Add GM for hard stop gradients

gm/hardstop_gradients.cpp

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/*
+ * This GM presents a variety of different gradients with different
+ * tile modes. Each entry in the table is a rectangle with a linear
+ * gradient that spans from its left edge to its right edge. The rows
+ * in the table represent different color/position configurations,
+ * while the columns in the table represent different tile modes. In
+ * order to highlight the differences between tile modes, the gradient
+ * starts and ends at 30 pixel inset from either side of the rectangle.
+ *
+ *                      | Clamp         Repeat          Mirror
+ * _____________________|___________________________________________
+ * 2-color              | rect00        rect01          rect02
+ * 3-color              | rect10        rect11          rect12
+ * 5-color hard stop    | rect20        rect21          rect22
+ * 5-color edge case 1  | rect30        rect31          rect32
+ * 5-color edge case 2  | rect40        rect41          rect42
+ *
+ * The LAST two t-values in edge case 1 are the same, while the FIRST 
+ * two t-values in edge case 2 are the same.
+ */
+
+#include "gm.h"
+
+#include "SkGradientShader.h"
+
+const int WIDTH  = 500;
+const int HEIGHT = 500;
+
+const int NUM_ROWS = 5;
+const int NUM_COLS = 3;
+
+const int CELL_WIDTH  = WIDTH  / NUM_COLS;
+const int CELL_HEIGHT = HEIGHT / NUM_ROWS;
+
+const int PAD_WIDTH  = 3;
+const int PAD_HEIGHT = 3;
+
+const int RECT_WIDTH  = CELL_WIDTH  - (2 * PAD_WIDTH);
+const int RECT_HEIGHT = CELL_HEIGHT - (2 * PAD_HEIGHT);
+
+static void shadeRect(SkCanvas* canvas, sk_sp<SkShader> shader, int cell_row, int cell_col) {
+    SkPaint paint;
+    paint.setShader(shader);
+    
+    SkRect rect = SkRect::MakeXYWH(cell_col * CELL_WIDTH  + PAD_WIDTH,
+                                   cell_row * CELL_HEIGHT + PAD_HEIGHT,
+                                   RECT_WIDTH,
+                                   RECT_HEIGHT);
+
+    canvas->drawRect(rect, paint);
+}
+
+static void createGradientPoints(int cell_row, int cell_col, SkPoint points[2]) {
+    const int X_OFFSET = 30;
+
+    int x0 = cell_col     * CELL_WIDTH + PAD_WIDTH + X_OFFSET;
+    int x1 = (cell_col+1) * CELL_WIDTH - PAD_WIDTH - X_OFFSET;
+    int y  = cell_row * CELL_HEIGHT + PAD_HEIGHT + RECT_HEIGHT/2;
+
+    points[0] = SkPoint::Make(x0, y);
+    points[1] = SkPoint::Make(x1, y);
+}
+
+class HardstopGradientShaderGM : public skiagm::GM {
+public:
+    HardstopGradientShaderGM() {
+
+    }
+
+protected:
+    SkString onShortName() override {
+        return SkString("hardstop_gradients");
+    }
+
+    SkISize onISize() override {
+        return SkISize::Make(512, 512);
+    }
+
+    void onDraw(SkCanvas* canvas) {
+        SkPoint points[2];
+
+        SkColor colors[] = {
+            SK_ColorRED,
+            SK_ColorGREEN,
+            SK_ColorBLUE,
+            SK_ColorYELLOW,
+            SK_ColorMAGENTA,
+        };
+
+        SkScalar row3[] = {0.00f, 0.25f, 0.50f, 0.50f, 1.00f};
+        SkScalar row4[] = {0.00f, 0.25f, 0.50f, 1.00f, 1.00f};
+        SkScalar row5[] = {0.00f, 0.00f, 0.50f, 0.50f, 1.00f};
+
+        SkScalar* positions[] = {
+            nullptr,
+            nullptr,
+            row3,
+            row4,
+            row5,
+        };
+
+        int num_gradient_colors[] = {
+            2,
+            3,
+            5,
+            5,
+            5,
+        };
+
+        SkShader::TileMode tilemodes[] = {
+            SkShader::kClamp_TileMode,
+            SkShader::kRepeat_TileMode,
+            SkShader::kMirror_TileMode,
+        };
+
+        for (int cell_row = 0; cell_row < NUM_ROWS; cell_row++) {

[tomhudson, 2016/06/22 15:54:06]

Nit: revisit this function after reading https://skia.org/dev/contrib/style.

Otherwise LGTM

+            for (int cell_col = 0; cell_col < NUM_COLS; cell_col++) {
+                createGradientPoints(cell_row, cell_col, points);
+
+                auto shader = SkGradientShader::MakeLinear(
+                                points,
+                                colors,
+                                positions[cell_row],
+                                num_gradient_colors[cell_row],
+                                tilemodes[cell_col],
+                                0,
+                                nullptr);
+
+                shadeRect(canvas, shader, cell_row, cell_col);
+            }
+        } 
+    }
+
+private:
+    typedef skiagm::GM INHERITED;
+};
+
+DEF_GM(return new HardstopGradientShaderGM;)