Question

I am trying to change the alpha value of a bitmap per pixel in a for loop. The Bitmap is created from a createBitmap(source,x,y,w,h) of another bitmap. I’ve done a little test but I can’t seem to alter the alpha. Is it the setPixel command or the fact the bitmap it isn’t ARGB?

I want to create a simple fade out effect in the end but for now I am not referencing original pixel colors just green with half alpha. Thanks if you can help 🙂

        _left[1] = Bitmap.createBitmap(TestActivity.photo, 0, 0, 256, 256);


        for (int i = 0; i < _left[1].getWidth(); i++)
            for (int t = 0; t < _left[1].getHeight(); t++) {

                int a = (_left[1].getWidth() / 2) - i;
                int b = (_left[1].getHeight() / 2) - t;
                double dist = Math.sqrt((a*a) + (b*b));

                if (dist > 20) _left[1].setPixel(i, t, Color.argb(128, 0, 255, 0)); 

            }

UPDATE :

Okay this is the result I came up with if anyone wants to take a bitmap and fade out radially. But yes it is VERY SLOW without arrays… Thanks Reuben for a step in the right direction

public void fadeBitmap (Bitmap input, double fadeStartPercent, double fadeEndPercent, Bitmap output) {

    Bitmap tempalpha = Bitmap.createBitmap(input.getWidth(), input.getHeight(), Bitmap.Config.ARGB_8888 );

    Canvas printcanvas = new Canvas(output);

    int radius = input.getWidth() / 2;
    double fadelength = (radius * (fadeEndPercent / 100));
    double fadestart = (radius * (fadeStartPercent / 100));

    for (int i = 0; i < input.getWidth(); i++)
        for (int t = 0; t < input.getHeight(); t++) {

            int a = (input.getWidth() / 2) - i;
            int b = (input.getHeight() / 2) - t;
            double dist = Math.sqrt((a*a) + (b*b));

            if (dist <= fadestart) {
                tempalpha.setPixel(i,t,Color.argb(255, 255, 255, 255));
            } else {
                int fadeoff = 255 - (int) ((dist - fadestart) * (255/(fadelength - fadestart)));
                if (dist > radius * (fadeEndPercent / 100)) fadeoff = 0;
                tempalpha.setPixel(i,t,Color.argb(fadeoff, 255, 255, 255));
            }

        }

    Paint alphaP = new Paint();
    alphaP.setAntiAlias(true);
    alphaP.setXfermode(new PorterDuffXfermode(Mode.DST_IN));

//  printcanvas.setBitmap();
    printcanvas.drawBitmap(input, 0, 0, null);
    printcanvas.drawBitmap(tempalpha, 0, 0, alphaP);

}

Answer 1

The version of Bitmap.createBitmap() you are using returns an immutable bitmap. Bitmap.setPixel() will have no effect.

setPixel is appallingly slow anyway. Aim to use setPixels(), or, best of all, find a better way than manipulating bitmap pixels directly. I expect you could do something clever with a separate alpha-only bitmap and the right PorterDuff mode.