Create alpha gradient on image to create fade effect

问题

How can I apply an alpha gradient on an image so that it fades linearly? Right now, I'm creating rectangles of unit width and using it to draw the bitmap with a paint object with alpha value being changed in a loop. I only did it since I couldn't think of anything else. So a neater way would be better.

Bitmap bitmap = BitmapFactory.decodeStream(is);
Bitmap bmp = Bitmap.createScaledBitmap(bitmap, 100, 151, true));
bitmap.recycle();

Rect Rect1 = new Rect(0, 0, 100, 100);
Rect Rect2 = new Rect(100, 0, 101, 100);

Paint paint = new Paint();

canvas.drawBitmap(bmp, Rect1, Rect1, null);
while (paint.getAlpha() != 0) {
    paint.setAlpha(paint.getAlpha() - 5);
    canvas.drawBitmap(bmp, Rect2, Rect2, paint);
    Rect2.set(Rect2.left + 1, Rect2.top, Rect2.right + 1, Rect2.bottom);
}

Something like this

P.S. I'm trying to do this for a live wallpaper.

回答1:

Probably, desired behaviour could be meet with only views manipulations. For it You should:

Prepare shape drawable

res/drawable/gradient_shape.xml:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient
            android:startColor="#00FFFFFF"
            android:endColor="#FFFFFFFF"
            android:type="linear" />
</shape>

Define layout: activity_main.xml:

<ImageView
    android:id="@+id/photo"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/photo"
    android:src="@drawable/gradient_shape" />

here drawable/photo is just jpeg in drawables folder;

Some suggests to add the following code to the activity (it, actually, depends on device native configuration and seems to be redundant for nowadays > 3.0 devices):
```
public void onAttachedToWindow() {
     super.onAttachedToWindow();
     Window window = getWindow();
     window.setFormat(PixelFormat.RGBA_8888);
 }
```

After that I've observed the following on my 4.1 device:

It looks like gradient. I'm still not sure if it's what You're looking for.

来源：https://stackoverflow.com/questions/12151727/create-alpha-gradient-on-image-to-create-fade-effect

标签

android

android-canvas

live-wallpaper