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 /* -*- Mode: Java; c-basic-offset: 4; tab-width: 20; indent-tabs-mode: nil; -*-

  * This Source Code Form is subject to the terms of the Mozilla Public

  * License, v. 2.0. If a copy of the MPL was not distributed with this file,

  * You can obtain one at http://mozilla.org/MPL/2.0/. */

 package org.mozilla.gecko.gfx;

 import org.mozilla.gecko.GeckoAppShell;

 import org.mozilla.gecko.PrefsHelper;

 import org.mozilla.gecko.util.FloatUtils;

 import org.json.JSONArray;

 import android.graphics.PointF;

 import android.graphics.RectF;

 import android.util.FloatMath;

 import android.util.Log;

 import java.util.HashMap;

 import java.util.Map;

 final class DisplayPortCalculator {

     private static final String LOGTAG = "GeckoDisplayPort";

     private static final PointF ZERO_VELOCITY = new PointF(0, 0);

     // Keep this in sync with the TILEDLAYERBUFFER_TILE_SIZE defined in gfx/layers/TiledLayerBuffer.h

     private static final int TILE_SIZE = 256;

     private static final String PREF_DISPLAYPORT_STRATEGY = "gfx.displayport.strategy";

     private static final String PREF_DISPLAYPORT_FM_MULTIPLIER = "gfx.displayport.strategy_fm.multiplier";

     private static final String PREF_DISPLAYPORT_FM_DANGER_X = "gfx.displayport.strategy_fm.danger_x";

     private static final String PREF_DISPLAYPORT_FM_DANGER_Y = "gfx.displayport.strategy_fm.danger_y";

     private static final String PREF_DISPLAYPORT_VB_MULTIPLIER = "gfx.displayport.strategy_vb.multiplier";

     private static final String PREF_DISPLAYPORT_VB_VELOCITY_THRESHOLD = "gfx.displayport.strategy_vb.threshold";

     private static final String PREF_DISPLAYPORT_VB_REVERSE_BUFFER = "gfx.displayport.strategy_vb.reverse_buffer";

     private static final String PREF_DISPLAYPORT_VB_DANGER_X_BASE = "gfx.displayport.strategy_vb.danger_x_base";

     private static final String PREF_DISPLAYPORT_VB_DANGER_Y_BASE = "gfx.displayport.strategy_vb.danger_y_base";

     private static final String PREF_DISPLAYPORT_VB_DANGER_X_INCR = "gfx.displayport.strategy_vb.danger_x_incr";

     private static final String PREF_DISPLAYPORT_VB_DANGER_Y_INCR = "gfx.displayport.strategy_vb.danger_y_incr";

     private static final String PREF_DISPLAYPORT_PB_VELOCITY_THRESHOLD = "gfx.displayport.strategy_pb.threshold";

     private static DisplayPortStrategy sStrategy = new VelocityBiasStrategy(null);

     static DisplayPortMetrics calculate(ImmutableViewportMetrics metrics, PointF velocity) {

         return sStrategy.calculate(metrics, (velocity == null ? ZERO_VELOCITY : velocity));

     }

     static boolean aboutToCheckerboard(ImmutableViewportMetrics metrics, PointF velocity, DisplayPortMetrics displayPort) {

         if (displayPort == null) {

             return true;

         }

         return sStrategy.aboutToCheckerboard(metrics, (velocity == null ? ZERO_VELOCITY : velocity), displayPort);

     }

     static boolean drawTimeUpdate(long millis, int pixels) {

         return sStrategy.drawTimeUpdate(millis, pixels);

     }

     static void resetPageState() {

         sStrategy.resetPageState();

     }

     static void initPrefs() {

         final String[] prefs = { PREF_DISPLAYPORT_STRATEGY,

                                  PREF_DISPLAYPORT_FM_MULTIPLIER,

                                  PREF_DISPLAYPORT_FM_DANGER_X,

                                  PREF_DISPLAYPORT_FM_DANGER_Y,

                                  PREF_DISPLAYPORT_VB_MULTIPLIER,

                                  PREF_DISPLAYPORT_VB_VELOCITY_THRESHOLD,

                                  PREF_DISPLAYPORT_VB_REVERSE_BUFFER,

                                  PREF_DISPLAYPORT_VB_DANGER_X_BASE,

                                  PREF_DISPLAYPORT_VB_DANGER_Y_BASE,

                                  PREF_DISPLAYPORT_VB_DANGER_X_INCR,

                                  PREF_DISPLAYPORT_VB_DANGER_Y_INCR,

                                  PREF_DISPLAYPORT_PB_VELOCITY_THRESHOLD };

         PrefsHelper.getPrefs(prefs, new PrefsHelper.PrefHandlerBase() {

             private Map<String, Integer> mValues = new HashMap<String, Integer>();

             @Override public void prefValue(String pref, int value) {

                 mValues.put(pref, value);

             }

             @Override public void finish() {

                 setStrategy(mValues);

             }

         });

     }

     /**

      * Set the active strategy to use.

      * See the gfx.displayport.strategy pref in mobile/android/app/mobile.js to see the

      * mapping between ints and strategies.

