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POOptimize Android canvas background drawing by caching matrix
 

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  1. POOptimize Android canvas background drawing by caching matrix
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    <p>I have a SurfaceView canvas I'm drawing a scene into. The canvas is transformed to accomodate the logical scene size which is 1024x768 (while the actual screen is 800x480), and also to support zoom/scroll navigation on it. Since I did not want black stripes on the sides (fitting 4:3 to 16:9) I'm using a different background bitmap which has "stretched" sides to cover the ratio difference. Another twist in the plot, is that I need to support both phones and tables. For tablets I'm loading a higher quality (size) image and for phones I'm scaling down the background bitmap when loading it from disk. Below is code that does what I want (although probably not the best way), and in particular, I have a feeling the background calculations can be cached into a matrix to be used later in <code>canvas.drawBitmap(Bitmap, Matrix, Paint);</code></p> <p>(Also, I couldn't get the equivalent positioning of drawBitmap(Bitmap, left, top, Paint) by translating the canvas with these offsets, I'm also curious why).</p> <p>This is the draw routine:</p> <pre><code>public void draw(Canvas canvas) { canvas.save(); float midX = width/2; float midY = height/2; // zoom out to fit logical view, and translate for future // sprite drawing with logical coordinates // x,y and zoom are calculated in on init and updated in touch events canvas.translate(x, y); canvas.scale(zoom, zoom, midX, midY); // background part if(back != null &amp;&amp; !back.isRecycled()) { // todo: these can probably be pre-calculated for optimization // todo: but so far I couldn't get it right.. if(bgMatrix != null) { // this is where I'm thinking of using the cached matrix canvas.drawBitmap(back, bgMatrix, paint); } else { float offsetW = (width-back.getWidth())/2; float offsetH = (height-back.getHeight())/2; canvas.save(); // bgScaleFactor is calculated upon setting the bg bitmap // it says by how much we need to scale the image to fill the canvas // taking into account the image (possible) downscale canvas.scale(bgScaleFactor, bgScaleFactor, midX, midY); // this doesn't work: canvas.postTranslate(offsetW, offsetH) and use 0,0 for next draw canvas.drawBitmap(back, offsetW, offsetH, paint); // todo: here I would like to save a matrix which represents // how the back bitmap was drawn onto the canvas // so that next time these calculations can be avoided // this fails: bgMatrix = canvas.getMatrix(); canvas.restore(); } // draw scene shapes on transformed canvas if(shapes != null){ shapes.onDraw(canvas); } canvas.restore(); } </code></pre>
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