Blending two images together with multiply and % opacity

Question

I'm trying to blend two images together with Android, using a Multiply-like blending mode.

// Prepare -------------------------------

// Create source images
Bitmap img1 = ...
Bitmap img2 = ...

// Create result image
Bitmap result = ...
Canvas canvas = new Canvas();
canvas.setBitmap(result);

// Get proper display reference
BitmapDrawable drawable = new BitmapDrawable(getResources(), result);
ImageView imageView = (ImageView)findViewById(R.id.imageBlend1);
imageView.setImageDrawable(drawable);


// Apply -------------------------------

// Draw base
canvas.drawBitmap(img1, 0, 0, null);

// Draw overlay
Paint paint = new Paint();
paint.setXfermode(new PorterDuffXfermode(Mode.MULTIPLY));
paint.setShader(new BitmapShader(img2, TileMode.CLAMP, TileMode.CLAMP));

canvas.drawRect(0, 0, img2.getWidth(), img2.getHeight(), paint);

This works, but I don't have control over the "amount" of multiply that is done - it's always a complete multiply transfer. Ideally, a 0% multiply would be the same as the base image (img1) without any change, but a 100% multiply would be the result I get with the code above.

paint.setAlpha() doesn't seem to work for this.

Any other way to set the % opacity of the new 'layer'?

P.S. There are some methods to make multiply work with this I guess (using a LightingColorFilter) by pre-multiplying and offsetting the color to white, but it's very specific to the multiplymode .. I'm trying to find a way to apply the opacity/% thing to all other transfer modes too.

Answer 1

I was implementing photo filters similar to what iOS app of ours does. They do it something like source bitmap + mask bitmap + blend mode + alpha value. To achieve identical behaviour I just increased the alpha of mask. Here's what my code finally looks:

public static Bitmap blend(Bitmap back, Bitmap front, BlendMethod method, float alpha) {
    if (alpha != 1.0F) {
        front = makeTransparent(front, Math.round(alpha * 255));
    }

    Bitmap.Config config = back.getConfig();
    int width = back.getWidth();
    int height = back.getHeight();

    if (width != front.getWidth() || height != front.getHeight()) {
        Log.e(TAG, "Arrays must be of identical size! Do bitmap scaling prior to blending.");
        return null;
    }

    int[] frontArr = new int[height * width], backArr = new int[height * width], resultArr;

    back.getPixels(backArr, 0, width, 0, 0, width, height);
    front.getPixels(frontArr, 0, width, 0, 0, width, height);

    resultArr = jniBlend(frontArr, backArr, alpha, method.toInt());
    return Bitmap.createBitmap(resultArr, width, height, config);
}

public static Bitmap blend(Bitmap back, Bitmap front, BlendMethod method) {
    return blend(back, front, method, 1F);
}

public static Bitmap makeTransparent(Bitmap src, int value) {
    int width = src.getWidth();
    int height = src.getHeight();
    Bitmap transBitmap = Bitmap.createBitmap(width, height, Bitmap.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;
}

Note that jniBlend is some method I wrote on my own, it behaves like stock PorterDuff modes in Java.

Method makeTransparent is not mine - found it here: (answer from Ruban)

Answer 2

I needed to do something like that a while ago, and I found this post about Color Channels a lot enlightening. (But I'm afraid this is related to what you described in your "PS")

Link above in archive.org, thanks to @1j01