      */

     static boolean setStrategy(Map<String, Integer> prefs) {

         Integer strategy = prefs.get(PREF_DISPLAYPORT_STRATEGY);

         if (strategy == null) {

             return false;

         }

         switch (strategy) {

             case 0:

                 sStrategy = new FixedMarginStrategy(prefs);

                 break;

             case 1:

                 sStrategy = new VelocityBiasStrategy(prefs);

                 break;

             case 2:

                 sStrategy = new DynamicResolutionStrategy(prefs);

                 break;

             case 3:

                 sStrategy = new NoMarginStrategy(prefs);

                 break;

             case 4:

                 sStrategy = new PredictionBiasStrategy(prefs);

                 break;

             default:

                 Log.e(LOGTAG, "Invalid strategy index specified");

                 return false;

         }

         Log.i(LOGTAG, "Set strategy " + sStrategy.toString());

         return true;

     }

     private static float getFloatPref(Map<String, Integer> prefs, String prefName, int defaultValue) {

         Integer value = (prefs == null ? null : prefs.get(prefName));

         return (float)(value == null || value < 0 ? defaultValue : value) / 1000f;

     }

     private static abstract class DisplayPortStrategy {

         /** Calculates a displayport given a viewport and panning velocity. */

         public abstract DisplayPortMetrics calculate(ImmutableViewportMetrics metrics, PointF velocity);

         /** Returns true if a checkerboard is about to be visible and we should not throttle drawing. */

         public abstract boolean aboutToCheckerboard(ImmutableViewportMetrics metrics, PointF velocity, DisplayPortMetrics displayPort);

         /** Notify the strategy of a new recorded draw time. Return false to turn off draw time recording. */

         public boolean drawTimeUpdate(long millis, int pixels) { return false; }

         /** Reset any page-specific state stored, as the page being displayed has changed. */

         public void resetPageState() {}

     }

     /**

      * Return the dimensions for a rect that has area (width*height) that does not exceed the page size in the

      * given metrics object. The area in the returned FloatSize may be less than width*height if the page is

      * small, but it will never be larger than width*height.

      * Note that this process may change the relative aspect ratio of the given dimensions.

      */

     private static FloatSize reshapeForPage(float width, float height, ImmutableViewportMetrics metrics) {

         // figure out how much of the desired buffer amount we can actually use on the horizontal axis

         float usableWidth = Math.min(width, metrics.getPageWidth());

         // if we reduced the buffer amount on the horizontal axis, we should take that saved memory and

         // use it on the vertical axis

         float extraUsableHeight = (float)Math.floor(((width - usableWidth) * height) / usableWidth);

         float usableHeight = Math.min(height + extraUsableHeight, metrics.getPageHeight());

         if (usableHeight < height && usableWidth == width) {

             // and the reverse - if we shrunk the buffer on the vertical axis we can add it to the horizontal

             float extraUsableWidth = (float)Math.floor(((height - usableHeight) * width) / usableHeight);

             usableWidth = Math.min(width + extraUsableWidth, metrics.getPageWidth());

         }

         return new FloatSize(usableWidth, usableHeight);

     }

     /**

      * Expand the given rect in all directions by a "danger zone". The size of the danger zone on an axis

      * is the size of the view on that axis multiplied by the given multiplier. The expanded rect is then

      * clamped to page bounds and returned.

      */

     private static RectF expandByDangerZone(RectF rect, float dangerZoneXMultiplier, float dangerZoneYMultiplier, ImmutableViewportMetrics metrics) {

         // calculate the danger zone amounts in pixels

         float dangerZoneX = metrics.getWidth() * dangerZoneXMultiplier;

         float dangerZoneY = metrics.getHeight() * dangerZoneYMultiplier;

         rect = RectUtils.expand(rect, dangerZoneX, dangerZoneY);

         // clamp to page bounds

         return clampToPageBounds(rect, metrics);

     }

     /**

      * Expand the given margins such that when they are applied on the viewport, the resulting rect

      * does not have any partial tiles, except when it is clipped by the page bounds. This assumes

      * the tiles are TILE_SIZE by TILE_SIZE and start at the origin, such that there will always be

      * a tile at (0,0)-(TILE_SIZE,TILE_SIZE)).

      */

     private static DisplayPortMetrics getTileAlignedDisplayPortMetrics(RectF margins, float zoom, ImmutableViewportMetrics metrics) {

         float left = metrics.viewportRectLeft - margins.left;

         float top = metrics.viewportRectTop - margins.top;

         float right = metrics.viewportRectRight + margins.right;

         float bottom = metrics.viewportRectBottom + margins.bottom;

         left = Math.max(metrics.pageRectLeft, TILE_SIZE * FloatMath.floor(left / TILE_SIZE));

         top = Math.max(metrics.pageRectTop, TILE_SIZE * FloatMath.floor(top / TILE_SIZE));

         right = Math.min(metrics.pageRectRight, TILE_SIZE * FloatMath.ceil(right / TILE_SIZE));

         bottom = Math.min(metrics.pageRectBottom, TILE_SIZE * FloatMath.ceil(bottom / TILE_SIZE));

         return new DisplayPortMetrics(left, top, right, bottom, zoom);

     }

     /**

      * Adjust the given margins so if they are applied on the viewport in the metrics, the resulting rect

      * does not exceed the page bounds. This code will maintain the total margin amount for a given axis;

      * it assumes that margins.left + metrics.getWidth() + margins.right is less than or equal to

      * metrics.getPageWidth(); and the same for the y axis.

