Android: bitmap to RGBA and back

Question

I'm trying to write a couple of methods to convert an Android Bitmap to an RGBA byte array and then back to a Bitmap. The problem is that I don't seem to hit the formula, because the colors are always coming back wrong. I have tried with several different assumptions but to no avail.

So, this is the method to convert from Bitmap to RGBA that I think is fine:

public static byte[] bitmapToRgba(Bitmap bitmap) {
    int[] pixels = new int[bitmap.getWidth() * bitmap.getHeight()];
    byte[] bytes = new byte[pixels.length * 4];
    bitmap.getPixels(pixels, 0, bitmap.getWidth(), 0, 0, bitmap.getWidth(), bitmap.getHeight());
    int i = 0;
    for (int pixel : pixels) {
        // Get components assuming is ARGB
        int A = (pixel >> 24) & 0xff;
        int R = (pixel >> 16) & 0xff;
        int G = (pixel >> 8) & 0xff;
        int B = pixel & 0xff;
        bytes[i++] = (byte) R;
        bytes[i++] = (byte) G;
        bytes[i++] = (byte) B;
        bytes[i++] = (byte) A;
    }
    return bytes;
}

And this is the method aimed at creating back a bitmap from those bytes that is not working as expected:

public static Bitmap bitmapFromRgba(int width, int height, byte[] bytes) {
    int[] pixels = new int[bytes.length / 4];
    int j = 0;

    // It turns out Bitmap.Config.ARGB_8888 is in reality RGBA_8888!
    // Source: https://stackoverflow.com/a/47982505/1160360
    // Now, according to my own experiments, it seems it is ABGR... this sucks.
    // So we have to change the order of the components

    for (int i = 0; i < pixels.length; i++) {
        byte R = bytes[j++];
        byte G = bytes[j++];
        byte B = bytes[j++];
        byte A = bytes[j++];

        int pixel = (A << 24) | (B << 16) | (G << 8) | R;
        pixels[i] = pixel;
    }

    Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
    bitmap.copyPixelsFromBuffer(IntBuffer.wrap(pixels));
    return bitmap;
}

That's my last implementation, though I have tried several different ones without success. I'm assuming createBitmap expects ABGR in spite of specifying ARGB_8888 because I have done experiments hardcoding all the pixels to things like:

0xff_ff_00_00 -> got blue 
0xff_00_ff_00 -> got green
0xff_00_00_ff -> got red

Anyway maybe that assumption is wrong and a consequence of some other mistaken one before.

I think the main problem may be related to the use of signed numeric values, since there are no unsigned ones in Java (well, there's something in Java 8+ but on one hand I don't think it should be necessary to use these, and on the other it is not supported by older Android versions that I need to support).

Any help will be very appreciated.

Thanks a lot in advance!

It seems the problem with the wrong color component ordering is related to copyPixelsFromBuffer. Maybe everything boils down to that. I've replaced it with setPixels and results are different. Something related to little/big indian? — Fran Marzoa, Jan 17 '20 at 13:19

score 7 · Accepted Answer · answered Jan 17 '20 at 13:27

I solved it myself. There are a number of issues but all these began with this line:

bitmap.copyPixelsFromBuffer(IntBuffer.wrap(pixels));

That seems to be mixing up the color components in the wrong way. Maybe it's something related to byte order (little/big indian stuff), in any case I worked it around using setPixels instead:

bitmap.setPixels(pixels, 0, width, 0, 0, width, height);

This is the final code that's working as expected, just in case it's useful for someone else:

public static byte[] bitmapToRgba(Bitmap bitmap) {
    if (bitmap.getConfig() != Bitmap.Config.ARGB_8888)
        throw new IllegalArgumentException("Bitmap must be in ARGB_8888 format");
    int[] pixels = new int[bitmap.getWidth() * bitmap.getHeight()];
    byte[] bytes = new byte[pixels.length * 4];
    bitmap.getPixels(pixels, 0, bitmap.getWidth(), 0, 0, bitmap.getWidth(), bitmap.getHeight());
    int i = 0;
    for (int pixel : pixels) {
        // Get components assuming is ARGB
        int A = (pixel >> 24) & 0xff;
        int R = (pixel >> 16) & 0xff;
        int G = (pixel >> 8) & 0xff;
        int B = pixel & 0xff;
        bytes[i++] = (byte) R;
        bytes[i++] = (byte) G;
        bytes[i++] = (byte) B;
        bytes[i++] = (byte) A;
    }
    return bytes;
}

public static Bitmap bitmapFromRgba(int width, int height, byte[] bytes) {
    int[] pixels = new int[bytes.length / 4];
    int j = 0;

    for (int i = 0; i < pixels.length; i++) {
        int R = bytes[j++] & 0xff;
        int G = bytes[j++] & 0xff;
        int B = bytes[j++] & 0xff;
        int A = bytes[j++] & 0xff;

        int pixel = (A << 24) | (R << 16) | (G << 8) | B;
        pixels[i] = pixel;
    }


    Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
    bitmap.setPixels(pixels, 0, width, 0, 0, width, height);
    return bitmap;
}

Android: bitmap to RGBA and back

1 Answers1