如何更改位图的不透明度？

Question

我有一张位图：

Bitmap bitmap = BitmapFactory.decodeFile("some/arbitrary/path/image.jpg");

但我不会向用户显示图像。我希望 alpha 为 100（255 中）。如果这不可能，我可以设置Bitmap 的不透明度吗？

Answer 1

如果您使用 Drawable 来显示图像，您可以按如下方式更改 alpha：

private Drawable mTriangle;
mTriangle = context.getResources().getDrawable(R.drawable.triangle_arrow_for_radar);

...

protected void onDraw(Canvas canvas)
{
    // Draw the triangle arrow
    float imageTargetWidth = getWidth() / 15;
    float scale = mTriangle.getIntrinsicWidth() / imageTargetWidth;

    int imgWidth  = (int)(imageTargetWidth);
    int imgHeight = (int)(mTriangle.getIntrinsicHeight() / scale);

    if (mTriangle != null)
    {
        mTriangle.setBounds(getWidth() / 2 - imgWidth / 2, getHeight() / 2 -       imgHeight / 2, getWidth() / 2 + imgWidth / 2, getHeight() / 2 + imgHeight / 2);

        mTriangle.setAlpha(150); // from (transparent) to 255 (opaque)
        mTriangle.draw(canvas);
    }
}

Answer 2

https://dzone.com/articles/adjusting-opacity-android提议：

/**
 * @param bitmap The source bitmap.
 * @param opacity a value between 0 (completely transparent) and 255 (completely
 * opaque).
 * @return The opacity-adjusted bitmap.  If the source bitmap is mutable it will be
 * adjusted and returned, otherwise a new bitmap is created.
 */
private Bitmap adjustOpacity(Bitmap bitmap, int opacity)
{
    Bitmap mutableBitmap = bitmap.isMutable()
                       ? bitmap
                       : bitmap.copy(Bitmap.Config.ARGB_8888, true);
    Canvas canvas = new Canvas(mutableBitmap);
    int colour = (opacity & 0xFF) << 24;
    canvas.drawColor(colour, PorterDuff.Mode.DST_IN);
    return mutableBitmap;
}

注意DST_IN可以修改（而不是重置）已经透明图片的透明度，也就是可以让图片越来越透明。

Answer 3

您也可以尝试使用BitmapDrawable 代替Bitmap。这是否对您有用取决于您使用位图的方式...

编辑

当一位评论者询问他如何存储带有 alpha 的位图时，这里有一些代码：

// lets create a new empty bitmap
Bitmap newBitmap = Bitmap.createBitmap(originalBitmap.getWidth(), originalBitmap.getHeight(), Bitmap.Config.ARGB_8888);
// create a canvas where we can draw on
Canvas canvas = new Canvas(newBitmap);
// create a paint instance with alpha
Paint alphaPaint = new Paint();
alphaPaint.setAlpha(42);
// now lets draw using alphaPaint instance
canvas.drawBitmap(originalBitmap, 0, 0, alphaPaint);

// now lets store the bitmap to a file - the canvas has drawn on the newBitmap, so we can just store that one
// please add stream handling with try/catch blocks
FileOutputStream fos = new FileOutputStream(new File("/awesome/path/to/bitmap.png"));
newBitmap.compress(Bitmap.CompressFormat.PNG, 100, fos);

Answer 4

public Bitmap makeTransparent(Bitmap src, int value) {  
    int width = src.getWidth();
    int height = src.getHeight();
       Bitmap transBitmap = Bitmap.createBitmap(width, height, Config.ARGB_8888);
       Canvas canvas = new Canvas(transBitmap);
       canvas.drawARGB(0, 0, 0, 0);
        // config paint
        final Paint paint = new Paint();
        paint.setAlpha(value);
        canvas.drawBitmap(src, 0, 0, paint);    
        return transBitmap;
}

Answer 5

Bitmap bgr = BitmapFactory.decodeResource(getResources(),R.drawable.main_logo_2);       
Paint transparentpainthack = new Paint();
transparentpainthack.setAlpha(100);
canvas.drawBitmap(bgr, 0, 0, transparentpainthack);

Answer 6

据我所知，不能在位图本身上设置不透明度或其他颜色过滤器。使用图像时需要设置 alpha：

如果您使用的是 ImageView，则有 ImageView.setAlpha()。

如果您使用的是 Canvas，则需要使用 Paint.setAlpha()：

Paint paint = new Paint();
paint.setAlpha(100);
canvas.drawBitmap(bitmap, src, dst, paint);

此外，结合 WarrenFaith 的回答，如果您将在需要可绘制对象的地方使用位图，您可以使用 BitmapDrawable.setAlpha()。