Android: Converting a Bitmap to a Monochrome Bitmap (1 Bit per Pixel)

Question

I want to print a Bitmap to a mobile Bluetooth Printer (Bixolon SPP-R200) - the SDK doesn't offer direkt methods to print an in-memory image. So I thought about converting a Bitmap like this:

Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);

To a Monochrome Bitmap. I am drawing black text on above given Bitmap using a Canvas, which works well. However, when I convert the above Bitmap to a ByteArray, the printer seems to be unable to handle those bytes. I suspect I need an Array with one Bit per Pixel (a Pixel would be either white = 1 or black = 0).

As there seems to be no convenient, out of the box way to do that, one idea I had was to use:

bitmap.getPixels(pixels, offset, stride, x, y, width, height)

to Obtain the pixels. I assume, I'd have to use it as follows:

int width = bitmap.getWidth();
int height = bitmap.getHeight();

int [] pixels = new int [width * height];
bitmap.getPixels(pixels, 0, width, 0, 0, width, height);

However - I am not sure about a few things:

• In getPixels - does it make sense to simply pass the width as the "Stride" argument?
• I guess I'd have to evaluate the color information of each pixel and either switch it to black or white (And I'd write this value in a new target byte array which I would ultimately pass to the printer)?
• How to best evaluate each pixel color information in order to decide that it should be black or white? (The rendered Bitmap is black pain on a white background)

Does this approach make sense at all? Is there an easier way? It's not enough to just make the bitmap black & white, the main issue is to reduce the color information for each pixel into one bit.

UPDATE

As suggested by Reuben I'll first convert the Bitmap to a monochrome Bitmap. and then I'll iterate over each pixel:

    int width = bitmap.getWidth();
    int height = bitmap.getHeight();

    int[] pixels = new int[width * height];
    bitmap.getPixels(pixels, 0, width, 0, 0, width, height);

    // Iterate over height
    for (int y = 0; y < height; y++) {
        int offset = y * height;
        // Iterate over width
        for (int x = 0; x < width; x++) {
            int pixel = bitmap.getPixel(x, y);
        }
    }

Now Reuben suggested to "read the lowest byte of each 32-bit pixel" - that would relate to my question about how to evaluate the pixel color. My last question in this regard: Do I get the lowest byte by simply doing this:

// Using the pixel from bitmap.getPixel(x,y)
int lowestByte = pixel & 0xff;

Answer 1

You can convert the image to monochrome 32bpp using a ColorMatrix.

Bitmap bmpMonochrome = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
Canvas canvas = new Canvas(bmpMonochrome);
ColorMatrix ma = new ColorMatrix();
ma.setSaturation(0);
Paint paint = new Paint();
paint.setColorFilter(new ColorMatrixColorFilter(ma));
canvas.drawBitmap(bmpSrc, 0, 0, paint);

That simplifies the color->monochrome conversion. Now you can just do a getPixels() and read the lowest byte of each 32-bit pixel. If it's <128 it's a 0, otherwise it's a 1.