ValueError: operands could not be broadcast together with shapes (720,1280) (720,1281)

I tried to use the following program to extract slides from a video of a powerpoint:

import os
import subprocess

import numpy as np
import pandas as pd
import av

def genweights(frame: np.ndarray) -> np.ndarray:
    """Creqte a meshgrid for weights as exp(-z^2)"""
    ymax, xmax = frame.shape
    normfactor = ymax // 2
    # y is -1 to 1; x is in same scale but only on rightmost 2/3
    y = np.arange(-ymax//2, ymax-ymax//2) / normfactor
    x = np.arange(-xmax*2//3, xmax-xmax*2//3) / normfactor
    xx, yy = np.meshgrid(x, y)
    weights = np.exp(-(xx**2 + yy**2))
    weights[:, :xmax//3] = 0  # left one-third = ignore
    return weights

def penalty(prev, this, weight) -> np.ndarray:
    """Compute the pixel-based penalty of two different images"""
    this = np.square(this > 128)   # convert grayscale into B&W
    prev = np.square(prev > 128)
    return np.square(this - prev) * weights

MOVIE = "hesabi.mp4"
next_t = 2
incr = 2
prev = weight = None
threshold = 1e-3

files = []
container = av.open(MOVIE)
container.seek(0)
for frame in container.decode(video=0):
    if frame.is_corrupt or frame.time < next_t:
        continue
    next_t += incr
    # Extract
    this = frame.to_ndarray(format="gray")
    if prev is None:
        # initialization for comparison, and skip this frame
        weights = genweights(this)    # reusable weight matrix
        prev = np.zeros_like(this)
        continue
    # Compare and save frame as JPEG image
    score = np.mean(penalty(prev, this, weights))
    prev = this
    if score > threshold:
        filename = "screen-{:02.0f}m{:.0f}s.jpg".format(frame.time//60, frame.time%60)
        files.append(filename)
        frame.to_image().save(filename)

# convert image into PDF and remove temp files
subprocess.run(["convert"]+files+["deck.pdf"])
for f in files:
    os.unlink(f)

However, when I run it I get the following error:

Traceback (most recent call last):
  File "scap.py", line 47, in <module>
    score = np.mean(penalty(prev, this, weights))
  File "scap.py", line 24, in penalty
    return np.square(this - prev) * weights
ValueError: operands could not be broadcast together with shapes (720,1280) (720,1281)

Please guide me how I can solve it.

Answer

Integer division will round down rather than rounding towards zero. If you replace:

xmax-xmax*2//3

with:

xmax + (-xmax)*2//3

this should solve it.

Parentheses shown for clarity but are not needed.

>>> xmax
1280
>>> np.arange(-xmax*2//3, xmax - xmax*2//3).shape
(1281,)
>>> np.arange(-xmax*2//3, xmax + -xmax*2//3).shape
(1280,)

You should also do similarly with the ymax. Instead of:

np.arange(-ymax//2, ymax-ymax//2)

use:

np.arange(-ymax//2, ymax + -ymax//2)

Advertisement

Answer