DataMatrix-encoding with zxing only generates 14px bitmap

Question 1

Here is a small example how you can change your method for the conversion from BitMatrix to Bitmap. The method does the scaling of the BitMatrix.

int BLACK = 0xFF000000;
int WHITE = 0xFFFFFFFF;

// change the values to your needs
int requestedWidth = 300;
int requestedHeight = 300;

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

// calculating the scaling factor
int pixelsize = requestedWidth/width;
if (pixelsize > requestedHeight/height)
{
  pixelsize = requestedHeight/height;
}

int[] pixels = new int[requestedWidth * requestedHeight];
// All are 0, or black, by default
for (int y = 0; y < height; y++) {
  int offset = y * requestedWidth * pixelsize;

  // scaling pixel height
  for (int pixelsizeHeight = 0; pixelsizeHeight < pixelsize; pixelsizeHeight++, offset+=requestedWidth) {
    for (int x = 0; x < width; x++) {
      int color = bitMatrix.get(x, y) ? BLACK : WHITE;

      // scaling pixel width
      for (int pixelsizeWidth = 0; pixelsizeWidth < pixelsize; pixelsizeWidth++) {
        pixels[offset + x * pixelsize + pixelsizeWidth] = color;
      }
    }
  }
}
Bitmap bitmap = Bitmap.createBitmap(requestedWidth, requestedHeight, Bitmap.Config.ARGB_8888);
bitmap.setPixels(pixels, 0, requestedWidth, 0, 0, requestedWidth, requestedHeight);

// I could only test it with BufferedImage and a modified version of the zxing J2SE client
// BufferedImage bitmap = new BufferedImage(requestedWidth, requestedHeight, BufferedImage.TYPE_INT_ARGB);
// bitmap.getRaster().setDataElements(0, 0, requestedWidth, requestedHeight, pixels);

Question 2

As I remember, the Data Matrix encoder is kind of the odd man out since the code came from a different place (barcode4j). It will use the minimum appropriate dimensions and assumes the caller will scale the graphic as desired.

You can do that here -- just set the ImageView to scale its contents. That shouldn't hurt for any of the encoded barcodes, as long as you do not enable anti-aliasing.

There is also a EncodeHintType.MIN_SIZE hint which will set the minimum size, just for Data Matrix.

Question 3

I know that this question is about Android, but I think could be classified as generic.
I'm working with Swift and the the wrapper ZXingObjC. I obtain this error using DataMatrix encoding:

'Can't find a symbol arrangement that matches the message. Data codewords: 5'

So, debugging code, I arrived to class ZXDataMatrixSymbolInfo, on method:

+ (ZXDataMatrixSymbolInfo *)lookup:(int)dataCodewords shape:(ZXDataMatrixSymbolShapeHint)shape minSize:(ZXDimension *)minSize

When happens?

I write a custom swift wrapper (CoderEncoder) for my project, with this method:

static func encode(string code: String, encoding: CoderEncorderType, size: CGSize = CGSizeMake(200, 200)) -> UIImage? {
    let writer: ZXWriter = ZXMultiFormatWriter.writer() as! ZXWriter
    do {
        let format = CodeEncoder.getEncoding(type: encoding) 
        let result = try writer.encode(code, format: format, width: Int32(size.width), height: Int32(size.height))
        let cgImage = ZXImage(matrix: result, onColor: UIColor.blackColor().CGColor, offColor: UIColor.clearColor().CGColor).cgimage
        return UIImage(CGImage: cgImage)
    } catch let error as NSError  {
        ERLog(what: error)
    }
    return nil
}

So, when I call:

CoderEncoder.encode("foo", .DataMatrix)

error occurs.

Basically, in ZXDataMatrixWriter maxSize is nil so you need to specify a dimension from 10 to 144.

or!! or specify the shape:

ZXDataMatrixSymbolShapeHintForceSquare
ZXDataMatrixSymbolShapeHintForceRectangle

Question 4

Is there a simple way to somehow scale a bitmatrix?

Yes, there is. Problem is that using Android automatic scaling hits some problems:

Android uses bi-linear filtering when scaling
bi-linear filtering cannot be disabled when using Hardware Acceleration

The approach I've taken is to scale manually by an integer factor the Bitmap. Here's how to convert a BitMatrix to an unscaled Bitmap:

 /**
 * Create an unscaled bitmap (1 square = 1 pixel) from a bitmatrix
 *
 * @param bitMatrix the bitmatrix to convert
 * @return a unscaled bitmap
 */
public Bitmap drawUnscaledBitmap(BitMatrix bitMatrix) {
    final int bmWidth = bitMatrix.getWidth();
    final int bmHeight = bitMatrix.getHeight();
    final Bitmap bitmap = Bitmap.createBitmap(bmWidth, bmHeight, Bitmap.Config.ARGB_8888);
    int[] pixels = new int[bmWidth * bmHeight];
    for (int y = 0; y < bmHeight; y++) {
        int offset = y * bmWidth;
        for (int x = 0; x < bmWidth; x++) {
            pixels[offset + x] = bitMatrix.get(x, y) ? Color.BLACK : Color.WHITE;
        }
    }
    bitmap.setPixels(pixels, 0, bmWidth, 0, 0, bmWidth, bmHeight);
    return bitmap;
}

and here's how to scale up a Bitmap to (almost) fit a desired rectangle size:

/**
 * Upscale a bitmap (maintaining ratio) by the biggest integer scaling factor that allows the bitmap to still fit in the given size
 *
 * @param bitmap the bitmap to upscale
 * @param size   the size that the output scaled bitmap needs to fit in
 * @return the scaled bitmap if the integer scaling factor is > 1; the original bitmap otherwise
 */
public Bitmap drawScaledBitmap(Bitmap bitmap, Size size) {
    final int bmWidth = bitmap.getWidth();
    final int bmHeight = bitmap.getHeight();
    final int wScalingFactor = size.getWidth() / bmWidth;
    final int hScalingFactor = size.getHeight() / bmHeight;
    final int scalingFactor = Math.min(wScalingFactor, hScalingFactor);
    return scalingFactor > 1 ? Bitmap.createScaledBitmap(bitmap, bmWidth * scalingFactor, bmHeight * scalingFactor, false) : bitmap;
}