Rework Overstroke GM to show differences between expected and actual

Draw the perpendiculars of the curve with the stroke width. See Cary's
SampleQuadStroker.cpp for what we are attempting to mimic.

BUG=589769,skia:5405,skia:5406
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2218773003

Review-Url: https://codereview.chromium.org/2218773003

1 files changed, 73 insertions, 35 deletions

diff --git a/gm/OverStroke.cpp b/gm/OverStroke.cpp
index 0c5679786c..f7134cd26c 100644
--- a/gm/OverStroke.cpp
+++ b/gm/OverStroke.cpp
@@ -13,6 +13,12 @@
  * overlap and create holes. There is not a really great algorithm for this
  * and several other 2D graphics engines have the same bug.
  *
+ * If we run this using Nvidia Path Renderer with:
+ * `path/to/dm --match OverStroke -w gm_out --gpu --config nvpr16`
+ * then we get correct results, so that is a possible direction of attack -
+ * use the GPU and a completely different algorithm to get correctness in
+ * Skia.
+ *
  * See crbug.com/589769 skbug.com/5405 skbug.com/5406
  */
 
@@ -20,14 +26,28 @@
 #include "gm.h"
 #include "SkPaint.h"
 #include "SkPath.h"
+#include "SkPathMeasure.h"
+
+const SkScalar OVERSTROKE_WIDTH = 500.0f;
+const SkScalar NORMALSTROKE_WIDTH = 3.0f;
 
 //////// path and paint builders
 
+SkPaint make_normal_paint() {
+    SkPaint p;
+    p.setAntiAlias(true);
+    p.setStyle(SkPaint::kStroke_Style);
+    p.setStrokeWidth(NORMALSTROKE_WIDTH);
+    p.setColor(SK_ColorBLUE);
+
+    return p;
+}
+
 SkPaint make_overstroke_paint() {
     SkPaint p;
     p.setAntiAlias(true);
     p.setStyle(SkPaint::kStroke_Style);
-    p.setStrokeWidth(500);
+    p.setStrokeWidth(OVERSTROKE_WIDTH);
 
     return p;
 }
@@ -38,6 +58,7 @@ SkPath quad_path() {
     path.lineTo(100, 0);
     path.quadTo(50, -40,
                 0, 0);
+    path.close();
 
     return path;
 }
@@ -62,19 +83,50 @@ SkPath oval_path() {
     return path;
 }
 
+SkPath ribs_path(SkPath path, SkScalar radius) {
+    SkPath ribs;
+
+    const SkScalar spacing = 5.0f;
+    float accum = 0.0f;
+
+    SkPathMeasure meas(path, false);
+    SkScalar length = meas.getLength();
+    SkPoint pos;
+    SkVector tan;
+    while (accum < length) {
+        if (meas.getPosTan(accum, &pos, &tan)) {
+            tan.scale(radius);
+            tan.rotateCCW();
+
+            ribs.moveTo(pos.x() + tan.x(), pos.y() + tan.y());
+            ribs.lineTo(pos.x() - tan.x(), pos.y() - tan.y());
+        }
+        accum += spacing;
+    }
+
+    return ribs;
+}
+
+void draw_ribs(SkCanvas *canvas, SkPath path) {
+    SkPath ribs = ribs_path(path, OVERSTROKE_WIDTH/2.0f);
+    SkPaint p = make_normal_paint();
+    p.setStrokeWidth(1);
+    p.setColor(SK_ColorBLUE);
+    p.setColor(SK_ColorGREEN);
+
+    canvas->drawPath(ribs, p);
+}
+
 ///////// quads
 
 void draw_small_quad(SkCanvas *canvas) {
     // scaled so it's visible
-    canvas->scale(8, 8);
-
-    SkPaint p;
-    p.setAntiAlias(true);
-    p.setStyle(SkPaint::kStroke_Style);
-    p.setStrokeWidth(3);
+    // canvas->scale(8, 8);
 
+    SkPaint p = make_normal_paint();
     SkPath path = quad_path();
 
