Class AlignmentPatternFinder

java.lang.Object

com.google.zxing.qrcode.detector.AlignmentPatternFinder

final class AlignmentPatternFinder
extends Object

This class attempts to find alignment patterns in a QR Code. Alignment patterns look like finder patterns but are smaller and appear at regular intervals throughout the image.

At the moment this only looks for the bottom-right alignment pattern.

This is mostly a simplified copy of FinderPatternFinder. It is copied, pasted and stripped down here for maximum performance but does unfortunately duplicate some code.

This class is thread-safe but not reentrant. Each thread must allocate its own object.

Field Summary

Fields

Modifier and Type	Field	Description
private final int[]	crossCheckStateCount
private final int	height
private final BitMatrix	image
private final float	moduleSize
private final List<AlignmentPattern>	possibleCenters
private final ResultPointCallback	resultPointCallback
private final int	startX
private final int	startY
private final int	width

Constructor Summary

Constructors

Constructor	Description
AlignmentPatternFinder(BitMatrix image, int startX, int startY, int width, int height, float moduleSize, ResultPointCallback resultPointCallback)	Creates a finder that will look in a portion of the whole image.

Method Summary

Modifier and Type	Method	Description
private static float	centerFromEnd(int[] stateCount, int end)	Given a count of black/white/black pixels just seen and an end position, figures the location of the center of this black/white/black run.
private float	crossCheckVertical(int startI, int centerJ, int maxCount, int originalStateCountTotal)	After a horizontal scan finds a potential alignment pattern, this method "cross-checks" by scanning down vertically through the center of the possible alignment pattern to see if the same proportion is detected.
(package private) AlignmentPattern	find()	This method attempts to find the bottom-right alignment pattern in the image.
private boolean	foundPatternCross(int[] stateCount)
private AlignmentPattern	handlePossibleCenter(int[] stateCount, int i, int j)	This is called when a horizontal scan finds a possible alignment pattern.

Methods inherited from class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Details

image

private final BitMatrix image

possibleCenters

private final List<AlignmentPattern> possibleCenters

startX

private final int startX

startY

private final int startY

width

private final int width

height

private final int height

moduleSize

private final float moduleSize

crossCheckStateCount

private final int[] crossCheckStateCount

resultPointCallback

private final ResultPointCallback resultPointCallback

Constructor Details

AlignmentPatternFinder

AlignmentPatternFinder(BitMatrix image,
 int startX,
 int startY,
 int width,
 int height,
 float moduleSize,
 ResultPointCallback resultPointCallback)

Creates a finder that will look in a portion of the whole image.

Parameters:

image - image to search

startX - left column from which to start searching

startY - top row from which to start searching

width - width of region to search

height - height of region to search

moduleSize - estimated module size so far

Method Details

find

AlignmentPattern find()
               throws NotFoundException

This method attempts to find the bottom-right alignment pattern in the image. It is a bit messy since it's pretty performance-critical and so is written to be fast foremost.

Returns:

AlignmentPattern if found

Throws:

NotFoundException - if not found

centerFromEnd

private static float centerFromEnd(int[] stateCount,
 int end)

Given a count of black/white/black pixels just seen and an end position, figures the location of the center of this black/white/black run.

foundPatternCross

private boolean foundPatternCross(int[] stateCount)

Parameters:

stateCount - count of black/white/black pixels just read

Returns:

true iff the proportions of the counts is close enough to the 1/1/1 ratios used by alignment patterns to be considered a match

crossCheckVertical

private float crossCheckVertical(int startI,
 int centerJ,
 int maxCount,
 int originalStateCountTotal)

After a horizontal scan finds a potential alignment pattern, this method "cross-checks" by scanning down vertically through the center of the possible alignment pattern to see if the same proportion is detected.

Parameters:

startI - row where an alignment pattern was detected

centerJ - center of the section that appears to cross an alignment pattern

maxCount - maximum reasonable number of modules that should be observed in any reading state, based on the results of the horizontal scan

Returns:

vertical center of alignment pattern, or Float.NaN if not found

handlePossibleCenter

private AlignmentPattern handlePossibleCenter(int[] stateCount,
 int i,
 int j)

This is called when a horizontal scan finds a possible alignment pattern. It will cross check with a vertical scan, and if successful, will see if this pattern had been found on a previous horizontal scan. If so, we consider it confirmed and conclude we have found the alignment pattern.

Parameters:

stateCount - reading state module counts from horizontal scan

i - row where alignment pattern may be found

j - end of possible alignment pattern in row

Returns:

AlignmentPattern if we have found the same pattern twice, or null if not