聊聊zxing的qrcode

摘要：序本文主要研究下的的一些代码。可以看到这里重新计算了输出的宽度和高度，是取了的宽高两边的与输入参数的目标宽高取最大值。也就是说如果的宽高大于目标的宽高，则以的宽高为准，这种情况下的基本跟输出的一致。

本文主要研究下zxing的qrcode的一些代码。

        
            com.google.zxing
            core
            3.3.1
        
        
            com.google.zxing
            javase
            3.3.1
        

core-3.3.1-sources.jar!/com/google/zxing/qrcode/QRCodeWriter.java
QRCodeWriter的encode方法进行编码，转换为BitMatrix

  @Override
  public BitMatrix encode(String contents,
                          BarcodeFormat format,
                          int width,
                          int height,
                          Map hints) throws WriterException {

    if (contents.isEmpty()) {
      throw new IllegalArgumentException("Found empty contents");
    }

    if (format != BarcodeFormat.QR_CODE) {
      throw new IllegalArgumentException("Can only encode QR_CODE, but got " + format);
    }

    if (width < 0 || height < 0) {
      throw new IllegalArgumentException("Requested dimensions are too small: " + width + "x" +
          height);
    }

    ErrorCorrectionLevel errorCorrectionLevel = ErrorCorrectionLevel.L;
    int quietZone = QUIET_ZONE_SIZE;
    if (hints != null) {
      if (hints.containsKey(EncodeHintType.ERROR_CORRECTION)) {
        errorCorrectionLevel = ErrorCorrectionLevel.valueOf(hints.get(EncodeHintType.ERROR_CORRECTION).toString());
      }
      if (hints.containsKey(EncodeHintType.MARGIN)) {
        quietZone = Integer.parseInt(hints.get(EncodeHintType.MARGIN).toString());
      }
    }

    QRCode code = Encoder.encode(contents, errorCorrectionLevel, hints);
    return renderResult(code, width, height, quietZone);
  }

core-3.3.1-sources.jar!/com/google/zxing/qrcode/encoder/QRCode.java

public final class QRCode {

  public static final int NUM_MASK_PATTERNS = 8;

  private Mode mode;
  private ErrorCorrectionLevel ecLevel;
  private Version version;
  private int maskPattern;
  private ByteMatrix matrix;

  public QRCode() {
    maskPattern = -1;
  }

  public Mode getMode() {
    return mode;
  }

  public ErrorCorrectionLevel getECLevel() {
    return ecLevel;
  }

  public Version getVersion() {
    return version;
  }

  public int getMaskPattern() {
    return maskPattern;
  }

  public ByteMatrix getMatrix() {
    return matrix;
  }

  @Override
  public String toString() {
    StringBuilder result = new StringBuilder(200);
    result.append("<<
");
    result.append(" mode: ");
    result.append(mode);
    result.append("
 ecLevel: ");
    result.append(ecLevel);
    result.append("
 version: ");
    result.append(version);
    result.append("
 maskPattern: ");
    result.append(maskPattern);
    if (matrix == null) {
      result.append("
 matrix: null
");
    } else {
      result.append("
 matrix:
");
      result.append(matrix);
    }
    result.append(">>
");
    return result.toString();
  }

  public void setMode(Mode value) {
    mode = value;
  }

  public void setECLevel(ErrorCorrectionLevel value) {
    ecLevel = value;
  }

  public void setVersion(Version version) {
    this.version = version;
  }

  public void setMaskPattern(int value) {
    maskPattern = value;
  }

  public void setMatrix(ByteMatrix value) {
    matrix = value;
  }

  // Check if "mask_pattern" is valid.
  public static boolean isValidMaskPattern(int maskPattern) {
    return maskPattern >= 0 && maskPattern < NUM_MASK_PATTERNS;
  }

}

是由Encoder类encode而来

core/3.3.1/core-3.3.1-sources.jar!/com/google/zxing/qrcode/encoder/Encoder.java

 public static QRCode encode(String content,
                              ErrorCorrectionLevel ecLevel,
                              Map hints) throws WriterException {

    // Determine what character encoding has been specified by the caller, if any
    String encoding = DEFAULT_BYTE_MODE_ENCODING;
    boolean hasEncodingHint = hints != null && hints.containsKey(EncodeHintType.CHARACTER_SET);
    if (hasEncodingHint) {
      encoding = hints.get(EncodeHintType.CHARACTER_SET).toString();
    }

    // Pick an encoding mode appropriate for the content. Note that this will not attempt to use
    // multiple modes / segments even if that were more efficient. Twould be nice.
    Mode mode = chooseMode(content, encoding);

    // This will store the header information, like mode and
    // length, as well as "header" segments like an ECI segment.
    BitArray headerBits = new BitArray();

    // Append ECI segment if applicable
    if (mode == Mode.BYTE && (hasEncodingHint || !DEFAULT_BYTE_MODE_ENCODING.equals(encoding))) {
      CharacterSetECI eci = CharacterSetECI.getCharacterSetECIByName(encoding);
      if (eci != null) {
        appendECI(eci, headerBits);
      }
    }

    // (With ECI in place,) Write the mode marker
    appendModeInfo(mode, headerBits);

    // Collect data within the main segment, separately, to count its size if needed. Don"t add it to
    // main payload yet.
    BitArray dataBits = new BitArray();
    appendBytes(content, mode, dataBits, encoding);

    Version version;
    if (hints != null && hints.containsKey(EncodeHintType.QR_VERSION)) {
      int versionNumber = Integer.parseInt(hints.get(EncodeHintType.QR_VERSION).toString());
      version = Version.getVersionForNumber(versionNumber);
      int bitsNeeded = calculateBitsNeeded(mode, headerBits, dataBits, version);
      if (!willFit(bitsNeeded, version, ecLevel)) {
        throw new WriterException("Data too big for requested version");
      }
    } else {
      version = recommendVersion(ecLevel, mode, headerBits, dataBits);
    }

