how to correctly convert OccupancyGrid format message to image ?

edit

Hello,all

I want to convert OccupancyGrid format message to 'mono8' formt image. Actually,I subscribed /move_base/local_costmap/costmap topic ,but the result look like weird, compared with map showed in rviz,it was inversed and rotated . here is code relative converting proccess ：

 def callback(self,data):
     self.width = data.info.width
     self.height = data.info.height
     self.resolution = data.info.resolution
     self.length = len(data.data)
     #self.min_line = []

     #creat an mat to load costmap
     costmap_mat = cv.CreateMat(self.height,self.width,cv.CV_8UC1)

     for i in range(1,self.height):
         for j in range(1,self.width):
            cv.Set2D(costmap_mat,i-1,j-1,255-int(float(data.data[(i-1)*self.width+j])/100*255))

Any answer is appreciated .-

edit：

the code given by @Stefan Kohlbrecher shed light on my problem . It was wrong with order i-1 and j-1。here is modified code and it works,

for i in range(1,self.height):
            for  j in range(1,self.width):
               cv.Set2D(costmap_mat,self.width-j,self.height-i,255-int(float(data.data[(i-1)*self.width+j])/100*255))

Althrough my problem was solved , I am still confused that the sequence is self.height-1--->0 rather than 0--->self.height-1 .