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

android.graphics.Matrix 類提供二維坐标變換（平移，旋轉，縮放等）使用Matrix通常的方法是直接指定所需的Matrix的值，另外一種如果已知坐标變換前後幾個點的坐标，你可以根據這些點的映射關系算出對應的變換矩陣。但Matrix提供了一種更為簡潔的方法：

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

提供指定變換前（src)和變換後(dst)的坐标對，Matrix自動幫你計算出實作這些坐标變換對于的Matrix。每個坐标的格式為[x0,y0,x1,y1 ...]兩個float值代表一個點。

本例使用1 個點 （偏移變換） 2個點（旋轉/縮放） ，3個點（旋轉/剪切），4個點（透視變換） 使用polyToPoly 計算出對應的Matrix，然後使用計算出的Matrix來繪制圖形：

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();