Android ApiDemos示例解析(74):Graphics->PolyToPoly

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android.graphics.Matrix 类提供二维坐标变换（平移，旋转，缩放等）使用Matrix通常的方法是直接指定所需的Matrix的值，另外一种如果已知坐标变换前后几个点的坐标，你可以根据这些点的映射关系算出对应的变换矩阵。但Matrix提供了一种更为简洁的方法：

public boolean setPolyToPoly(float[] src, int srcIndex, float[] dst, int dstIndex, int pointCount)

提供指定变换前（src)和变换后(dst)的坐标对，Matrix自动帮你计算出实现这些坐标变换对于的Matrix。每个坐标的格式为[x0,y0,x1,y1 ...]两个float值代表一个点。

本例使用1 个点（偏移变换） 2个点（旋转/缩放），3个点（旋转/剪切），4个点（透视变换）使用polyToPoly 计算出对应的Matrix，然后使用计算出的Matrix来绘制图形：

private void doDraw(Canvas canvas, float src[], float dst[]) { canvas.save(); mMatrix.setPolyToPoly(src, 0, dst, 0, src.length >> 1); canvas.concat(mMatrix);  mPaint.setColor(Color.GRAY); mPaint.setStyle(Paint.Style.STROKE); canvas.drawRect(0, 0, 64, 64, mPaint); canvas.drawLine(0, 0, 64, 64, mPaint); canvas.drawLine(0, 64, 64, 0, mPaint);  mPaint.setColor(Color.RED); mPaint.setStyle(Paint.Style.FILL); // how to draw the text center on our square // centering in X is easy... use alignment (and X at midpoint) float x = 64/2; // centering in Y, we need to measure ascent/descent first float y = 64/2 - (mFontMetrics.ascent + mFontMetrics.descent)/2; canvas.drawText(src.length/2 + "", x, y, mPaint);  canvas.restore();} .... canvas.save();canvas.translate(10, 10);// translate (1 point)doDraw(canvas, new float[] { 0, 0 }, new float[] { 5, 5 });canvas.restore(); canvas.save();canvas.translate(160, 10);// rotate/uniform-scale (2 points)doDraw(canvas, new float[] { 32, 32, 64, 32 }, new float[] { 32, 32, 64, 48 });canvas.restore(); canvas.save();canvas.translate(10, 110);// rotate/skew (3 points)doDraw(canvas, new float[] { 0, 0, 64, 0, 0, 64 }, new float[] { 0, 0, 96, 0, 24, 64 });canvas.restore(); canvas.save();canvas.translate(160, 110);// perspective (4 points)doDraw(canvas, new float[] { 0, 0, 64, 0, 64, 64, 0, 64 }, new float[] { 0, 0, 96, 0, 64, 96, 0, 64 });canvas.restore();