通过缓存矩阵优化Android画布背景绘图

我有一张SurfaceView画布，我正在画一个场景。 画布经过变换以适应逻辑场景大小1024x768（而实际屏幕为800x480），并支持其上的缩放/滚动导航。 因为我不希望在侧面有黑色条纹（适合4:3到16:9），所以我使用了一个不同的背景位图，它有“拉伸”的侧面来覆盖比率差异。 情节的另一个转折点是，我需要同时支持手机和桌子。对于平板电脑，我正在加载更高质量（大小）的图像；对于手机，我正在从磁盘加载背景位图时缩小背景位图。 下面是我想要的代码（虽然可能不是最好的方式），特别是，我感觉背景计算可以缓存到矩阵中，稍后在

canvas.drawBitmap（位图、矩阵、绘画）中使用

（此外，我无法通过使用这些偏移量转换画布来获得drawBitmap（位图、左、上、绘制）的等效位置，我也很好奇为什么）

这是绘图例程：

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 && !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();
}