ALPHA_8 bitmaps and getPixel

Question

I am trying to load a movement map from a PNG image. In order to save memory after I load the bitmap I do something like that.

 
 `Bitmap mapBmp = tempBmp.         


        

Answer 1

I developed solution with PNGJ library, to read image from assets and then create Bitmap with Config.ALPHA_8.

import ar.com.hjg.pngj.IImageLine;
import ar.com.hjg.pngj.ImageLineHelper;
import ar.com.hjg.pngj.PngReader;

public Bitmap getAlpha8BitmapFromAssets(String file) {

    Bitmap result = null;

    try {

        PngReader pngr = new PngReader(getAssets().open(file));
        int channels = pngr.imgInfo.channels;
        if (channels < 3 || pngr.imgInfo.bitDepth != 8)
            throw new RuntimeException("This method is for RGB8/RGBA8 images");

        int bytes = pngr.imgInfo.cols * pngr.imgInfo.rows;
        ByteBuffer buffer = ByteBuffer.allocate(bytes);

        for (int row = 0; row < pngr.imgInfo.rows; row++) { 

            IImageLine l1 = pngr.readRow();

            for (int j = 0; j < pngr.imgInfo.cols; j++) {

                int original_color = ImageLineHelper.getPixelARGB8(l1, j);

                byte x = (byte) Color.alpha(original_color);

                buffer.put(row * pngr.imgInfo.cols + j, x ^= 0xff);

            }

        }

        pngr.end();

        result = Bitmap.createBitmap(pngr.imgInfo.cols,pngr.imgInfo.rows, Bitmap.Config.ALPHA_8);
        result.copyPixelsFromBuffer(buffer);

    } catch (IOException e) {
        Log.e(LOG_TAG, e.getMessage());
    }

    return result;

}

I also invert alpha values, because of my particular needs. This code is only tested for API 21.