Allow small strokes, fixed textBoxBounds()

- added test for multiple stroke widths
- allow stroke width down to 1px size, then scale with alpha
- fixed textBoxBounds() in hi-dpi

example/demo.c

@@ -915,6 +915,27 @@ void drawParagraph(struct NVGcontext* vg, float x, float y, float width, float h
 	nvgRestore(vg);
 }
 
+void drawWidths(struct NVGcontext* vg, float x, float y, float width)
+{
+	int i;
+
+	nvgSave(vg);
+
+	nvgStrokeColor(vg, nvgRGBA(0,0,0,255));
+
+	for (i = 0; i < 20; i++) {
+		float w = (i+0.5f)*0.1f;
+		nvgStrokeWidth(vg, w);
+		nvgBeginPath(vg);
+		nvgMoveTo(vg, x,y);
+		nvgLineTo(vg, x+width,y+width*0.3f);
+		nvgStroke(vg);
+		y += 10;
+	}
+
+	nvgRestore(vg);
+}
+
 void renderDemo(struct NVGcontext* vg, float mx, float my, float width, float height,
 				float t, int blowup, struct DemoData* data)
 {
@@ -928,6 +949,9 @@ void renderDemo(struct NVGcontext* vg, float mx, float my, float width, float he
 	// Line joints
 	drawLines(vg, 50, height-50, 600, 50, t);
 
+	// Line width;
+	drawWidths(vg, 10, 50, 30);
+
 	nvgSave(vg);
 	if (blowup) {
 		nvgRotate(vg, sinf(t*0.3f)*5.0f/180.0f*NVG_PI);

src/nanovg.c

@@ -1840,17 +1840,27 @@ void nvgStroke(struct NVGcontext* ctx)
 {
 	struct NVGstate* state = nvg__getState(ctx);
 	float scale = nvg__getAverageScale(state->xform);
-	float strokeWidth = nvg__clampf(state->strokeWidth * scale, 1.0f, 20.0f);
+	float strokeWidth = nvg__clampf(state->strokeWidth * scale, 0.0f, 20.0f);
+	struct NVGpaint strokePaint = state->stroke;
 	const struct NVGpath* path;
 	int i;
 
+	if (strokeWidth < ctx->fringeWidth) {
+		// If the stroke width is less than pixel size, use alpha to emulate coverate.
+		// Since coverage is area, scale by alpha*alpha.
+		float alpha = nvg__clampf(strokeWidth / ctx->fringeWidth, 0.0f, 1.0f);
+		strokePaint.innerColor.a *= alpha*alpha;
+		strokePaint.outerColor.a *= alpha*alpha;
+		strokeWidth = ctx->fringeWidth;
+	}
+
 	nvg__flattenPaths(ctx);
 	if (ctx->params.edgeAntiAlias)
 		nvg__expandStrokeAndFill(ctx, NVG_STROKE|NVG_CAPS, strokeWidth*0.5f + ctx->fringeWidth*0.5f, state->lineCap, state->lineJoin, state->miterLimit);
 	else
 		nvg__expandStrokeAndFill(ctx, NVG_STROKE|NVG_CAPS, strokeWidth*0.5f, state->lineCap, state->lineJoin, state->miterLimit);
 
-	ctx->params.renderStroke(ctx->params.userPtr, &state->stroke, &state->scissor, ctx->fringeWidth,
+	ctx->params.renderStroke(ctx->params.userPtr, &strokePaint, &state->scissor, ctx->fringeWidth,
 							 strokeWidth, ctx->cache->paths, ctx->cache->npaths);
 
 	// Count triangles
@@ -2359,10 +2369,10 @@ void nvgTextBoxBounds(struct NVGcontext* ctx, float x, float y, float breakRowWi
 	state->textAlign = oldAlign;
 
 	if (bounds != NULL) {
-		bounds[0] = minx * invscale;
-		bounds[1] = miny * invscale;
-		bounds[2] = maxx * invscale;
-		bounds[3] = maxy * invscale;
+		bounds[0] = minx;
+		bounds[1] = miny;
+		bounds[2] = maxx;
+		bounds[3] = maxy;
 	}
 }