + t*p[2].y};
+
+	vec2 q2 = {(1-t)*p[2].x + t*p[3].x,
+	           (1-t)*p[2].y + t*p[3].y};
+
+	vec2 r0 = {(1-t)*q0.x + t*q1.x,
+	           (1-t)*q0.y + t*q1.y};
+
+	vec2 r1 = {(1-t)*q1.x + t*q2.x,
+	           (1-t)*q1.y + t*q2.y};
+
+	vec2 s = {(1-t)*r0.x + t*r1.x,
+	          (1-t)*r0.y + t*r1.y};;
+
+	outLeft[0] = p[0];
+	outLeft[1] = q0;
+	outLeft[2] = r0;
+	outLeft[3] = s;
+
+	outRight[0] = s;
+	outRight[1] = r1;
+	outRight[2] = q2;
+	outRight[3] = p[3];
+}
+
+void mg_gl_encode_stroke_line(mg_gl_encoding_context* context, vec2* p)
+{
+	f32 width = context->primitive->attributes.width;
+
+	vec2 v = {p[1].x-p[0].x, p[1].y-p[0].y};
+	vec2 n = {v.y, -v.x};
+	f32 norm = sqrt(n.x*n.x + n.y*n.y);
+	vec2 offset = vec2_mul(0.5*width/norm, n);
+
+	vec2 left[2] = {vec2_add(p[0], offset), vec2_add(p[1], offset)};
+	vec2 right[2] = {vec2_add(p[1], vec2_mul(-1, offset)), vec2_add(p[0], vec2_mul(-1, offset))};
+	vec2 joint0[2] = {vec2_add(p[0], vec2_mul(-1, offset)), vec2_add(p[0], offset)};
+	vec2 joint1[2] = {vec2_add(p[1], offset), vec2_add(p[1], vec2_mul(-1, offset))};
+
+	mg_gl_canvas_encode_element(context, MG_PATH_LINE, right);
+
+	mg_gl_canvas_encode_element(context, MG_PATH_LINE, left);
+	mg_gl_canvas_encode_element(context, MG_PATH_LINE, joint0);
+	mg_gl_canvas_encode_element(context, MG_PATH_LINE, joint1);
+}
+
+enum { MG_HULL_CHECK_SAMPLE_COUNT = 5 };
+
+void mg_gl_encode_stroke_quadratic(mg_gl_encoding_context* context, vec2* p)
+{
+	f32 width = context->primitive->attributes.width;
+	f32 tolerance = minimum(context->primitive->attributes.tolerance, 0.5 * width);
+
+	//NOTE: check for degenerate line case
+	const f32 equalEps = 1e-3;
+	if(vec2_close(p[0], p[1], equalEps))
+	{
+		mg_gl_encode_stroke_line(context, p+1);
+		return;
+	}
+	else if(vec2_close(p[1], p[2], equalEps))
+	{
+		mg_gl_encode_stroke_line(context, p);
+		return;
+	}
+
+	vec2 leftHull[3];
+	vec2 rightHull[3];
+
+	if(  !mg_offset_hull(3, p, leftHull, width/2)
+	  || !mg_offset_hull(3, p, rightHull, -width/2))
+	{
+		//TODO split and recurse
+		//NOTE: offsetting the hull failed, split the curve
+		vec2 splitLeft[3];
+		vec2 splitRight[3];
+		mg_quadratic_split(p, 0.5, splitLeft, splitRight);
+		mg_gl_encode_stroke_quadratic(context, splitLeft);
+		mg_gl_encode_stroke_quadratic(context, splitRight);
+	}
+	else
+	{
+		f32 checkSamples[MG_HULL_CHECK_SAMPLE_COUNT] = {1./6, 2./6, 3./6, 4./6, 5./6};
+
+		f32 d2LowBound = Square(0.5 * width - tolerance);
+		f32 d2HighBound = Square(0.5 * width + tolerance);
+
+		f32 maxOvershoot = 0;
+		f32 maxOvershootParameter = 0;
+
+		for(int i=0; i<MG_HULL_CHECK_SAMPLE_COUNT; i++)
+		{
+			f32 t = checkSamples[i];
+
+			vec2 c = mg_quadratic_get_point(p, t);
+			vec2 cp =  mg_quadratic_get_point(leftHull, t);
+			vec2 cn =  mg_quadratic_get_point(rightHull, t);
+
+			f32 positiveDistSquare = Square(c.x - cp.x) + Square(c.y - cp.y);
+			f32 negativeDistSquare = Square(c.x - cn.x) + Square(c.y - cn.y);
+
+			f32 positiveOvershoot = maximum(positiveDistSquare - d2HighBound, d2LowBound - positiveDistSquare);
+			f32 negativeOvershoot = maximum(negativeDistSquare - d2HighBound, d2LowBound - negativeDistSquare);
+
