Upstream oodles of Chrome for Android code into Chromium.

chrome/android/java_staging/src/org/chromium/chrome/browser/widget/FadingShadow.java

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.widget;
+
+import android.graphics.Canvas;
+import android.graphics.Color;
+import android.graphics.LinearGradient;
+import android.graphics.Matrix;
+import android.graphics.Paint;
+import android.graphics.Shader;
+import android.view.View;
+
+/**
+ * This class draws a variable-sized shadow at the top or bottom edge of a view. This is just like
+ * a fading edge, except that the shadow color can have an alpha component, whereas a fading edge
+ * color must be opaque.
+ */
+public class FadingShadow {
+
+    public static final int POSITION_TOP = 0;
+    public static final int POSITION_BOTTOM = 1;
+
+    private static final int SMOOTH_ALGORITHM_INTERPOLATION_POINTS_NUM = 8;
+
+    private Paint mShadowPaint = new Paint();
+    private Matrix mShadowMatrix = new Matrix();
+    private Shader mShadowShader;
+
+    /**
+     * @param shadowColor The color of the shadow, e.g. 0x11000000.
+     */
+    public FadingShadow(int shadowColor) {
+        final int n = SMOOTH_ALGORITHM_INTERPOLATION_POINTS_NUM;
+        float[] positions = new float[n];
+        int[] colors = new int[n];
+        int transparentShadowColor = shadowColor & 0x00FFFFFF;
+        int shadowAlpha = Color.alpha(shadowColor);
+
+        // Piece-wise linear interpolation of the smooth cubic function below.
+        for (int i = 0; i < n; ++i) {
+            float x = (float) i / (n - 1);
+            // Polynomial computation by Estrin's scheme.
+            float value = (1.0f - 2.2f * x) + (1.8f - 0.6f * x) * (x * x);
+
+            positions[i] = x;
+            colors[i] = (Math.round(shadowAlpha * value) << 24) | transparentShadowColor;
+        }
+
+        mShadowShader = new LinearGradient(0, 0, 0, 1, colors, positions, Shader.TileMode.CLAMP);
+
+    }
+
+    /**
+     * Draws a shadow at the top or bottom of a view. This should be called from dispatchDraw() so
+     * the shadow is drawn on top of the view's children.
+     *
+     * @param view The View in which to draw the shadow.
+     * @param canvas The canvas on which to draw.
+     * @param position Where to draw the shadow: either POSITION_TOP or POSITION_BOTTOM.
+     * @param shadowHeight The maximum height of the shadow, in pixels.
+     * @param shadowStrength A value between 0 and 1 indicating the relative size of the shadow. 0
+     *                       means no shadow at all. 1 means a full height shadow.
+     */
+    public void drawShadow(View view, Canvas canvas, int position, float shadowHeight,
+            float shadowStrength) {
+        float scaledShadowHeight = Math.max(0.0f, Math.min(1.0f, shadowStrength)) * shadowHeight;
+        if (scaledShadowHeight < 1.0f) return;
+
+        int left = view.getScrollX();
+        int right = left + view.getRight();
+
+        if (position == POSITION_BOTTOM) {
+            int bottom = view.getScrollY() + view.getBottom() - view.getTop();
+            mShadowMatrix.setScale(1, scaledShadowHeight);
+            mShadowMatrix.postRotate(180);
+            mShadowMatrix.postTranslate(left, bottom);
+            mShadowShader.setLocalMatrix(mShadowMatrix);
+            mShadowPaint.setShader(mShadowShader);
+            canvas.drawRect(left, bottom - scaledShadowHeight, right, bottom, mShadowPaint);
+        } else if (position == POSITION_TOP) {
+            int top = view.getScrollY();
+            mShadowMatrix.setScale(1, scaledShadowHeight);
+            mShadowMatrix.postTranslate(left, top);
+            mShadowShader.setLocalMatrix(mShadowMatrix);
+            mShadowPaint.setShader(mShadowShader);
+            canvas.drawRect(left, top, right, top + scaledShadowHeight, mShadowPaint);
+        }
+    }
+}