如何从图像中获取 select 像素并在 Android 编程中降低该像素的不透明度

Question

I want to select a pixel from an image that is displayed in the Image View and then change the opacity of the same pixel without changing the opacity of the rest of the image pixels. For example, I select a point with position x = 50 and y = 70, and I just want the opacity to change in this point and not change the opacity elsewhere.How can I do that?

Answer 1

Here's what I managed to do:

// Decode the bitmap resource and make sure it's mutable.
BitmapFactory.Options opts = new BitmapFactory.Options();
opts.inMutable = true;
Bitmap bm = BitmapFactory.decodeResource(getResources(), R.drawable.black, opts);

imv.setImageBitmap(bm);

imv.setOnTouchListener((v, event) -> {
    if (event.getAction() == MotionEvent.ACTION_DOWN) {
        int x = (int) event.getX();
        int y = (int) event.getY();

        // Get the color of the touched pixel and change only the alpha component.
        int color = bm.getPixel(x, y);
        int alpha = color >>> 24;
        alpha /= 2;  // Alpha will be divided by two in the final color.
        color = color & 0xFFFFFF | alpha << 24;

        // Change the pixel color on the bitmap and update the ImageView.
        bm.setPixel(x, y, color);
        imv.setImageBitmap(bm);
        imv.invalidate();
        return true;
    }
    return false;
});

It's a little more than what you asked but I needed to test it. When you click on the ImageView, the opacity (alpha) of the pixel you clicked on is halved.

That's actually barely perceptible on most phones because the screen has a very high density.