+			f32 overshoot = maximum(positiveOvershoot, negativeOvershoot);
+
+			if(overshoot > maxOvershoot)
+			{
+				maxOvershoot = overshoot;
+				maxOvershootParameter = t;
+			}
+		}
+
+		if(maxOvershoot > 0)
+		{
+			vec2 splitLeft[3];
+			vec2 splitRight[3];
+			mg_quadratic_split(p, maxOvershootParameter, splitLeft, splitRight);
+			mg_gl_encode_stroke_quadratic(context, splitLeft);
+			mg_gl_encode_stroke_quadratic(context, splitRight);
+		}
+		else
+		{
+			vec2 tmp = leftHull[0];
+			leftHull[0] = leftHull[2];
+			leftHull[2] = tmp;
+
+			mg_gl_canvas_encode_element(context, MG_PATH_QUADRATIC, rightHull);
+			mg_gl_canvas_encode_element(context, MG_PATH_QUADRATIC, leftHull);
+
+			vec2 joint0[2] = {rightHull[2], leftHull[0]};
+			vec2 joint1[2] = {leftHull[2], rightHull[0]};
+			mg_gl_canvas_encode_element(context, MG_PATH_LINE, joint0);
+			mg_gl_canvas_encode_element(context, MG_PATH_LINE, joint1);
+		}
+	}
+}
+
+void mg_gl_encode_stroke_cubic(mg_gl_encoding_context* context, vec2* p)
+{
+	f32 width = context->primitive->attributes.width;
+	f32 tolerance = minimum(context->primitive->attributes.tolerance, 0.5 * width);
+
+	//NOTE: check degenerate line cases
+	f32 equalEps = 1e-3;
+
+	if( (vec2_close(p[0], p[1], equalEps) && vec2_close(p[2], p[3], equalEps))
+	  ||(vec2_close(p[0], p[1], equalEps) && vec2_close(p[1], p[2], equalEps))
+	  ||(vec2_close(p[1], p[2], equalEps) && vec2_close(p[2], p[3], equalEps)))
+	{
+		vec2 line[2] = {p[0], p[3]};
+		mg_gl_encode_stroke_line(context, line);
+		return;
+	}
+	else if(vec2_close(p[0], p[1], equalEps) && vec2_close(p[1], p[3], equalEps))
+	{
+		vec2 line[2] = {p[0], vec2_add(vec2_mul(5./9, p[0]), vec2_mul(4./9, p[2]))};
+		mg_gl_encode_stroke_line(context, line);
+		return;
+	}
+	else if(vec2_close(p[0], p[2], equalEps) && vec2_close(p[2], p[3], equalEps))
+	{
+		vec2 line[2] = {p[0], vec2_add(vec2_mul(5./9, p[0]), vec2_mul(4./9, p[1]))};
+		mg_gl_encode_stroke_line(context, line);
+		return;
+	}
+
+	vec2 leftHull[4];
+	vec2 rightHull[4];
+
+	if(  !mg_offset_hull(4, p, leftHull, width/2)
+	  || !mg_offset_hull(4, p, rightHull, -width/2))
+	{
+		//TODO split and recurse
+		//NOTE: offsetting the hull failed, split the curve
+		vec2 splitLeft[4];
+		vec2 splitRight[4];
+		mg_cubic_split(p, 0.5, splitLeft, splitRight);
+		mg_gl_encode_stroke_cubic(context, splitLeft);
+		mg_gl_encode_stroke_cubic(context, splitRight);
+	}
+	else
+	{
+		f32 checkSamples[MG_HULL_CHECK_SAMPLE_COUNT] = {1./6, 2./6, 3./6, 4./6, 5./6};
+
+		f32 d2LowBound = Square(0.5 * width - tolerance);
+		f32 d2HighBound = Square(0.5 * width + tolerance);
+
+		f32 maxOvershoot = 0;
+		f32 maxOvershootParameter = 0;
+
+		for(int i=0; i<MG_HULL_CHECK_SAMPLE_COUNT; i++)
+		{
+			f32 t = checkSamples[i];
+
+			vec2 c = mg_cubic_get_point(p, t);
+			vec2 cp =  mg_cubic_get_point(leftHull, t);
+			vec2 cn =  mg_cubic_get_point(rightHull, t);
+
+			f32 positiveDistSquare = Square(c.x - cp.x) + Square(c.y - cp.y);
+			f32 negativeDistSquare = Square(c.x - cn.x) + Square(c.y - cn.y);
+
+			f32 positiveOvershoot = maximum(positiveDistSquare - d2HighBound, d2LowBound - positiveDistSquare);
