How convert byte array RGB image into greyscale

Question

I use com.google.zxing core to scan an image. RGBLuminanceSource constructor gets int array of pixels, convert it into byte array and "convert the entire image to a greyscale array". I have byte array of pixels and i would like only to convert it.

Can anyone help me with with the gray-scale conversion?

package com.google.zxing;

/**
 * This class is used to help decode images from files which arrive as RGB data from
 * an ARGB pixel array. It does not support rotation.
 *
 * @author dswitkin@google.com (Daniel Switkin)
 * @author Betaminos
 */
public final class RGBLuminanceSource extends LuminanceSource {

  private final byte[] luminances;
  private final int dataWidth;
  private final int dataHeight;
  private final int left;
  private final int top;

  public RGBLuminanceSource(int width, int height, int[] pixels) {
    super(width, height);

    dataWidth = width;
    dataHeight = height;
    left = 0;
    top = 0;

    // In order to measure pure decoding speed, we convert the entire image to a greyscale array
    // up front, which is the same as the Y channel of the YUVLuminanceSource in the real app.
    //
    // Total number of pixels suffices, can ignore shape
    int size = width * height;
    luminances = new byte[size];
    for (int offset = 0; offset < size; offset++) {
      int pixel = pixels[offset];
      int r = (pixel >> 16) & 0xff; // red
      int g2 = (pixel >> 7) & 0x1fe; // 2 * green
      int b = pixel & 0xff; // blue
      // Calculate green-favouring average cheaply
      luminances[offset] = (byte) ((r + g2 + b) / 4);
    }
  }

    /**
     * My constructor
     */
  public RGBLuminanceSource(byte[] pixels,
                             int dataWidth,
                             int dataHeight
                        ) {
    this(pixels,dataWidth,dataHeight,0,0,dataWidth,dataHeight);

    // WHAT SHOULD I DO HERE TO "convert the entire image to a greyscale array"?




  }

  private RGBLuminanceSource(byte[] pixels,
                             int dataWidth,
                             int dataHeight,
                             int left,
                             int top,
                             int width,
                             int height) {
    super(width, height);
    if (left + width > dataWidth || top + height > dataHeight) {
      throw new IllegalArgumentException("Crop rectangle does not fit within image data.");
    }
    this.luminances = pixels;
    this.dataWidth = dataWidth;
    this.dataHeight = dataHeight;
    this.left = left;
    this.top = top;
  }

  @Override
  public byte[] getRow(int y, byte[] row) {
    if (y < 0 || y >= getHeight()) {
      throw new IllegalArgumentException("Requested row is outside the image: " + y);
    }
    int width = getWidth();
    if (row == null || row.length < width) {
      row = new byte[width];
    }
    int offset = (y + top) * dataWidth + left;
    System.arraycopy(luminances, offset, row, 0, width);
    return row;
  }

  @Override
  public byte[] getMatrix() {
    int width = getWidth();
    int height = getHeight();

    // If the caller asks for the entire underlying image, save the copy and give them the
    // original data. The docs specifically warn that result.length must be ignored.
    if (width == dataWidth && height == dataHeight) {
      return luminances;
    }

    int area = width * height;
    byte[] matrix = new byte[area];
    int inputOffset = top * dataWidth + left;

    // If the width matches the full width of the underlying data, perform a single copy.
    if (width == dataWidth) {
      System.arraycopy(luminances, inputOffset, matrix, 0, area);
      return matrix;
    }

    // Otherwise copy one cropped row at a time.
    for (int y = 0; y < height; y++) {
      int outputOffset = y * width;
      System.arraycopy(luminances, inputOffset, matrix, outputOffset, width);
      inputOffset += dataWidth;
    }
    return matrix;
  }

  @Override
  public boolean isCropSupported() {
    return true;
  }

  @Override
  public LuminanceSource crop(int left, int top, int width, int height) {
    return new RGBLuminanceSource(luminances,
                                  dataWidth,
                                  dataHeight,
                                  this.left + left,
                                  this.top + top,
                                  width,
                                  height);
  }

}

Answer 1

Assuming your pixels bytes are RGB byte interleaved format, you could do:

public RGBLuminanceSource(byte[] pixels, int width, int height) {
    super(width, height);

    dataWidth = width;
    dataHeight = height;
    left = 0;
    top = 0;

    // Total number of pixels suffices, can ignore shape
    int size = width * height;
    luminances = new byte[size];
    for (int offset = 0; offset < size; offset++) {
        int r  =  pixels[offset * 3    ] & 0xff; // red
        int g2 = (pixels[offset * 3 + 1] & 0xff) << 1 // 2 * green
        int b  =  pixels[offset * 3 + 2] & 0xff; // blue

        // Calculate green-favouring average cheaply
        luminances[offset] = (byte) ((r + g2 + b) / 4);
}

The calculation isn't really the standard way of computing luminance (grayscale) form RGB values, but should produce the same results as with the int packed RGB version.