2D 线链码
链码是一种无损压缩技术，用于在图像中表示对象。物体任何连续边界的坐标都可以表示为一串数字，其中每个数字代表一个特定的方向，连线上的下一个点就在这个方向上。一个点作为参考/起点，在绘制由链生成的点时，可以重新绘制原始图形。
本文描述了如何生成二维直线的 8 邻域链码。在矩形网格中，一个点最多可以有 8 个周围的点，如下所示。线上的下一个点必须是这 8 个周围点之一。每个方向都分配了一个代码。使用这个代码，我们可以找到下一个应该绘制的周围点。
链码可以通过对每个方向使用条件语句来生成，但是对于具有大量方向的系统(三维网格可以有多达 26 个方向)来描述变得非常乏味。相反，我们使用散列函数。计算并散列两个连续点的 X( 
)和 Y( 
)坐标之差，以生成这两个点之间的链码的密钥。
链码列表:![[5, 6, 7, 4, -1, 0, 3, 2, 1]](/Uploads/Picture/2022-05-08/6277984ecb240.png "Rendered by QuickLaTeX.com")
哈希函数:
散列表:-
|  |  |  | ![chainCode[C]](img/f094e3ca2f98b24c8e14a2aaf796e040.png "Rendered by QuickLaTeX.com") | | --- | --- | --- | --- | | one | Zero | five | Zero | | one | one | eight | one | | Zero | one | seven | Two | | -1 | one | six | three | | -1 | Zero | three | four | | -1 | -1 | Zero | five | | Zero | -1 | one | six | | one | -1 | Two | seven | 该函数不生成值 4，因此会在其中存储一个伪值。 示例:Input : (2, -3), (-4, 2)
Output : Chain code for the straight line from (2, -3) to (-4, 2) is 333433
Input : (-7, -4), (9, 3)
Output : Chain code for the straight line from (-7, -4) to (9, 3) is 0101010100101010
# Python3 code for generating 8-neighbourhood chain
# code for a 2-D line
codeList = [5, 6, 7, 4, -1, 0, 3, 2, 1]
# This function generates the chaincode
# for transition between two neighbour points
def getChainCode(x1, y1, x2, y2):
dx = x2 - x1
dy = y2 - y1
hashKey = 3 * dy + dx + 4
return codeList[hashKey]
'''This function generates the list of
chaincodes for given list of points'''
def generateChainCode(ListOfPoints):
chainCode = []
for i in range(len(ListOfPoints) - 1):
a = ListOfPoints[i]
b = ListOfPoints[i + 1]
chainCode.append(getChainCode(a[0], a[1], b[0], b[1]))
return chainCode
'''This function generates the list of points for
a straight line using Bresenham's Algorithm'''
def Bresenham2D(x1, y1, x2, y2):
ListOfPoints = []
ListOfPoints.append([x1, y1])
xdif = x2 - x1
ydif = y2 - y1
dx = abs(xdif)
dy = abs(ydif)
if(xdif > 0):
xs = 1
else:
xs = -1
if (ydif > 0):
ys = 1
else:
ys = -1
if (dx > dy):
# Driving axis is the X-axis
p = 2 * dy - dx
while (x1 != x2):
x1 += xs
if (p >= 0):
y1 += ys
p -= 2 * dx
p += 2 * dy
ListOfPoints.append([x1, y1])
else:
# Driving axis is the Y-axis
p = 2 * dx-dy
while(y1 != y2):
y1 += ys
if (p >= 0):
x1 += xs
p -= 2 * dy
p += 2 * dx
ListOfPoints.append([x1, y1])
return ListOfPoints
def DriverFunction():
(x1, y1) = (-9, -3)
(x2, y2) = (10, 1)
ListOfPoints = Bresenham2D(x1, y1, x2, y2)
chainCode = generateChainCode(ListOfPoints)
chainCodeString = "".join(str(e) for e in chainCode)
print ('Chain code for the straight line from', (x1, y1),
'to', (x2, y2), 'is', chainCodeString)
DriverFunction()
Chain code for the straight line from (-9, -3) to (10, 1) is 0010000100010000100
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