      */

     private static RectF shiftMarginsForPageBounds(RectF margins, ImmutableViewportMetrics metrics) {

         // check how much we're overflowing in each direction. note that at most one of leftOverflow

         // and rightOverflow can be greater than zero, and at most one of topOverflow and bottomOverflow

         // can be greater than zero, because of the assumption described in the method javadoc.

         float leftOverflow = metrics.pageRectLeft - (metrics.viewportRectLeft - margins.left);

         float rightOverflow = (metrics.viewportRectRight + margins.right) - metrics.pageRectRight;

         float topOverflow = metrics.pageRectTop - (metrics.viewportRectTop - margins.top);

         float bottomOverflow = (metrics.viewportRectBottom + margins.bottom) - metrics.pageRectBottom;

         // if the margins overflow the page bounds, shift them to other side on the same axis

         if (leftOverflow > 0) {

             margins.left -= leftOverflow;

             margins.right += leftOverflow;

         } else if (rightOverflow > 0) {

             margins.right -= rightOverflow;

             margins.left += rightOverflow;

         }

         if (topOverflow > 0) {

             margins.top -= topOverflow;

             margins.bottom += topOverflow;

         } else if (bottomOverflow > 0) {

             margins.bottom -= bottomOverflow;

             margins.top += bottomOverflow;

         }

         return margins;

     }

     /**

      * Clamp the given rect to the page bounds and return it.

      */

     private static RectF clampToPageBounds(RectF rect, ImmutableViewportMetrics metrics) {

         if (rect.top < metrics.pageRectTop) rect.top = metrics.pageRectTop;

         if (rect.left < metrics.pageRectLeft) rect.left = metrics.pageRectLeft;

         if (rect.right > metrics.pageRectRight) rect.right = metrics.pageRectRight;

         if (rect.bottom > metrics.pageRectBottom) rect.bottom = metrics.pageRectBottom;

         return rect;

     }

     /**

      * This class implements the variation where we basically don't bother with a a display port.

      */

     private static class NoMarginStrategy extends DisplayPortStrategy {

         NoMarginStrategy(Map<String, Integer> prefs) {

             // no prefs in this strategy

         }

         @Override

         public DisplayPortMetrics calculate(ImmutableViewportMetrics metrics, PointF velocity) {

             return new DisplayPortMetrics(metrics.viewportRectLeft,

                     metrics.viewportRectTop,

                     metrics.viewportRectRight,

                     metrics.viewportRectBottom,

                     metrics.zoomFactor);

         }

         @Override

         public boolean aboutToCheckerboard(ImmutableViewportMetrics metrics, PointF velocity, DisplayPortMetrics displayPort) {

             return true;

         }

         @Override

         public String toString() {

             return "NoMarginStrategy";

         }

     }

     /**

      * This class implements the variation where we use a fixed-size margin on the display port.

      * The margin is always 300 pixels in all directions, except when we are (a) approaching a page

      * boundary, and/or (b) if we are limited by the page size. In these cases we try to maintain

      * the area of the display port by (a) shifting the buffer to the other side on the same axis,

      * and/or (b) increasing the buffer on the other axis to compensate for the reduced buffer on

      * one axis.

      */

     private static class FixedMarginStrategy extends DisplayPortStrategy {

         // The length of each axis of the display port will be the corresponding view length

         // multiplied by this factor.

         private final float SIZE_MULTIPLIER;

         // If the visible rect is within the danger zone (measured as a fraction of the view size

         // from the edge of the displayport) we start redrawing to minimize checkerboarding.

         private final float DANGER_ZONE_X_MULTIPLIER;

         private final float DANGER_ZONE_Y_MULTIPLIER;

         FixedMarginStrategy(Map<String, Integer> prefs) {

             SIZE_MULTIPLIER = getFloatPref(prefs, PREF_DISPLAYPORT_FM_MULTIPLIER, 2000);

             DANGER_ZONE_X_MULTIPLIER = getFloatPref(prefs, PREF_DISPLAYPORT_FM_DANGER_X, 100);

             DANGER_ZONE_Y_MULTIPLIER = getFloatPref(prefs, PREF_DISPLAYPORT_FM_DANGER_Y, 200);

         }

         @Override

         public DisplayPortMetrics calculate(ImmutableViewportMetrics metrics, PointF velocity) {

             float displayPortWidth = metrics.getWidth() * SIZE_MULTIPLIER;

             float displayPortHeight = metrics.getHeight() * SIZE_MULTIPLIER;

             // we need to avoid having a display port that is larger than the page, or we will end up

             // painting things outside the page bounds (bug 729169). we simultaneously need to make

             // the display port as large as possible so that we redraw less. reshape the display

             // port dimensions to accomplish this.

