一、关于PorterDuffXfermode
java.lang.Object
android.graphics.Xfermode
android.graphics.PorterDuffXfermode
PorterDuffXfermode继承自Xfermode是个简单图像合成类，，源码中位于graphics下。当图像交叉绘制时，可通过PorterDuffXfermode指定Paint的绘制规则，从而达到图像的合成效果。
与常见的类命名方式不同，PorterDuffXfermode中Porter，Duff指的是Thomas Porter和Tom Duff以及其发表于1984年论文《Compositingdigitalimages》(该论文可在ACM Digital Library上花费15美元购得， 详见https://dl.acm.org/purchase.cfm?id=808606&CFID=486449268&CFTOKEN=73298988)
关于PorterDuffXfermode的使用，可以参见API Demos中在Xfermodes.java。
Xfermodes.java源码如下:
public class Xfermodes extends Activity {// create a bitmap with a circle, used for the "dst" imagestatic Bitmap makeDst(int w, int h) {Bitmap bm = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);Canvas c = new Canvas(bm);Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);p.setColor(0xFFFFCC44);c.drawOval(new RectF(0, 0, w * 3 / 4, h * 3 / 4), p);return bm;}// create a bitmap with a rect, used for the "src" imagestatic Bitmap makeSrc(int w, int h) {Bitmap bm = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);Canvas c = new Canvas(bm);Paint p = new Paint(Paint.ANTI_ALIAS_FLAG);p.setColor(0xFF66AAFF);c.drawRect(w / 3, h / 3, w * 19 / 20, h * 19 / 20, p);return bm;}@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);this.requestWindowFeature(Window.FEATURE_NO_TITLE);setContentView(new SampleView(this));}private static class SampleView extends View {private static final int W = 64;private static final int H = 64;private static final int ROW_MAX = 4; // number of samples per rowprivate Bitmap mSrcB;private Bitmap mDstB;private Shader mBG;// background checker-board patternprivate static final Xfermode[] sModes = {new PorterDuffXfermode(PorterDuff.Mode.CLEAR),new PorterDuffXfermode(PorterDuff.Mode.SRC),new PorterDuffXfermode(PorterDuff.Mode.DST),new PorterDuffXfermode(PorterDuff.Mode.SRC_OVER),new PorterDuffXfermode(PorterDuff.Mode.DST_OVER),new PorterDuffXfermode(PorterDuff.Mode.SRC_IN),new PorterDuffXfermode(PorterDuff.Mode.DST_IN),new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT),new PorterDuffXfermode(PorterDuff.Mode.DST_OUT),new PorterDuffXfermode(PorterDuff.Mode.SRC_ATOP),new PorterDuffXfermode(PorterDuff.Mode.DST_ATOP),new PorterDuffXfermode(PorterDuff.Mode.XOR),new PorterDuffXfermode(PorterDuff.Mode.DARKEN),new PorterDuffXfermode(PorterDuff.Mode.LIGHTEN),new PorterDuffXfermode(PorterDuff.Mode.MULTIPLY),new PorterDuffXfermode(PorterDuff.Mode.SCREEN)};private static final String[] sLabels = {"Clear", "Src", "Dst", "SrcOver","DstOver", "SrcIn", "DstIn", "SrcOut","DstOut", "SrcATop", "DstATop", "Xor","Darken", "Lighten", "Multiply", "Screen"};public SampleView(Context context) {super(context);mSrcB = makeSrc(W, H);mDstB = makeDst(W, H);// make a ckeckerboard patternBitmap bm = Bitmap.createBitmap(new int[]{0xFFFFFFFF, 0xFFCCCCCC,0xFFCCCCCC, 0xFFFFFFFF}, 2, 2,Bitmap.Config.RGB_565);mBG = new BitmapShader(bm,Shader.TileMode.REPEAT,Shader.TileMode.REPEAT);Matrix m = new Matrix();m.setScale(6, 6);mBG.setLocalMatrix(m);}@Overrideprotected void onDraw(Canvas canvas) {canvas.drawColor(Color.WHITE);Paint labelP = new Paint(Paint.ANTI_ALIAS_FLAG);labelP.setTextAlign(Paint.Align.CENTER);Paint paint = new Paint();paint.setFilterBitmap(false);canvas.translate(15, 35);int x = 0;int y = 0;for (int i = 0; i < sModes.length; i++) {// draw the borderpaint.setStyle(Paint.Style.STROKE);paint.setShader(null);canvas.drawRect(x – 0.5f, y – 0.5f,x + W + 0.5f, y + H + 0.5f, paint);// draw the checker-board patternpaint.setStyle(Paint.Style.FILL);paint.setShader(mBG);canvas.drawRect(x, y, x + W, y + H, paint);// draw the src/dst example into our offscreen bitmapint sc = canvas.saveLayer(x, y, x + W, y + H, null,Canvas.MATRIX_SAVE_FLAG |Canvas.CLIP_SAVE_FLAG |Canvas.HAS_ALPHA_LAYER_SAVE_FLAG |Canvas.FULL_COLOR_LAYER_SAVE_FLAG |Canvas.CLIP_TO_LAYER_SAVE_FLAG);canvas.translate(x, y);canvas.drawBitmap(mDstB, 0, 0, paint);//yellowpaint.setXfermode(sModes[i]);canvas.drawBitmap(mSrcB, 0, 0, paint);//bluepaint.setXfermode(null);canvas.restoreToCount(sc);// draw the labelcanvas.drawText(sLabels[i],x + W / 2, y – labelP.getTextSize() / 2, labelP);x += W + 10;// wrap around when we've drawn enough for one rowif ((i % ROW_MAX) == ROW_MAX – 1) {x = 0;y += H + 30;}}}}}
运行结果：
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