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POAndroid image transformation with matrix, translate touch coordinates back
 

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  1. POAndroid image transformation with matrix, translate touch coordinates back
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    <p>I'm building a "navigation type" app for android.</p> <p>For the navigation part I'm building an Activity where the user can move and zoom the map (which is a bitmap) using touch events, and also the map rotate around the center of the screen using the compass.</p> <p>I'm using Matrix to scale, transpose and rotate the image, and than I draw it to the canvas.</p> <p>Here is the code called on loading of the view, to center the image in the screen:</p> <pre><code> image = new Matrix(); image.setScale(zoom, zoom); image_center = new PointF(bmp.getWidth() / 2, bmp.getHeight() / 2); float centerScaledWidth = image_center.x * zoom; float centerScaledHeigth = image_center.y * zoom; image.postTranslate( screen_center.x - centerScaledWidth, screen_center.y - centerScaledHeigth); </code></pre> <p>The rotation of the image is doing using the <em>postRotate</em> method.</p> <p>Then in the <em>onDraw()</em> method I only call</p> <pre><code> canvas.drawBitmap(bmp, image, drawPaint); </code></pre> <p>The problem is that, when the user touch the screen, I want to get the point touched on the image, but apparently I can't get the correct position. I tried to invert the <strong>image</strong> matrix and translate the touched points, it isn't working.</p> <p>Do somebody know how to translate the point coordinates?</p> <p><strong>EDIT</strong></p> <p>I'm using this code for traslation. <em>dx</em> and <em>dy</em> are translation values get from the <em>onTouch</em> listener. *new_center* is an array of float values in this form {x0, y0, x1, y1...}</p> <pre><code> Matrix translated = new Matrix(); Matrix inverted = new Matrix(); translated.set(image); translated.postTranslate(dx, dy); translated.invert(inverted); inverted.mapPoints(new_center); translated.mapPoints(new_center); Log.i("new_center", new_center[0]+" "+new_center[1]); </code></pre> <p>Actually I tried using as *new_center = {0,0}*:</p> <p>appling only the <em>translated</em> matrix, I get as espected the distance between the (0,0) point of the bmp and the (0,0) point of the screen, but it seems to not take account of the rotation.</p> <p>Appling the <em>inverted</em> matrix to the points I get those results, moving the image in every possible way.</p> <pre><code> 12-26 13:26:08.481: I/new_center(11537): 1.9073486E-6 -1.4901161E-7 12-26 13:26:08.581: I/new_center(11537): 0.0 -3.874302E-7 12-26 13:26:08.631: I/new_center(11537): 1.9073486E-6 1.2516975E-6 12-26 13:26:08.781: I/new_center(11537): -1.9073486E-6 -5.364418E-7 12-26 13:26:08.951: I/new_center(11537): 0.0 2.682209E-7 12-26 13:26:09.093: I/new_center(11537): 0.0 7.003546E-7 </code></pre> <p>Instead I was especting the coordinates translated on the image.</p> <p>Is it correct my line of thoughts?</p>
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