Advanced Frame Differencing Example

# This work is licensed under the MIT license.
# Copyright (c) 2013-2023 OpenMV LLC. All rights reserved.
# https://github.com/openmv/openmv/blob/master/LICENSE
#
# Advanced Frame Differencing Example
#
# This example demonstrates using frame differencing with your OpenMV Cam. This
# example is advanced because it performs a background update to deal with the
# background image changing overtime.

import csi
import image
import time

TRIGGER_THRESHOLD = 5

BG_UPDATE_FRAMES = 50  # How many frames before blending.
BG_UPDATE_BLEND = 128  # How much to blend by... ([0-255]==[0.0-1.0]).

csi0 = csi.CSI()
csi0.reset()  # Reset and initialize the sensor.
csi0.pixformat(csi.RGB565)  # or csi.RGB565
csi0.framesize(csi.QVGA)  # or csi.QQVGA (or others)
csi0.snapshot(time=2000)  # Let new settings take affect.
csi0.auto_whitebal(False)  # Turn off white balance.
clock = time.clock()  # Tracks FPS.

# Create a second frame buffer on the heap.
extra_fb = image.Image(csi0.width(), csi0.height(), csi0.pixformat())

print("About to save background image...")
csi0.snapshot(time=2000)  # Give the user time to get ready.
extra_fb.draw_image(csi0.snapshot())
print("Saved background image - Now frame differencing!")

triggered = False

frame_count = 0
while True:
    clock.tick()  # Track elapsed milliseconds between snapshots().
    img = csi0.snapshot()  # Take a picture and return the image.

    frame_count += 1
    if frame_count > BG_UPDATE_FRAMES:
        frame_count = 0
        # Blend in new frame. We're doing 255-alpha here because we want to
        # blend the new frame into the background. Not the background into the
        # new frame which would be just alpha. Blend replaces each pixel by
        # ((NEW*(alpha))+(OLD*(255-alpha)))/255. So, a low alpha results in
        # low blending of the new image while a high alpha results in high
        # blending of the new image. We need to reverse that for this update.
        img.blend(extra_fb, alpha=(255 - BG_UPDATE_BLEND))
        extra_fb.draw_image(img)

    # Replace the image with the "abs(NEW-OLD)" frame difference.
    img.difference(extra_fb)

    hist = img.get_histogram()
    # This code below works by comparing the 99th percentile value (e.g. the
    # non-outlier max value against the 90th percentile value (e.g. a non-max
    # value. The difference between the two values will grow as the difference
    # image seems more pixels change.
    diff = hist.get_percentile(0.99).l_value - hist.get_percentile(0.90).l_value
    triggered = diff > TRIGGER_THRESHOLD

    print(clock.fps(), triggered)  # Note: Your OpenMV Cam runs about half as fast while
    # connected to your computer. The FPS should increase once disconnected.