openmv求取物体偏移角

import sensor, image, time
from machine import UART,Timer
from fpioa_manager import fm
#映射串口引脚
fm.register(6, fm.fpioa.UART1_RX, force=True)
fm.register(7, fm.fpioa.UART1_TX, force=True)
#初始化串口
uart = UART(UART.UART1, 115200, read_buf_len=4096)


yellow_threshold   = (25, 91, 20, 80, 9, 67)


sensor.reset()
sensor.set_pixformat(sensor.RGB565)
sensor.set_framesize(sensor.QQVGA) #160*120
sensor.skip_frames(10)
sensor.set_auto_whitebal(False)
clock = time.clock()
cx=160/2
K=450
while(True):
    clock.tick() # Track elapsed milliseconds between snapshots().
    img = sensor.snapshot() # Take a picture and return the image.

    blobs = img.find_blobs([yellow_threshold])
    if len(blobs) == 1:
        # Draw a rect around the blob.
        b = blobs[0]
        img.draw_rectangle(b[0:4]) # rect
        img.draw_cross(b[5], b[6]) # cx, cy

        c=cx-b[5]
        if c > 20:
            print("在右")
            uart.write('left')
        if c < -20:
            print("在左")
            uart.write('right')
        Lm = (b[2]+b[3])/2
        length = K/Lm
        print(length)
        uart.write(str(length))
        time.sleep_ms(100)

能够实现检测色块的距离
返回色块中心坐标与图像中心坐标的像素差，但是一个是实际距离，另一个是像素，怎么才能得到色块的偏移角
下面是在星瞳科技的测距内容