             FloatSize usableSize = reshapeForPage(displayPortWidth, displayPortHeight, metrics);

             float horizontalBuffer = usableSize.width - metrics.getWidth();

             float verticalBuffer = usableSize.height - metrics.getHeight();

             // and now calculate the display port margins based on how much buffer we've decided to use and

             // the page bounds, ensuring we use all of the available buffer amounts on one side or the other

             // on any given axis. (i.e. if we're scrolled to the top of the page, the vertical buffer is

             // entirely below the visible viewport, but if we're halfway down the page, the vertical buffer

             // is split).

             RectF margins = new RectF();

             margins.left = horizontalBuffer / 2.0f;

             margins.right = horizontalBuffer - margins.left;

             margins.top = verticalBuffer / 2.0f;

             margins.bottom = verticalBuffer - margins.top;

             margins = shiftMarginsForPageBounds(margins, metrics);

             return getTileAlignedDisplayPortMetrics(margins, metrics.zoomFactor, metrics);

         }

         @Override

         public boolean aboutToCheckerboard(ImmutableViewportMetrics metrics, PointF velocity, DisplayPortMetrics displayPort) {

             // Increase the size of the viewport based on the danger zone multiplier (and clamp to page

             // boundaries), and intersect it with the current displayport to determine whether we're

             // close to checkerboarding.

             RectF adjustedViewport = expandByDangerZone(metrics.getViewport(), DANGER_ZONE_X_MULTIPLIER, DANGER_ZONE_Y_MULTIPLIER, metrics);

             return !displayPort.contains(adjustedViewport);

         }

         @Override

         public String toString() {

             return "FixedMarginStrategy mult=" + SIZE_MULTIPLIER + ", dangerX=" + DANGER_ZONE_X_MULTIPLIER + ", dangerY=" + DANGER_ZONE_Y_MULTIPLIER;

         }

     }

     /**

      * This class implements the variation with a small fixed-size margin with velocity bias.

      * In this variation, the default margins are pretty small relative to the view size, but

      * they are affected by the panning velocity. Specifically, if we are panning on one axis,

      * we remove the margins on the other axis because we are likely axis-locked. Also once

      * we are panning in one direction above a certain threshold velocity, we shift the buffer

      * so that it is almost entirely in the direction of the pan, with a little bit in the

      * reverse direction.

      */

     private static class VelocityBiasStrategy extends DisplayPortStrategy {

         // The length of each axis of the display port will be the corresponding view length

         // multiplied by this factor.

         private final float SIZE_MULTIPLIER;

         // The velocity above which we apply the velocity bias

         private final float VELOCITY_THRESHOLD;

         // How much of the buffer to keep in the reverse direction of the velocity

         private final float REVERSE_BUFFER;

         // If the visible rect is within the danger zone we start redrawing to minimize

         // checkerboarding. the danger zone amount is a linear function of the form:

         //    viewportsize * (base + velocity * incr)

         // where base and incr are configurable values.

         private final float DANGER_ZONE_BASE_X_MULTIPLIER;

         private final float DANGER_ZONE_BASE_Y_MULTIPLIER;

         private final float DANGER_ZONE_INCR_X_MULTIPLIER;

         private final float DANGER_ZONE_INCR_Y_MULTIPLIER;

         VelocityBiasStrategy(Map<String, Integer> prefs) {

             SIZE_MULTIPLIER = getFloatPref(prefs, PREF_DISPLAYPORT_VB_MULTIPLIER, 2000);

             VELOCITY_THRESHOLD = GeckoAppShell.getDpi() * getFloatPref(prefs, PREF_DISPLAYPORT_VB_VELOCITY_THRESHOLD, 32);

             REVERSE_BUFFER = getFloatPref(prefs, PREF_DISPLAYPORT_VB_REVERSE_BUFFER, 200);

             DANGER_ZONE_BASE_X_MULTIPLIER = getFloatPref(prefs, PREF_DISPLAYPORT_VB_DANGER_X_BASE, 1000);

             DANGER_ZONE_BASE_Y_MULTIPLIER = getFloatPref(prefs, PREF_DISPLAYPORT_VB_DANGER_Y_BASE, 1000);

             DANGER_ZONE_INCR_X_MULTIPLIER = getFloatPref(prefs, PREF_DISPLAYPORT_VB_DANGER_X_INCR, 0);

             DANGER_ZONE_INCR_Y_MULTIPLIER = getFloatPref(prefs, PREF_DISPLAYPORT_VB_DANGER_Y_INCR, 0);

         }

         /**

          * Split the given amounts into margins based on the VELOCITY_THRESHOLD and REVERSE_BUFFER values.

          * If the velocity is above the VELOCITY_THRESHOLD on an axis, split the amount into REVERSE_BUFFER

          * and 1.0 - REVERSE_BUFFER fractions. The REVERSE_BUFFER fraction is set as the margin in the

          * direction opposite to the velocity, and the remaining fraction is set as the margin in the direction

          * of the velocity. If the velocity is lower than VELOCITY_THRESHOLD, split the amount evenly into the

          * two margins on that axis.

