Create a new bitmap and draw new pixels into it

Question

I'm trying to make an app that will take two pictures you specify via editText, compare the colors of each pixel on both images and create a new picture (bitmap) (that you can save to the sd card) containing the differences between the two original pictures.

I'm having a problem with creating this new bitmap. How can I achieve my goal? I don't really know how to do this, do I create the new bitmap first and then write into it, or do I get the differences first and then draw a bitmap from that? The pictures will be approx. 300x300 px.

Answer 1

this code is just out of my head and untested but it should get you on the right track.

final int w1 = b1.getWidth();
final int w2 = b2.getWidth();
final int h1 = b1.getHeight();
final int h2 = b2.getHeight();
final int w = Math.max(w1, w2);
final int h = Math.max(h2, h2);

Bitmap compare = Bitmap.createBitmap(w, h, Config.ARGB_8888);

int color1, color2, a, r, g, b;

for (int x = 0; x < w; x++) {
    for (int y = 0; y < h; y++) {
        if (x < w1 && y < h1) {
            color1 = b1.getPixel(x, y);
        } else {
            color1 = Color.BLACK;
        }
        if (x < w2 && y < h2) {
            color2 = b2.getPixel(x, y);
        } else {
            color2 = Color.BLACK;
        }
        a = Math.abs(Color.alpha(color1) - Color.alpha(color2));
        r = Math.abs(Color.red(color1) - Color.red(color2));
        g = Math.abs(Color.green(color1) - Color.green(color2));
        b = Math.abs(Color.blue(color1) - Color.blue(color1));

        compare.setPixel(x, y, Color.argb(a, r, g, b));
    }
}
b1.recycle();
b2.recycle();