// Does not sharpen the data, as this call is intended to only be used by 2D Readers.
@Override
public BitMatrix getBlackMatrix() throws NotFoundException {
LuminanceSource source = getLuminanceSource();
int width = source.getWidth();
int height = source.getHeight();
BitMatrix matrix = new BitMatrix(width, height);
// Quickly calculates the histogram by sampling four rows from the image. This proved to be
// more robust on the blackbox tests than sampling a diagonal as we used to do.
initArrays(width);
int[] localBuckets = buckets;
for (int y = 1; y < 5; y++) {
int row = height * y / 5;
byte[] localLuminances = source.getRow(row, luminances);
int right = (width * 4) / 5;
for (int x = width / 5; x < right; x++) {
int pixel = localLuminances[x] & 0xff;
localBuckets[pixel >> LUMINANCE_SHIFT]++;
}
}
int blackPoint = estimateBlackPoint(localBuckets);
// We delay reading the entire image luminance until the black point estimation succeeds.
// Although we end up reading four rows twice, it is consistent with our motto of
// "fail quickly" which is necessary for continuous scanning.
byte[] localLuminances = source.getMatrix();
for (int y = 0; y < height; y++) {
int offset = y * width;
for (int x = 0; x < width; x++) {
int pixel = localLuminances[offset + x] & 0xff;
if (pixel < blackPoint) {
matrix.set(x, y);
}
}
}
return matrix;
}