          */

         private RectF velocityBiasedMargins(float xAmount, float yAmount, PointF velocity) {

             RectF margins = new RectF();

             if (velocity.x > VELOCITY_THRESHOLD) {

                 margins.left = xAmount * REVERSE_BUFFER;

             } else if (velocity.x < -VELOCITY_THRESHOLD) {

                 margins.left = xAmount * (1.0f - REVERSE_BUFFER);

             } else {

                 margins.left = xAmount / 2.0f;

             }

             margins.right = xAmount - margins.left;

             if (velocity.y > VELOCITY_THRESHOLD) {

                 margins.top = yAmount * REVERSE_BUFFER;

             } else if (velocity.y < -VELOCITY_THRESHOLD) {

                 margins.top = yAmount * (1.0f - REVERSE_BUFFER);

             } else {

                 margins.top = yAmount / 2.0f;

             }

             margins.bottom = yAmount - margins.top;

             return margins;

         }

         @Override

         public DisplayPortMetrics calculate(ImmutableViewportMetrics metrics, PointF velocity) {

             float displayPortWidth = metrics.getWidth() * SIZE_MULTIPLIER;

             float displayPortHeight = metrics.getHeight() * SIZE_MULTIPLIER;

             // but if we're panning on one axis, set the margins for the other axis to zero since we are likely

             // axis locked and won't be displaying that extra area.

             if (Math.abs(velocity.x) > VELOCITY_THRESHOLD && FloatUtils.fuzzyEquals(velocity.y, 0)) {

                 displayPortHeight = metrics.getHeight();

             } else if (Math.abs(velocity.y) > VELOCITY_THRESHOLD && FloatUtils.fuzzyEquals(velocity.x, 0)) {

                 displayPortWidth = metrics.getWidth();

             }

             // we need to avoid having a display port that is larger than the page, or we will end up

             // painting things outside the page bounds (bug 729169).

             displayPortWidth = Math.min(displayPortWidth, metrics.getPageWidth());

             displayPortHeight = Math.min(displayPortHeight, metrics.getPageHeight());

             float horizontalBuffer = displayPortWidth - metrics.getWidth();

             float verticalBuffer = displayPortHeight - metrics.getHeight();

             // split the buffer amounts into margins based on velocity, and shift it to

             // take into account the page bounds

             RectF margins = velocityBiasedMargins(horizontalBuffer, verticalBuffer, velocity);

             margins = shiftMarginsForPageBounds(margins, metrics);

             return getTileAlignedDisplayPortMetrics(margins, metrics.zoomFactor, metrics);

         }

         @Override

         public boolean aboutToCheckerboard(ImmutableViewportMetrics metrics, PointF velocity, DisplayPortMetrics displayPort) {

             // calculate the danger zone amounts based on the prefs

             float dangerZoneX = metrics.getWidth() * (DANGER_ZONE_BASE_X_MULTIPLIER + (velocity.x * DANGER_ZONE_INCR_X_MULTIPLIER));

             float dangerZoneY = metrics.getHeight() * (DANGER_ZONE_BASE_Y_MULTIPLIER + (velocity.y * DANGER_ZONE_INCR_Y_MULTIPLIER));

             // clamp it such that when added to the viewport, they don't exceed page size.

             // this is a prerequisite to calling shiftMarginsForPageBounds as we do below.

             dangerZoneX = Math.min(dangerZoneX, metrics.getPageWidth() - metrics.getWidth());

             dangerZoneY = Math.min(dangerZoneY, metrics.getPageHeight() - metrics.getHeight());

             // split the danger zone into margins based on velocity, and ensure it doesn't exceed

             // page bounds.

             RectF dangerMargins = velocityBiasedMargins(dangerZoneX, dangerZoneY, velocity);

             dangerMargins = shiftMarginsForPageBounds(dangerMargins, metrics);

             // we're about to checkerboard if the current viewport area + the danger zone margins

             // fall out of the current displayport anywhere.

             RectF adjustedViewport = new RectF(

                     metrics.viewportRectLeft - dangerMargins.left,

                     metrics.viewportRectTop - dangerMargins.top,

                     metrics.viewportRectRight + dangerMargins.right,

                     metrics.viewportRectBottom + dangerMargins.bottom);

             return !displayPort.contains(adjustedViewport);

         }

         @Override

         public String toString() {

             return "VelocityBiasStrategy mult=" + SIZE_MULTIPLIER + ", threshold=" + VELOCITY_THRESHOLD + ", reverse=" + REVERSE_BUFFER

                 + ", dangerBaseX=" + DANGER_ZONE_BASE_X_MULTIPLIER + ", dangerBaseY=" + DANGER_ZONE_BASE_Y_MULTIPLIER

                 + ", dangerIncrX=" + DANGER_ZONE_INCR_Y_MULTIPLIER + ", dangerIncrY=" + DANGER_ZONE_INCR_Y_MULTIPLIER;

         }

     }

     /**

      * This class implements the variation where we draw more of the page at low resolution while panning.

      * In this variation, as we pan faster, we increase the page area we are drawing, but reduce the draw

      * resolution to compensate. This results in the same device-pixel area drawn; the compositor then

      * scales this up to the viewport zoom level. This results in a large area of the page drawn but it

      * looks blurry. The assumption is that drawing extra that we never display is better than checkerboarding,

      * where we draw less but never even show it on the screen.

