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POCanvas Pinch-Zoom to Point Within Bounds
 

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    <p>I've been stuck on this problem for eight hours, so I figured it was time to get some help.</p> <p>Before I begin my problem, I'll just let it be known that I've been through this site and Google, and none of the answers I've found have helped. (<a href="https://stackoverflow.com/questions/8848523/canvas-zoom-goes-to-point-0-0">This</a> is one, <a href="https://stackoverflow.com/questions/6835224/android-bitmap-canvas-offset-after-scale">another</a>, and <a href="https://stackoverflow.com/questions/6624103/scaling-image-of-imageview-while-maintaining-center-point-in-same-place">another</a>.)</p> <p>Here's the deal: I have a class that extends <code>SurfaceView</code> (let's call it <code>MySurface</code>) and overrides many methods in it. Normally, it draws several squares and text boxes, which is all fine. As soon as a user starts touching, it converts to a <code>Bitmap</code>, then draws each frame that until the user releases.</p> <p>Here's the rub: I want to implement such a functionality that the user can place two fingers on the screen, pinch to zoom, and also pan around (but ONLY with two fingers down).</p> <p>I found a few implementations of pinch-to-zoom and adapted them to my <code>Canvas</code> object in <code>MySurface</code> via the following:</p> <pre><code>public void onDraw(Canvas canvas) { super.onDraw(canvas); canvas.save(); canvas.scale(mScaleVector.z, mScaleVector.z); // This is the scale factor as seen below canvas.translate(mScaleVector.x, mScaleVector.y); // These are offset values from 0,0, both working fine // Start draw code // ... // End draw code canvas.restore(); } private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener { @Override public boolean onScale(ScaleGestureDetector detector) { float factor = detector.getScaleFactor(); if (Math.abs(factor - 1.0f) &gt;= 0.0075f) { mScaleVector.z *= factor; mScaleVector.z = Math.max(MIN_ZOOM, Math.min(mScaleVector.z, MAX_ZOOM)); } // ... invalidate(); return true; } } public boolean onTouchEvent(MotionEvent event) { int action = event.getAction() &amp; MotionEvent.ACTION_MASK; int pointerIndex = (event.getAction() &amp; MotionEvent.ACTION_POINTER_INDEX_MASK) &gt;&gt; MotionEvent.ACTION_POINTER_INDEX_SHIFT; if (event.getPointerCount() == 2) { if (action == MotionEvent.ACTION_POINTER_DOWN &amp;&amp; pointerIndex == 1) { // The various pivot coordinate codes would belong here } } detector.onTouchEvent(event); // Calls the Scale Gesture Detector return true; } </code></pre> <p>While both elements work fine--the scrolling back and forth and the pinch-to-zoom--there is one large problem. The pinch-to-zoom, when used, zooms into the point <code>0,0</code>, instead of zooming into the finger point.</p> <p>I've tried a lot of ways to fix this:</p> <ul> <li>Using <code>canvas.scale(mScaleVector.z, mScaleVector.z, mScaleVector.x, mScaleVector.y);</code>; obviously, this produces unwanted results as the <code>mScaleVector</code> x and y values are 0-offsets.</li> <li>Managing a "pivot" coordinate that uses the same offset as the <code>translate()</code> method, but this produces either the same <code>0,0</code> issue, or jumping around when the view is touched.</li> <li>Numerous other things... I've done a lot with the aforementioned pivot coordinate, trying to base its location on the user's first touch, and moving it relative to that touch each successive gesture.</li> </ul> <p>Additionally, this canvas must be bounded, so the user cannot scroll forever. However, when I use the <code>.scale(sx, sy, px, py)</code> method, it pushes things beyond any bounds I set in <code>.translate()</code>.</p> <p>I'm... pretty much open to anything at this point. I know this functionality can be added, as it is seen in the Android 4.0 gallery (when viewing a single image). I've tried to track down the source code that handles this, to no avail.</p>
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    1. COI personally handle the two finger positions directly using the `onTouchEvent`. With this, you can easily find the mid point, and how much scaling is required.
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    2. COcan you please post the solution if you have solved the issue I am facing same problem, and my client is.... :(, or can you tell me how you have solve the problem. thank you
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