+    draw_ribs(canvas, path);
     canvas->drawPath(path, p);
 }
 
@@ -83,15 +135,14 @@ void draw_large_quad(SkCanvas *canvas) {
     SkPath path = quad_path();
 
     canvas->drawPath(path, p);
+    draw_ribs(canvas, path);
 }
 
 void draw_quad_fillpath(SkCanvas *canvas) {
     SkPath path = quad_path();
     SkPaint p = make_overstroke_paint();
 
-    SkPaint fillp;
-    fillp.setAntiAlias(true);
-    fillp.setStyle(SkPaint::kStroke_Style);
+    SkPaint fillp = make_normal_paint();
     fillp.setColor(SK_ColorMAGENTA);
 
     SkPath fillpath;
@@ -101,7 +152,7 @@ void draw_quad_fillpath(SkCanvas *canvas) {
 }
 
 void draw_stroked_quad(SkCanvas *canvas) {
-    canvas->translate(200, 0);
+    canvas->translate(400, 0);
     draw_large_quad(canvas);
     draw_quad_fillpath(canvas);
 }
@@ -109,16 +160,10 @@ void draw_stroked_quad(SkCanvas *canvas) {
 ////////// cubics
 
 void draw_small_cubic(SkCanvas *canvas) {
-    // scaled so it's visible
-    canvas->scale(8, 8);
-
-    SkPaint p;
-    p.setAntiAlias(true);
-    p.setStyle(SkPaint::kStroke_Style);
-    p.setStrokeWidth(3);
-
+    SkPaint p = make_normal_paint();
     SkPath path = cubic_path();
 
+    draw_ribs(canvas, path);
     canvas->drawPath(path, p);
 }
 
@@ -127,15 +172,14 @@ void draw_large_cubic(SkCanvas *canvas) {
     SkPath path = cubic_path();
 
     canvas->drawPath(path, p);
+    draw_ribs(canvas, path);
 }
 
 void draw_cubic_fillpath(SkCanvas *canvas) {
     SkPath path = cubic_path();
     SkPaint p = make_overstroke_paint();
 
-    SkPaint fillp;
-    fillp.setAntiAlias(true);
-    fillp.setStyle(SkPaint::kStroke_Style);
+    SkPaint fillp = make_normal_paint();
     fillp.setColor(SK_ColorMAGENTA);
 
     SkPath fillpath;
@@ -153,16 +197,11 @@ void draw_stroked_cubic(SkCanvas *canvas) {
 ////////// ovals
 
 void draw_small_oval(SkCanvas *canvas) {
-    // scaled so it's visible
-    canvas->scale(8, 8);
-
-    SkPaint p;
-    p.setAntiAlias(true);
-    p.setStyle(SkPaint::kStroke_Style);
-    p.setStrokeWidth(3);
+    SkPaint p = make_normal_paint();
 
     SkPath path = oval_path();
 
+    draw_ribs(canvas, path);
     canvas->drawPath(path, p);
 }
 
@@ -171,15 +210,14 @@ void draw_large_oval(SkCanvas *canvas) {
     SkPath path = oval_path();
 
     canvas->drawPath(path, p);
+    draw_ribs(canvas, path);
 }
 
 void draw_oval_fillpath(SkCanvas *canvas) {
     SkPath path = oval_path();
     SkPaint p = make_overstroke_paint();
 
-    SkPaint fillp;
-    fillp.setAntiAlias(true);
-    fillp.setStyle(SkPaint::kStroke_Style);
+    SkPaint fillp = make_normal_paint();
     fillp.setColor(SK_ColorMAGENTA);
 
     SkPath fillpath;
@@ -210,9 +248,9 @@ DEF_SIMPLE_GM(OverStroke, canvas, 500, 500) {
         int y = (int)(i / width);
 
         canvas->save();
-        canvas->translate(200.0f * x, 150.0f * y);
-        canvas->scale(0.25f, 0.25f);
-        canvas->translate(100.0f, 400.0f);
+        canvas->translate(150.0f * x, 150.0f * y);
+        canvas->scale(0.2f, 0.2f);
+        canvas->translate(300.0f, 400.0f);
 
         examples[i](canvas);