      */

     private static class DynamicResolutionStrategy extends DisplayPortStrategy {

         // The length of each axis of the display port will be the corresponding view length

         // multiplied by this factor.

         private static final float SIZE_MULTIPLIER = 1.5f;

         // The velocity above which we start zooming out the display port to keep up

         // with the panning.

         private static final float VELOCITY_EXPANSION_THRESHOLD = GeckoAppShell.getDpi() / 16f;

         // How much we increase the display port based on velocity. Assuming no friction and

         // splitting (see below), this should be be the number of frames (@60fps) between us

         // calculating the display port and the draw of the *next* display port getting composited

         // and displayed on the screen. This is because the timeline looks like this:

         //      Java: pan pan pan pan pan pan ! pan pan pan pan pan pan !

         //     Gecko:   \-> draw -> composite /   \-> draw -> composite /

         // The display port calculated on the first "pan" gets composited to the screen at the

         // first exclamation mark, and remains on the screen until the second exclamation mark.

         // In order to avoid checkerboarding, that display port must be able to contain all of

         // the panning until the second exclamation mark, which encompasses two entire draw/composite

         // cycles.

         // If we take into account friction, our velocity multiplier should be reduced as the

         // amount of pan will decrease each time. If we take into account display port splitting,

         // it should be increased as the splitting means some of the display port will be used to

         // draw in the opposite direction of the velocity. For now I'm assuming these two cancel

         // each other out.

         private static final float VELOCITY_MULTIPLIER = 60.0f;

         // The following constants adjust how biased the display port is in the direction of panning.

         // When panning fast (above the FAST_THRESHOLD) we use the fast split factor to split the

         // display port "buffer" area, otherwise we use the slow split factor. This is based on the

         // assumption that if the user is panning fast, they are less likely to reverse directions

         // and go backwards, so we should spend more of our display port buffer in the direction of

         // panning.

         private static final float VELOCITY_FAST_THRESHOLD = VELOCITY_EXPANSION_THRESHOLD * 2.0f;

         private static final float FAST_SPLIT_FACTOR = 0.95f;

         private static final float SLOW_SPLIT_FACTOR = 0.8f;

         // The following constants are used for viewport prediction; we use them to estimate where

         // the viewport will be soon and whether or not we should trigger a draw right now. "soon"

         // in the previous sentence really refers to the amount of time it would take to draw and

         // composite from the point at which we do the calculation, and that is not really a known

         // quantity. The velocity multiplier is how much we multiply the velocity by; it has the

         // same caveats as the VELOCITY_MULTIPLIER above except that it only needs to take into account

         // one draw/composite cycle instead of two. The danger zone multiplier is a multiplier of the

         // viewport size that we use as an extra "danger zone" around the viewport; if this danger

         // zone falls outside the display port then we are approaching the point at which we will

         // checkerboard, and hence should start drawing. Note that if DANGER_ZONE_MULTIPLIER is

         // greater than (SIZE_MULTIPLIER - 1.0f), then at zero velocity we will always be in the

         // danger zone, and thus will be constantly drawing.

         private static final float PREDICTION_VELOCITY_MULTIPLIER = 30.0f;

         private static final float DANGER_ZONE_MULTIPLIER = 0.20f; // must be less than (SIZE_MULTIPLIER - 1.0f)

         DynamicResolutionStrategy(Map<String, Integer> prefs) {

             // ignore prefs for now

         }

         @Override

         public DisplayPortMetrics calculate(ImmutableViewportMetrics metrics, PointF velocity) {

             float displayPortWidth = metrics.getWidth() * SIZE_MULTIPLIER;

             float displayPortHeight = metrics.getHeight() * SIZE_MULTIPLIER;

             // for resolution calculation purposes, we need to know what the adjusted display port dimensions

             // would be if we had zero velocity, so calculate that here before we increase the display port

             // based on velocity.

             FloatSize reshapedSize = reshapeForPage(displayPortWidth, displayPortHeight, metrics);

             // increase displayPortWidth and displayPortHeight based on the velocity, but maintaining their

             // relative aspect ratio.

             if (velocity.length() > VELOCITY_EXPANSION_THRESHOLD) {

                 float velocityFactor = Math.max(Math.abs(velocity.x) / displayPortWidth,

                                                 Math.abs(velocity.y) / displayPortHeight);

                 velocityFactor *= VELOCITY_MULTIPLIER;

                 displayPortWidth += (displayPortWidth * velocityFactor);

                 displayPortHeight += (displayPortHeight * velocityFactor);

             }

             // at this point, displayPortWidth and displayPortHeight are how much of the page (in device pixels)

             // we want to be rendered by Gecko. Note here "device pixels" is equivalent to CSS pixels multiplied

             // by metrics.zoomFactor

             // we need to avoid having a display port that is larger than the page, or we will end up

             // painting things outside the page bounds (bug 729169). we simultaneously need to make

             // the display port as large as possible so that we redraw less. reshape the display

             // port dimensions to accomplish this. this may change the aspect ratio of the display port,

             // but we are assuming that this is desirable because the advantages from pre-drawing will

             // outweigh the disadvantages from any buffer reallocations that might occur.

