繪制透明漸變，Alpha透明度從0到1

Question

我在Android中有一個動態生成的位圖，我希望從頂部邊緣羽化，使邊框區域在頂部完全透明，並逐漸變為完全不透明，略低於下方。

創建均勻完全透明的上邊緣

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漸變而不是完全透明的孔？

你會如何實現這樣的目標：

[在這種情況下只是最頂端]

Answer 1

看一下這個例子： 觸摸時使位圖的某些區域透明

這是一種使用漸變繪制的方法：

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));

            }

Answer 2

如果您可以接受白色邊緣而不是透明，請嘗試屏幕模式。 在PorterDuff.Mode.SCREEN模式下，白色像素保持白色，黑色像素變為不可見。 創建一個疊加位圖，其邊緣為白色，中間為黑色並與圖像混合。 這將創建一個位圖，白色邊緣淡入中間的照片。

Answer 3

Romain Guy使用了與你想要達到的效果非常相似的褪色，除了這種效果之外，他還添加了圓角