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.

Or simply, i want to remove all pixels of one color from bitmap. I havie tried every colorfilter, and xfermode with no luck, is there any other possibility to remove color other that doing it pixel by pixel?

Answer 1

This works for removing a certain color from a bitmap. The main part is the use of AvoidXfermode. It should also work if trying to change one color to another color.

I should add that this answers the question title of removing a color from a bitmap. The specific question is probably better solved using PorterDuff Xfermode like the OP said.

// start with a Bitmap bmp

// make a mutable copy and a canvas from this mutable bitmap
Bitmap mb = bmp.copy(Bitmap.Config.ARGB_8888, true);
Canvas c = new Canvas(mb);

// get the int for the colour which needs to be removed
Paint p = new Paint();
p.setARGB(255, 255, 0, 0); // ARGB for the color, in this case red
int removeColor = p.getColor(); // store this color's int for later use

// Next, set the alpha of the paint to transparent so the color can be removed.
// This could also be non-transparent and be used to turn one color into another color            
p.setAlpha(0);

// then, set the Xfermode of the pain to AvoidXfermode
// removeColor is the color that will be replaced with the pain't color
// 0 is the tolerance (in this case, only the color to be removed is targetted)
// Mode.TARGET means pixels with color the same as removeColor are drawn on
p.setXfermode(new AvoidXfermode(removeColor, 0, AvoidXfermode.Mode.TARGET));

// draw transparent on the "brown" pixels
c.drawPaint(p);

// mb should now have transparent pixels where they were red before

Answer 2

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.