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我用的openmv4 H7 ，官网介绍有1M的ram，我用VGA灰度方式读图片，然后在图片里寻找圆，会报内存溢出。
计算一下VGA是640*480，灰度一个字节，大概是300K，远远小于1M。手册上介绍说只有100K的缓冲区，有一个sensor.alloc_extra_fb(width, he ight, pixformat)函数重新分配缓冲区。这一个怎么用？

import sensor, image, time

rect = (0, 0, 340, 240)

sensor.reset()                      # Reset and initialize the sensor.
sensor.set_pixformat(sensor.GRAYSCALE) # Set pixel format to RGB565 (or GRAYSCALE)
sensor.set_framesize(sensor.VGA)   # Set frame size to QVGA (320x240)
sensor.alloc_extra_fb(640, 480, sensor.GRAYSCALE)

#sensor.set_windowing(rect)

sensor.skip_frames(time = 2000)     # Wait for settings take effect.


while(True):
    
    img = sensor.snapshot()         # Take a picture and return the image.
                                # to the IDE. The FPS should increase once disconnected.
    for c in img.find_circles(roi=rect, x_stride=2, y_stride=1,threshold = 3800, x_margin = 10,
    y_margin = 10, r_margin = 10,r_min = 110, r_max = 120, r_step = 2):
        img.draw_circle(c.x(), c.y(), c.r(), color = 255)