Question

I’ve got this loop that is run thousands of times (so needs to be efficient). It changes the value of the bitmap pixel.

I want to be able to run thought the loop and “switch” a certain group of pixels to alpha and then switch them back at a later point.

My question is.
How do I switch the values? So say 0xFFCC1BE0 becomes 0x00CC1BE0 and then if I want to switch back to 0xFFCC1BE0 I simply take the 00 and turn it too FF.

I can’t make two bitmaps as I run out of memory 🙁

Anyhow here’s what I’ve got so far:

private void setTransparencyOnLightMap(float WidthPercentage, float LeftPosition)
{
    int blankPixel = 0x00000000;
    int savedPixel = 0x00000000;
    int desiredAlpha = 200; //Can also be 0x00
    //Find away of turning alhpa off and on.

    for(int BMx = 0; BMx < mLightMap.getWidth(); BMx++)
    {
        for(int BMy = 0; BMy < mLightMap.getHeight(); BMy++)
        {
            if(mLightMap.getPixel(BMx, BMy) != blankPixel) //Make sure don't overwrite blank transparent pixels.
            {
                savedPixel = mLightMap.getPixel(BMx,BMy);
                savedPixel = savedPixel | (desiredAlpha << 24);

                mLightMap.setPixel(BMx, BMy, savedPixel);
            }   
        }
    }
}

Answer 1

You could switch the alpha of a pixel like so:

savedPixel = savedPixel & 0x00FFFFFF;
savedPixel = savedPixel | (desiredAlpha << 24);

The first line zeros out the 8 most significant bits of savedPixel (these are the bits where the alpha is held). The second line sets the 8 most significant bits of savedPixel to desiredAlpha. Note that desiredAlpha must be between 0 and 255 (These are the ints that can be stored in 8 bits).

Note that this uses bitwise operators, (&, |, <<) which are very efficient.