+			f32 negativeOvershoot = maximum(negativeDistSquare - d2HighBound, d2LowBound - negativeDistSquare);
+
+			f32 overshoot = maximum(positiveOvershoot, negativeOvershoot);
+
+			if(overshoot > maxOvershoot)
+			{
+				maxOvershoot = overshoot;
+				maxOvershootParameter = t;
+			}
+		}
+
+		if(maxOvershoot > 0)
+		{
+			vec2 splitLeft[4];
+			vec2 splitRight[4];
+			mg_cubic_split(p, maxOvershootParameter, splitLeft, splitRight);
+			mg_gl_encode_stroke_cubic(context, splitLeft);
+			mg_gl_encode_stroke_cubic(context, splitRight);
+		}
+		else
+		{
+			vec2 tmp = leftHull[0];
+			leftHull[0] = leftHull[3];
+			leftHull[3] = tmp;
+			tmp = leftHull[1];
+			leftHull[1] = leftHull[2];
+			leftHull[2] = tmp;
+
+			mg_gl_canvas_encode_element(context, MG_PATH_CUBIC, rightHull);
+			mg_gl_canvas_encode_element(context, MG_PATH_CUBIC, leftHull);
+
+			vec2 joint0[2] = {rightHull[3], leftHull[0]};
+			vec2 joint1[2] = {leftHull[3], rightHull[0]};
+			mg_gl_canvas_encode_element(context, MG_PATH_LINE, joint0);
+			mg_gl_canvas_encode_element(context, MG_PATH_LINE, joint1);
+		}
+	}
+}
+
+void mg_gl_encode_stroke_element(mg_gl_encoding_context* context,
+                                  mg_path_elt* element,
+                                  vec2 currentPoint,
+                                  vec2* startTangent,
+                                  vec2* endTangent,
+                                  vec2* endPoint)
+{
+	vec2 controlPoints[4] = {currentPoint, element->p[0], element->p[1], element->p[2]};
+	int endPointIndex = 0;
+
+	switch(element->type)
+	{
+		case MG_PATH_LINE:
+			mg_gl_encode_stroke_line(context, controlPoints);
+			endPointIndex = 1;
+			break;
+
+		case MG_PATH_QUADRATIC:
+			mg_gl_encode_stroke_quadratic(context, controlPoints);
+			endPointIndex = 2;
+			break;
+
+		case MG_PATH_CUBIC:
+			mg_gl_encode_stroke_cubic(context, controlPoints);
+			endPointIndex = 3;
+			break;
+
+		case MG_PATH_MOVE:
+			ASSERT(0, "should be unreachable");
+			break;
+	}
+
+	//NOTE: ensure tangents are properly computed even in presence of coincident points
+	//TODO: see if we can do this in a less hacky way
+
+	for(int i=1; i<4; i++)
+	{
+		if(  controlPoints[i].x != controlPoints[0].x
+		  || controlPoints[i].y != controlPoints[0].y)
+		{
+			*startTangent = (vec2){.x = controlPoints[i].x - controlPoints[0].x,
+			                       .y = controlPoints[i].y - controlPoints[0].y};
+			break;
+		}
+	}
+	*endPoint = controlPoints[endPointIndex];
+
+	for(int i=endPointIndex-1; i>=0; i++)
+	{
+		if(  controlPoints[i].x != endPoint->x
+		  || controlPoints[i].y != endPoint->y)
+		{
+			*endTangent = (vec2){.x = endPoint->x - controlPoints[i].x,
+			                     .y = endPoint->y - controlPoints[i].y};
+			break;
+		}
+	}
+	DEBUG_ASSERT(startTangent->x != 0 || startTangent->y != 0);
+}
+
+void mg_gl_stroke_cap(mg_gl_encoding_context* context,
+                       vec2 p0,
+                       vec2 direction)
+{
+	mg_attributes* attributes = &context->primitive->attributes;
+
+	//NOTE(martin): compute the tangent and normal vectors (multiplied by half width) at the cap point
+	f32 dn = sqrt(Square(direction.x) + Square(direction.y));
+	f32 alpha = 0.5 * attributes->width/dn;
