Assemble an image hidden among multiple video frames.

crop_frames.py

'''
	Construct a single image by stacking the bottom READ_HORIZ_PX
	of each source frame of form image-XXXX.jpeg.

	To create source frames of form image-XXXX.jpeg from a video file:
	$ ffmpeg -i inputfile.avi image-%4d.jpeg
	
	This was helpful in extracting a QR code partially displayed 
	across many frames.
'''
import os
from PIL import Image

READ_HORIZ_PX = 2  # Number of horizontal lines to read from each source frame
FRAME_SKIP_INTERVAL = -1 # -1 = use all frames. 2 = skip every other frame, 3 = skip every third frame etc

# Source frame dimens
FRAME_WIDTH = 320
FRAME_HEIGHT = 240


CROP_BOX_BOTTOM_MARGIN = 0 # Turned out not to help
# Crop box for source frame
# 	      left, top, left+width, top+height
CROP_BOX = (0, FRAME_HEIGHT - READ_HORIZ_PX, FRAME_WIDTH, FRAME_HEIGHT - CROP_BOX_BOTTOM_MARGIN)

# Pasting box for destination frame. Will be adjusted after each paste
# 	      left, top, left+width, top+height
paste_box = (0, 0, FRAME_WIDTH, READ_HORIZ_PX - CROP_BOX_BOTTOM_MARGIN)
dst_filename = 'output'

dst_img = None


def increment_paste_box(src):
	new_src = (src[0], src[1] + READ_HORIZ_PX, src[2] , src[3] + READ_HORIZ_PX)
	return new_src

frame_num = 0
for prospect in os.listdir('./'):
	if 'image-' in prospect:
		frame_num +=1
		if FRAME_SKIP_INTERVAL != -1 and frame_num % FRAME_SKIP_INTERVAL == 0:
			continue

		frame = Image.open(prospect)

		if not dst_img:
			dst_img = Image.new(frame.mode, frame.size, "black")

		qr_region = frame.crop(CROP_BOX)
		dst_img.paste(qr_region, paste_box)
		paste_box = increment_paste_box(paste_box)

dst_path = dst_filename + '_' + str(READ_HORIZ_PX) +  '_' + str(FRAME_SKIP_INTERVAL) + '.jpeg'
dst_img.save(dst_path, 'JPEG')
print 'saved ' + dst_path