Open PNG-File, Edit Transparency (Alpha) and Save on Android with Java

Question

I need to write an Android-Application that uses an existing PNG-File, changes it's transparency (let's say 50%) and overwrites that file.

I already tried to open the file as a bitmap, change the paint-alpha there and save it again, but it always looses it's transparency value.

MyBitmap.compress(Bitmap.CompressFormat.PNG, 100, myOutputStream);

I already read that changing the quality-value to 0 might help, but that also didn't do the trick:

MyBitmap.compress(Bitmap.CompressFormat.PNG, 100, myOutputStream);

I can prove that my transparency-funktion works but when I try to save that image as a PNG, the transparency is lost. This is how I make the bitmap transparent:

Paint AlphaPaint = new Paint();
AlphaPaint.setAlpha(Math.round(Opacity * 255));
Canvas canvas = new Canvas(MyImage);
canvas.drawBitmap(..., ..., ..., AlphaPaint);

Any help is appreciated! Performance does not matter in this case, if using a library is the easiest way, that's definately ok. Thanks in advance!

Answer 1

Try this way just iterate through all the pixels and then reduce the alpha by whatever percentage you want and set back the pixel to that bitmap.

int A=0, B=0, C=0, D=0, color=0, ncolor=0;

for(int i=0; i<width; i++)
{
    for(int j=0; j<height; j++)
    {
        color = bitmap.getPixel(i,j);
        A = (color >> 24) & 0xff; // Get Alpha
        R = (color >> 16) & 0xff;
        G = (color >>  8) & 0xff;
        B = (color) & 0xff;
        A = A/2; //REDUCE BY HALF SO DIVIDE BY 2(50%)
        ncolor = (A & 0xff) << 24 | (R & 0xff) << 16 | (G & 0xff) << 8 | (B & 0xff);
        bitmap.setPixel(i,j,ncolor);
    }
}

Its just a basic idea how you can solve your problem. Hope this helps you out.. Thank You.