python - cv2中的可变fps(每秒帧数)

Question

我使用 cv2 从我拥有的不同帧创建视频。创建视频时，我无法更改 fps(每秒帧数)。我希望视频开始时速度慢但结束时速度快，这意味着开始时 fps 小但结束时 fps 大。但是，当我实例化 cv2.VideoWriter 时，我无法再更改 fps。我该怎么办？

可复制代码

import numpy as np
import cv2, os
import matplotlib

image_size = 200
def create_image_array(image_size):
  image_array = np.random.randn(image_size, image_size)
  row = np.random.randint(0, image_size)
  image_array[row, :] = 100
  return image_array

frame_numbers = 200
for i in range(frame_numbers):
  image_array = create_image_array(image_size)
  matplotlib.image.imsave(f'./shots/frame_{i:03d}.png', image_array)

def make_a_video(shots_folder, video_path):

    shots_folder = 'shots'
    fps = 25
    images = [img for img in os.listdir(shots_folder) if img.endswith(".png")]

    images = sorted(images)[:]
    frame = cv2.imread(os.path.join(shots_folder, images[0]))
    height, width, layers = frame.shape

    video = cv2.VideoWriter(video_path, cv2.VideoWriter_fourcc(*'mp4v'), fps, (width, height))

    for image in images:
        video.write(cv2.imread(os.path.join(shots_folder, image)))

    cv2.destroyAllWindows()
    video.release()

shots_folder = 'shots'
video_path = 'video.mp4'  
make_a_video(shots_folder, video_path)

Answer 1

将其想象成将一个视频复制到另一个视频。在执行此操作时，您实际上不想更改输出帧速率，而是更改输入帧速率。这是一个非常基本的示例，说明如何在写入恒定帧速率的流时连续更改读取帧速率。

示例生成200帧的样本视频，写入320帧的视频。前 100 帧以 5 fps 读取，接下来的 100 帧以从 5 到 25 fps 的递增速率读取，最后 120 帧以 25 fps 读取。

import math
import numpy as np
import cv2

image_size = 200
src_fps = 25
dst_fps = 25


# create sample frame with four rotating balls
def create_sample_frame(size, frame_no, fps):
    img = np.zeros((size, size, 3), dtype=np.uint8)
    ctr = size // 2

    for i in range(1, 5):
        phi = -frame_no / fps * i
        r = size * (0.5 - 0.1 * i)
        cv2.circle(img, (round(math.sin(phi) * r + ctr), round(math.cos(phi) * r + ctr)), size // 30, (0, 255, 0), -1)

    return img


frames = [create_sample_frame(image_size, i, src_fps) for i in range(200)]
height, width, layers = frames[0].shape
video = cv2.VideoWriter('video.mp4', cv2.VideoWriter_fourcc(*'mp4v'), dst_fps, (width, height))

# ramp array with number of destination frames, start fps of source, end fps of source
fps_ramp = [[100, 5, 5], [100, 5, 25], [120, 25, 25]]

src_pos = 0
dst_pos = 0

for n, start_src_fps, end_src_fps in fps_ramp:
    for i in range(n):
        print(f"writing source frame {int(src_pos)} to destination frame {dst_pos}")
        video.write(frames[round(src_pos)])
        dst_pos += 1
        cur_fps = (end_src_fps - start_src_fps) * (i / n) + start_src_fps
        src_pos += cur_fps / src_fps

video.release()

结果: