Android bitmap mask color, remove color

Question

I am creating bitmap, next i am drawing second solid color bitmap on top of it. And now i want to change first bitmap, so solid color that i drawed on it will be transparent

Answer 1

user487252's solution works like a charm up until API level 16 (Jelly Bean), after which AvoidXfermode does not seem to work at all.

In my particular use case, I have rendered a page of a PDF (via APV PDFView) into a pixel array int[] that I am going to pass into Bitmap.createBitmap( int[], int, int, Bitmap.Config ). This page contains line art drawn onto a white background, and I need to remove the background while preserving the anti-aliasing.

I couldn't find a Porter-Duff mode that did exactly what I wanted, so I ended up buckling and iterating through the pixels and transforming them one by one. The result was surprisingly simple and performant:

int [] pixels = ...;

for( int i = 0; i < pixels.length; i++ ) {
    // Invert the red channel as an alpha bitmask for the desired color.
    pixels[i] = ~( pixels[i] << 8 & 0xFF000000 ) & Color.BLACK;
}

Bitmap bitmap = Bitmap.createBitmap( pixels, width, height, Bitmap.Config.ARGB_8888 );

This is perfect for drawing line art, since any color can be used for the lines without losing the anti-aliasing. I'm using the red channel here, but you can use green by shifting 16 bits instead of 8, or blue by shifting 24.