             FloatSize usableSize = reshapeForPage(displayPortWidth, displayPortHeight, metrics);

             float horizontalBuffer = usableSize.width - metrics.getWidth();

             float verticalBuffer = usableSize.height - metrics.getHeight();

             // at this point, horizontalBuffer and verticalBuffer are the dimensions of the buffer area we have.

             // the buffer area is the off-screen area that is part of the display port and will be pre-drawn in case

             // the user scrolls there. we now need to split the buffer area on each axis so that we know

             // what the exact margins on each side will be. first we split the buffer amount based on the direction

             // we're moving, so that we have a larger buffer in the direction of travel.

             RectF margins = new RectF();

             margins.left = splitBufferByVelocity(horizontalBuffer, velocity.x);

             margins.right = horizontalBuffer - margins.left;

             margins.top = splitBufferByVelocity(verticalBuffer, velocity.y);

             margins.bottom = verticalBuffer - margins.top;

             // then, we account for running into the page bounds - so that if we hit the top of the page, we need

             // to drop the top margin and move that amount to the bottom margin.

             margins = shiftMarginsForPageBounds(margins, metrics);

             // finally, we calculate the resolution we want to render the display port area at. We do this

             // so that as we expand the display port area (because of velocity), we reduce the resolution of

             // the painted area so as to maintain the size of the buffer Gecko is painting into. we calculate

             // the reduction in resolution by comparing the display port size with and without the velocity

             // changes applied.

             // this effectively means that as we pan faster and faster, the display port grows, but we paint

             // at lower resolutions. this paints more area to reduce checkerboard at the cost of increasing

             // compositor-scaling and blurriness. Once we stop panning, the blurriness must be entirely gone.

             // Note that usable* could be less than base* if we are pinch-zoomed out into overscroll, so we

             // clamp it to make sure this doesn't increase our display resolution past metrics.zoomFactor.

             float scaleFactor = Math.min(reshapedSize.width / usableSize.width, reshapedSize.height / usableSize.height);

             float displayResolution = metrics.zoomFactor * Math.min(1.0f, scaleFactor);

             DisplayPortMetrics dpMetrics = new DisplayPortMetrics(

                 metrics.viewportRectLeft - margins.left,

                 metrics.viewportRectTop - margins.top,

                 metrics.viewportRectRight + margins.right,

                 metrics.viewportRectBottom + margins.bottom,

                 displayResolution);

             return dpMetrics;

         }

         /**

          * Split the given buffer amount into two based on the velocity.

          * Given an amount of total usable buffer on an axis, this will

          * return the amount that should be used on the left/top side of

          * the axis (the side which a negative velocity vector corresponds

          * to).

          */

         private float splitBufferByVelocity(float amount, float velocity) {

             // if no velocity, so split evenly

             if (FloatUtils.fuzzyEquals(velocity, 0)) {

                 return amount / 2.0f;

             }

             // if we're moving quickly, assign more of the amount in that direction

             // since is is less likely that we will reverse direction immediately

             if (velocity < -VELOCITY_FAST_THRESHOLD) {

                 return amount * FAST_SPLIT_FACTOR;

             }

             if (velocity > VELOCITY_FAST_THRESHOLD) {

                 return amount * (1.0f - FAST_SPLIT_FACTOR);

             }

             // if we're moving slowly, then assign less of the amount in that direction

             if (velocity < 0) {

                 return amount * SLOW_SPLIT_FACTOR;

             } else {

                 return amount * (1.0f - SLOW_SPLIT_FACTOR);

             }

         }

         @Override

         public boolean aboutToCheckerboard(ImmutableViewportMetrics metrics, PointF velocity, DisplayPortMetrics displayPort) {

             // Expand the viewport based on our velocity (and clamp it to page boundaries).

             // Then intersect it with the last-requested displayport to determine whether we're

             // close to checkerboarding.

             RectF predictedViewport = metrics.getViewport();

             // first we expand the viewport in the direction we're moving based on some

             // multiple of the current velocity.

             if (velocity.length() > 0) {

                 if (velocity.x < 0) {

                     predictedViewport.left += velocity.x * PREDICTION_VELOCITY_MULTIPLIER;

                 } else if (velocity.x > 0) {

                     predictedViewport.right += velocity.x * PREDICTION_VELOCITY_MULTIPLIER;

                 }

                 if (velocity.y < 0) {

                     predictedViewport.top += velocity.y * PREDICTION_VELOCITY_MULTIPLIER;

                 } else if (velocity.y > 0) {

                     predictedViewport.bottom += velocity.y * PREDICTION_VELOCITY_MULTIPLIER;

                 }

             }

             // then we expand the viewport evenly in all directions just to have an extra

             // safety zone. this also clamps it to page bounds.

             predictedViewport = expandByDangerZone(predictedViewport, DANGER_ZONE_MULTIPLIER, DANGER_ZONE_MULTIPLIER, metrics);

             return !displayPort.contains(predictedViewport);

         }

         @Override

         public String toString() {

             return "DynamicResolutionStrategy";

         }

     }

     /**

      * This class implements the variation where we use the draw time to predict where we will be when

      * a draw completes, and draw that instead of where we are now. In this variation, when our panning

      * speed drops below a certain threshold, we draw 9 viewports' worth of content so that the user can

      * pan in any direction without encountering checkerboarding.