    BitArray headerAndDataBits = new BitArray();
    headerAndDataBits.appendBitArray(headerBits);
    // Find "length" of main segment and write it
    int numLetters = mode == Mode.BYTE ? dataBits.getSizeInBytes() : content.length();
    appendLengthInfo(numLetters, version, mode, headerAndDataBits);
    // Put data together into the overall payload
    headerAndDataBits.appendBitArray(dataBits);

    Version.ECBlocks ecBlocks = version.getECBlocksForLevel(ecLevel);
    int numDataBytes = version.getTotalCodewords() - ecBlocks.getTotalECCodewords();

    // Terminate the bits properly.
    terminateBits(numDataBytes, headerAndDataBits);

    // Interleave data bits with error correction code.
    BitArray finalBits = interleaveWithECBytes(headerAndDataBits,
                                               version.getTotalCodewords(),
                                               numDataBytes,
                                               ecBlocks.getNumBlocks());

    QRCode qrCode = new QRCode();

    qrCode.setECLevel(ecLevel);
    qrCode.setMode(mode);
    qrCode.setVersion(version);

    //  Choose the mask pattern and set to "qrCode".
    int dimension = version.getDimensionForVersion();
    ByteMatrix matrix = new ByteMatrix(dimension, dimension);
    int maskPattern = chooseMaskPattern(finalBits, ecLevel, version, matrix);
    qrCode.setMaskPattern(maskPattern);

    // Build the matrix and set it to "qrCode".
    MatrixUtil.buildMatrix(finalBits, ecLevel, version, maskPattern, matrix);
    qrCode.setMatrix(matrix);

    return qrCode;
  }

这里重点看Version的这段

    Version version;
    if (hints != null && hints.containsKey(EncodeHintType.QR_VERSION)) {
      int versionNumber = Integer.parseInt(hints.get(EncodeHintType.QR_VERSION).toString());
      version = Version.getVersionForNumber(versionNumber);
      int bitsNeeded = calculateBitsNeeded(mode, headerBits, dataBits, version);
      if (!willFit(bitsNeeded, version, ecLevel)) {
        throw new WriterException("Data too big for requested version");
      }
    } else {
      version = recommendVersion(ecLevel, mode, headerBits, dataBits);
    }

这里计算version，同时判断content的大小，是否超出qrcode的容量，超出的话，抛出WriterException("Data too big for requested version")

willFit计算方法

/**
   * @return true if the number of input bits will fit in a code with the specified version and
   * error correction level.
   */
  private static boolean willFit(int numInputBits, Version version, ErrorCorrectionLevel ecLevel) {
      // In the following comments, we use numbers of Version 7-H.
      // numBytes = 196
      int numBytes = version.getTotalCodewords();
      // getNumECBytes = 130
      Version.ECBlocks ecBlocks = version.getECBlocksForLevel(ecLevel);
      int numEcBytes = ecBlocks.getTotalECCodewords();
      // getNumDataBytes = 196 - 130 = 66
      int numDataBytes = numBytes - numEcBytes;
      int totalInputBytes = (numInputBits + 7) / 8;
      return numDataBytes >= totalInputBytes;
  }

// Note that the input matrix uses 0 == white, 1 == black, while the output matrix uses
  // 0 == black, 255 == white (i.e. an 8 bit greyscale bitmap).
  private static BitMatrix renderResult(QRCode code, int width, int height, int quietZone) {
    ByteMatrix input = code.getMatrix();
    if (input == null) {
      throw new IllegalStateException();
    }
    int inputWidth = input.getWidth();
    int inputHeight = input.getHeight();
    int qrWidth = inputWidth + (quietZone * 2);
    int qrHeight = inputHeight + (quietZone * 2);
    int outputWidth = Math.max(width, qrWidth);
    int outputHeight = Math.max(height, qrHeight);

    int multiple = Math.min(outputWidth / qrWidth, outputHeight / qrHeight);
    // Padding includes both the quiet zone and the extra white pixels to accommodate the requested
    // dimensions. For example, if input is 25x25 the QR will be 33x33 including the quiet zone.
    // If the requested size is 200x160, the multiple will be 4, for a QR of 132x132. These will
    // handle all the padding from 100x100 (the actual QR) up to 200x160.
    int leftPadding = (outputWidth - (inputWidth * multiple)) / 2;
    int topPadding = (outputHeight - (inputHeight * multiple)) / 2;

    BitMatrix output = new BitMatrix(outputWidth, outputHeight);

    for (int inputY = 0, outputY = topPadding; inputY < inputHeight; inputY++, outputY += multiple) {
      // Write the contents of this row of the barcode
      for (int inputX = 0, outputX = leftPadding; inputX < inputWidth; inputX++, outputX += multiple) {
        if (input.get(inputX, inputY) == 1) {
          output.setRegion(outputX, outputY, multiple, multiple);
        }
      }
    }

    return output;
  }

这个renderResult根据QRCode信息来构造BitMatrix。可以看到这里重新计算了输出的宽度和高度，是取了qrcode的宽高+两边的quietZone与输入参数的目标宽高取最大值。也就是说如果qrcode的宽高大于目标的宽高，则以qrcode的宽高为准，这种情况下的quietZone基本跟输出的一致。
一般而言qrcode的宽高小于目标宽高的话，这种情况下quietZone就跟输出的不一致，需要经过重新放大，得到的才是最后输出的padding。

二维码最大能包含多少信息量？

使用zxing生成和识别二维码