+
+	vec2 n0 = {-alpha*direction.y,
+		    alpha*direction.x};
+
+	vec2 m0 = {alpha*direction.x,
+	           alpha*direction.y};
+
+	vec2 points[] = {{p0.x + n0.x, p0.y + n0.y},
+	                 {p0.x + n0.x + m0.x, p0.y + n0.y + m0.y},
+	                 {p0.x - n0.x + m0.x, p0.y - n0.y + m0.y},
+	                 {p0.x - n0.x, p0.y - n0.y},
+	                 {p0.x + n0.x, p0.y + n0.y}};
+
+	mg_gl_canvas_encode_element(context, MG_PATH_LINE, points);
+	mg_gl_canvas_encode_element(context, MG_PATH_LINE, points+1);
+	mg_gl_canvas_encode_element(context, MG_PATH_LINE, points+2);
+	mg_gl_canvas_encode_element(context, MG_PATH_LINE, points+3);
+}
+
+void mg_gl_stroke_joint(mg_gl_encoding_context* context,
+                         vec2 p0,
+                         vec2 t0,
+                         vec2 t1)
+{
+	mg_attributes* attributes = &context->primitive->attributes;
+
+	//NOTE(martin): compute the normals at the joint point
+	f32 norm_t0 = sqrt(Square(t0.x) + Square(t0.y));
+	f32 norm_t1 = sqrt(Square(t1.x) + Square(t1.y));
+
+	vec2 n0 = {-t0.y, t0.x};
+	n0.x /= norm_t0;
+	n0.y /= norm_t0;
+
+	vec2 n1 = {-t1.y, t1.x};
+	n1.x /= norm_t1;
+	n1.y /= norm_t1;
+
+	//NOTE(martin): the sign of the cross product determines if the normals are facing outwards or inwards the angle.
+	//              we flip them to face outwards if needed
+	f32 crossZ = n0.x*n1.y - n0.y*n1.x;
+	if(crossZ > 0)
+	{
+		n0.x *= -1;
+		n0.y *= -1;
+		n1.x *= -1;
+		n1.y *= -1;
+	}
+
+	//NOTE(martin): use the same code as hull offset to find mitter point...
+	/*NOTE(martin): let vector u = (n0+n1) and vector v = pIntersect - p1
+		then v = u * (2*offset / norm(u)^2)
+		(this can be derived from writing the pythagoras theorems in the triangles of the joint)
+	*/
+	f32 halfW = 0.5 * attributes->width;
+	vec2 u = {n0.x + n1.x, n0.y + n1.y};
+	f32 uNormSquare = u.x*u.x + u.y*u.y;
+	f32 alpha = attributes->width / uNormSquare;
+	vec2 v = {u.x * alpha, u.y * alpha};
+
+	f32 excursionSquare = uNormSquare * Square(alpha - attributes->width/4);
+
+	if(  attributes->joint == MG_JOINT_MITER
+	  && excursionSquare <= Square(attributes->maxJointExcursion))
+	{
+		//NOTE(martin): add a mitter joint
+		vec2 points[] = {p0,
+		                 {p0.x + n0.x*halfW, p0.y + n0.y*halfW},
+		                 {p0.x + v.x, p0.y + v.y},
+		                 {p0.x + n1.x*halfW, p0.y + n1.y*halfW},
+		                 p0};
+
+		mg_gl_canvas_encode_element(context, MG_PATH_LINE, points);
+		mg_gl_canvas_encode_element(context, MG_PATH_LINE, points+1);
+		mg_gl_canvas_encode_element(context, MG_PATH_LINE, points+2);
+		mg_gl_canvas_encode_element(context, MG_PATH_LINE, points+3);
+	}
+	else
+	{
+		//NOTE(martin): add a bevel joint
+		vec2 points[] = {p0,
+		                 {p0.x + n0.x*halfW, p0.y + n0.y*halfW},
+		                 {p0.x + n1.x*halfW, p0.y + n1.y*halfW},
+		                 p0};
+
+		mg_gl_canvas_encode_element(context, MG_PATH_LINE, points);
+		mg_gl_canvas_encode_element(context, MG_PATH_LINE, points+1);
+		mg_gl_canvas_encode_element(context, MG_PATH_LINE, points+2);
+	}
+}
+
+u32 mg_gl_encode_stroke_subpath(mg_gl_encoding_context* context,