      * Once the user is panning, we modify the displayport to encompass an area range of where we think

      * the user will be when the draw completes. This heuristic relies on both the estimated draw time

      * the panning velocity; unexpected changes in either of these values will cause the heuristic to

      * fail and show checkerboard.

      */

     private static class PredictionBiasStrategy extends DisplayPortStrategy {

         private static float VELOCITY_THRESHOLD;

         private int mPixelArea;         // area of the viewport, used in draw time calculations

         private int mMinFramesToDraw;   // minimum number of frames we take to draw

         private int mMaxFramesToDraw;   // maximum number of frames we take to draw

         PredictionBiasStrategy(Map<String, Integer> prefs) {

             VELOCITY_THRESHOLD = GeckoAppShell.getDpi() * getFloatPref(prefs, PREF_DISPLAYPORT_PB_VELOCITY_THRESHOLD, 16);

             resetPageState();

         }

         @Override

         public DisplayPortMetrics calculate(ImmutableViewportMetrics metrics, PointF velocity) {

             float width = metrics.getWidth();

             float height = metrics.getHeight();

             mPixelArea = (int)(width * height);

             if (velocity.length() < VELOCITY_THRESHOLD) {

                 // if we're going slow, expand the displayport to 9x viewport size

                 RectF margins = new RectF(width, height, width, height);

                 return getTileAlignedDisplayPortMetrics(margins, metrics.zoomFactor, metrics);

             }

             // figure out how far we expect to be

             float minDx = velocity.x * mMinFramesToDraw;

             float minDy = velocity.y * mMinFramesToDraw;

             float maxDx = velocity.x * mMaxFramesToDraw;

             float maxDy = velocity.y * mMaxFramesToDraw;

             // figure out how many pixels we will be drawing when we draw the above-calculated range.

             // this will be larger than the viewport area.

             float pixelsToDraw = (width + Math.abs(maxDx - minDx)) * (height + Math.abs(maxDy - minDy));

             // adjust how far we will get because of the time spent drawing all these extra pixels. this

             // will again increase the number of pixels drawn so really we could keep iterating this over

             // and over, but once seems enough for now.

             maxDx = maxDx * pixelsToDraw / mPixelArea;

             maxDy = maxDy * pixelsToDraw / mPixelArea;

             // and finally generate the displayport. the min/max stuff takes care of

             // negative velocities as well as positive.

             RectF margins = new RectF(

                 -Math.min(minDx, maxDx),

                 -Math.min(minDy, maxDy),

                 Math.max(minDx, maxDx),

                 Math.max(minDy, maxDy));

             return getTileAlignedDisplayPortMetrics(margins, metrics.zoomFactor, metrics);

         }

         @Override

         public boolean aboutToCheckerboard(ImmutableViewportMetrics metrics, PointF velocity, DisplayPortMetrics displayPort) {

             // the code below is the same as in calculate() but is awkward to refactor since it has multiple outputs.

             // refer to the comments in calculate() to understand what this is doing.

             float minDx = velocity.x * mMinFramesToDraw;

             float minDy = velocity.y * mMinFramesToDraw;

             float maxDx = velocity.x * mMaxFramesToDraw;

             float maxDy = velocity.y * mMaxFramesToDraw;

             float pixelsToDraw = (metrics.getWidth() + Math.abs(maxDx - minDx)) * (metrics.getHeight() + Math.abs(maxDy - minDy));

             maxDx = maxDx * pixelsToDraw / mPixelArea;

             maxDy = maxDy * pixelsToDraw / mPixelArea;

             // now that we have an idea of how far we will be when the draw completes, take the farthest

             // end of that range and see if it falls outside the displayport bounds. if it does, allow

             // the draw to go through

             RectF predictedViewport = metrics.getViewport();

             predictedViewport.left += maxDx;

             predictedViewport.top += maxDy;

             predictedViewport.right += maxDx;

             predictedViewport.bottom += maxDy;

             predictedViewport = clampToPageBounds(predictedViewport, metrics);

             return !displayPort.contains(predictedViewport);

         }

         @Override

         public boolean drawTimeUpdate(long millis, int pixels) {

             // calculate the number of frames it took to draw a viewport-sized area

             float normalizedTime = (float)mPixelArea * (float)millis / (float)pixels;

             int normalizedFrames = (int)FloatMath.ceil(normalizedTime * 60f / 1000f);

             // broaden our range on how long it takes to draw if the draw falls outside

             // the range. this allows it to grow gradually. this heuristic may need to

             // be tweaked into more of a floating window average or something.

             if (normalizedFrames <= mMinFramesToDraw) {

                 mMinFramesToDraw--;

             } else if (normalizedFrames > mMaxFramesToDraw) {

                 mMaxFramesToDraw++;

             } else {

                 return true;

             }

             Log.d(LOGTAG, "Widened draw range to [" + mMinFramesToDraw + ", " + mMaxFramesToDraw + "]");

             return true;

         }

         @Override

         public void resetPageState() {

             mMinFramesToDraw = 0;

             mMaxFramesToDraw = 2;

         }

         @Override

         public String toString() {

             return "PredictionBiasStrategy threshold=" + VELOCITY_THRESHOLD;

         }

     }

 }