如何使用LinearGradient着色器正确遮罩位图?

问题描述

我正在尝试使用底部的渐变alpha遮罩位图.渐变大小是固定的，并且与位图大小无关.但是它绘制不正确:渐变的底部在顶部，而不是顶部.怎么了?

I'm trying to mask Bitmap with gradient alpha at bottom. Gradient size are fixed and independed of Bitmap size. But it draws incorrect: bottom of gradient at top, than top.What's wrong?

有示例代码:

final int GRADIENT_HEIGHT = 32;

public Bitmap addGradient(Bitmap src) {
    int w = src.getWidth();
    int h = src.getHeight();
    Bitmap overlay = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(overlay);

    canvas.drawBitmap(src, 0, 0, null);

    Paint paint = new Paint();
    LinearGradient shader = new LinearGradient(0, 0, 0, GRADIENT_HEIGHT, 0xFFFFFFFF, 0x00FFFFFF, TileMode.REPEAT);
    paint.setShader(shader);
    paint.setXfermode(new PorterDuffXfermode(Mode.DST_IN));
    canvas.drawRect(0, h - GRADIENT_HEIGHT, w, h, paint);

    return overlay;
}

谢谢！

推荐答案

将LinearGradient更改为此:

Change your LinearGradient to this:

    LinearGradient shader = new LinearGradient(0,  h - GRADIENT_HEIGHT, 0, h, 0xFFFFFFFF, 0x00FFFFFF, Shader.TileMode.CLAMP);

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