+                                 mg_path_elt* elements,
+                                 mg_path_descriptor* path,
+                                 u32 startIndex,
+                                 vec2 startPoint)
+{
+	u32 eltCount = path->count;
+	DEBUG_ASSERT(startIndex < eltCount);
+
+	vec2 currentPoint = startPoint;
+	vec2 endPoint = {0, 0};
+	vec2 previousEndTangent = {0, 0};
+	vec2 firstTangent = {0, 0};
+	vec2 startTangent = {0, 0};
+	vec2 endTangent = {0, 0};
+
+	//NOTE(martin): encode first element and compute first tangent
+	mg_gl_encode_stroke_element(context, elements + startIndex, currentPoint, &startTangent, &endTangent, &endPoint);
+
+	firstTangent = startTangent;
+	previousEndTangent = endTangent;
+	currentPoint = endPoint;
+
+	//NOTE(martin): encode subsequent elements along with their joints
+
+	mg_attributes* attributes = &context->primitive->attributes;
+
+	u32 eltIndex = startIndex + 1;
+	for(;
+	    eltIndex<eltCount && elements[eltIndex].type != MG_PATH_MOVE;
+	    eltIndex++)
+	{
+		mg_gl_encode_stroke_element(context, elements + eltIndex, currentPoint, &startTangent, &endTangent, &endPoint);
+
+		if(attributes->joint != MG_JOINT_NONE)
+		{
+			mg_gl_stroke_joint(context, currentPoint, previousEndTangent, startTangent);
+		}
+		previousEndTangent = endTangent;
+		currentPoint = endPoint;
+	}
+	u32 subPathEltCount = eltIndex - startIndex;
+
+	//NOTE(martin): draw end cap / joint. We ensure there's at least two segments to draw a closing joint
+	if(  subPathEltCount > 1
+	  && startPoint.x == endPoint.x
+	  && startPoint.y == endPoint.y)
+	{
+		if(attributes->joint != MG_JOINT_NONE)
+		{
+			//NOTE(martin): add a closing joint if the path is closed
+			mg_gl_stroke_joint(context, endPoint, endTangent, firstTangent);
+		}
+	}
+	else if(attributes->cap == MG_CAP_SQUARE)
+	{
+		//NOTE(martin): add start and end cap
+		mg_gl_stroke_cap(context, startPoint, (vec2){-startTangent.x, -startTangent.y});
+		mg_gl_stroke_cap(context, endPoint, endTangent);
+	}
+	return(eltIndex);
+}
+
+void mg_gl_encode_stroke(mg_gl_encoding_context* context,
+                          mg_path_elt* elements,
+                          mg_path_descriptor* path)
+{
+	u32 eltCount = path->count;
+	DEBUG_ASSERT(eltCount);
+
+	vec2 startPoint = path->startPoint;
+	u32 startIndex = 0;
+
+	while(startIndex < eltCount)
+	{
+		//NOTE(martin): eliminate leading moves
+		while(startIndex < eltCount && elements[startIndex].type == MG_PATH_MOVE)
+		{
+			startPoint = elements[startIndex].p[0];
+			startIndex++;
+		}
+		if(startIndex < eltCount)
+		{
+			startIndex = mg_gl_encode_stroke_subpath(context, elements, path, startIndex, startPoint);
+		}
+	}
+}
+
+
 void mg_gl_render_batch(mg_gl_canvas_backend* backend,
                         mg_wgl_surface* surface,
                         int pathCount,
@@ -434,7 +1088,7 @@ void mg_gl_canvas_render(mg_canvas_backend* interface,
 
 			if(primitive->cmd == MG_CMD_STROKE)
 			{
-//TODO				mg_gl_render_stroke(&context, pathElements + primitive->path.startIndex, &primitive->path);
+				mg_gl_encode_stroke(&context, pathElements + primitive->path.startIndex, &primitive->path);
 			}
 			else
 			{