绘制透明渐变,Alpha透明度从0到1

Question

我在Android中有一个动态生成的位图,我想从顶部边缘羽化,使边框区域在顶部完全透明,并逐渐变为完全不透明,略低于下方.

创建均匀完全透明的顶边

transparentPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
transparentPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));
Shader shader = new LinearGradient(0, 0, 0, 20,
                    Color.TRANSPARENT, Color.TRANSPARENT, Shader.TileMode.CLAMP);
transparentPaint.setShader(shader);
// the bitmap is dynamically generated beforehand
Canvas c = new Canvas(bitmap);
c.drawRect(0, 0, bitmapWidth, 20, transparentPaint);

Alpha渐变而不是完全透明的孔？

你会如何实现这样的目标:

[在这种情况下只是最顶端]

Answer 1

看一下这个例子:触摸时使位图的某些区域透明

这是一种使用渐变绘制的方法:

Paint framePaint = new Paint();
for(int i = 1; i < 5; i++){
   setFramePaint(framePaint, i, imageW, imageH);
   myCanvas.drawPaint(framePaint);
}

...

private void setFramePaint(Paint p, int side, float iw, float ih){
                // paint, side of rect, image width, image height

                p.setShader(null);
                p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));

                float borderSize = 0.1f; //relative size of border
                //use the smaller image size to calculate the actual border size
                float bSize = (iw > ih)? ih * borderSize: ih * borderSize; 
                float g1x = 0;
                float g1y = 0;
                float g2x = 0;
                float g2y = 0;
                int c1 = 0, c2 = 0;

                if (side == 1){
                    //left
                    g1x = 0;
                    g1y = ih/2;
                    g2x = bSize;
                    g2y = ih/2;
                    c1 = Color.TRANSPARENT;
                    c2 = Color.BLACK;

                }else if(side == 2){
                    //top
                    g1x = iw/2;
                    g1y = 0;
                    g2x = iw/2;
                    g2y = bSize;
                    c1 = Color.TRANSPARENT;
                    c2 = Color.BLACK;


                }else if(side == 3){
                    //right
                    g1x = iw;
                    g1y = ih/2;
                    g2x = iw - bSize;
                    g2y = ih/2;
                    c1 = Color.TRANSPARENT;
                    c2 = Color.BLACK;


                }else if(side == 4){
                    //bottom
                    g1x = iw/2;
                    g1y = ih;
                    g2x = iw/2;
                    g2y = ih - bSize;
                    c1 = Color.TRANSPARENT;
                    c2 = Color.BLACK;
                }

                p.setShader(new LinearGradient(g1x, g1y, g2x, g2y, c1, c2, Shader.TileMode.CLAMP));

            }