python - 如何使用 pygame 正确实现音频延迟测试

Question

我有一个学校项目涉及用 Python 制作一个简单的节奏游戏。我目前正在尝试使用 pygame 将闪烁的点与已知 bpm(在本例中为 120)的歌曲同步。

闪烁的点一开始是同步的，但慢慢地与时间不同步。我在互联网上搜索了如何制作节奏游戏，发现我需要进行音频延迟测试。但是，我想出的代码并没有真正起作用，所以我真的很想得到一些帮助。我已经尝试删除 mp3 开头的间隙并将其导出为没有元数据的 ogg。

import sys, pygame, math, random, time

pygame.init()

size = width, height = 1000, 1000
speed = [2, 2]
black = 0, 0, 0

screen = pygame.display.set_mode(size)
pygame.display.update()
pygame.mixer.init()
pygame.mixer.music.load("metr.ogg")

continuer = True

bpm = 120

t = 60 / bpm

font = pygame.font.Font('freesansbold.ttf', 15)

lasttime = 0

delay = 0
red = (200, 0, 0)
green = (0, 200, 0)
colors = [red, red, red, red]
i = 0
av = []
drawn = False
clock = pygame.time.Clock()
pygame.mixer.music.play()
while continuer:

    time = (pygame.mixer.music.get_pos()) / 1000
    nextkey = lasttime + t + (delay / 1000)
    if time > nextkey:
        if drawn:
            screen.fill((0, 0, 0))
            drawn = False
        else:
            pygame.draw.circle(screen, colors[i], (500, 500), 20)
            drawn = True
            if i == 3:
                i = 0
            else:
                i += 1
        lasttime += t
    screen.blit(font.render('delay = ' + str(delay) + "ms", True, (255, 255, 255), (0, 0, 0)), (500, 600))
    for event in pygame.event.get():
        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_RIGHT:
                delay += 1
            elif event.key == pygame.K_h:
                av.append(nextkey - time)
                delay = sum(av) / len(av)
            elif event.key == pygame.K_LEFT:
                delay -= 1
            elif event.key == pygame.K_SPACE:
                continuer = False
    pygame.display.flip()
    clock.tick(60)

pygame.quit()

文件 metr.ogg 只是一个 120bpm 的节拍器

Answer 1

这是因为你的变量 nextkey 是不同变量的总和，每个变量都会包含一个小错误。所以你总是在添加小错误，直到它最终变得明显。

防止累积误差的方法是始终直接从源头进行计算。在你的情况下 pygame.mixer.music.get_pos()。

在下面的例子中，我们首先创建一个变量ms_per_beat。该值是一个常量，用于定义每个节拍应该经过多少毫秒。对于每一帧，我们根据歌曲播放的时间计算time_since_last_beat。一个粗略的、不完整的例子:

bpm = 120
beats_per_ms = (bpm / 60) / 1000
ms_per_beat  = 1 / beats_per_ms  # How many milliseconds each beat takes.
current_beat = 0                 # The beat we're currently on.
time_since_last_beat = 0         # How many milliseconds since last beat.

dot_display_time = 500  # Display the dot for 500 ms (half a second)
dot_timer = 0           # Keeps track on how long the dot has been displayed.
display_dot = False     # Whether to display the dot or not.

clock = pygame.time.Clock()

while running:
    dt = clock.tick(60)

    current_play_time_ms = pygame.mixer.music.get_pos() / 1000
    time_since_last_beat = current_play_time_ms - (current_beat * ms_per_beat)

    if time_since_last_beat >= ms_per_beat:
        print('Bop!')
        current_beat += 1
        display_dot = True
        dot_timer = dot_display_time

    screen.fill((0, 0, 0))

    if display_dot:
        dot_timer -= dt
        if dot_timer <= 0:
            display_dot = False
        pygame.draw.circle(screen, colors[i], (500, 500), 20)

    pygame.display.update()

上面的例子假设音频正好在节拍上开始。它还假设它实际上是 120 bpm。如果它是 120.1 bpm，它最终会失去同步。更合适的方法是分析音频的振幅峰值，然后只显示该点。如果您的音频只是节拍器，则可以使用 pygame.mixer.music.get_volume() 完成。

dot_display_time = 500  # Display the dot for 500 ms (half a second)
dot_timer = 0           # Keeps track on how long the dot has been displayed.
display_dot = False     # Whether to display the dot or not.

beat_volume_threshold = 0.7  # The volume the audio has to overcome to count as a beat.

clock = pygame.time.Clock()

while running:
    dt = clock.tick(60)

    if pygame.mixer.music.get_volume() >= beat_volume_threshold:
        print('Bop!')
        display_dot = True
        dot_timer = dot_display_time

    screen.fill((0, 0, 0))

    if display_dot:
        dot_timer -= dt
        if dot_timer <= 0:
            display_dot = False
        pygame.draw.circle(screen, colors[i], (500, 500), 20)

    pygame.display.update()

但是，我会推荐一种完全不同的方法。同步总是一件麻烦事，所以尽可能避免它。让您的游戏成为节拍器，而不是让您的游戏与节拍器同步。有一个特定时间过去时播放的“bop”音频。然后您的音频和图形将始终同步，因为它们使用相同的时钟。下面的示例在 pygame.time.set_timer 的帮助下播放安排用户事件的音频。

import pygame

# This fixes the latency issue with the pygame mixer.
pygame.mixer.pre_init(22050, -16, 2, 1024)
pygame.init()

PLAY_CLICK = pygame.USEREVENT + 1

screen = pygame.display.set_mode((400, 400))

dot_display_time = 200  # Display the dot for 500 ms (half a second)
dot_timer = 0           # Keeps track on how long the dot has been displayed.
display_dot = False     # Whether to display the dot or not.

bpm = 120
beats_per_ms = (bpm / 60) / 1000
ms_per_beat  = 1 / beats_per_ms  # How many milliseconds each beat takes.

clock = pygame.time.Clock()
sound = pygame.mixer.Sound('bop.wav')

pygame.time.set_timer(PLAY_CLICK, int(ms_per_beat))  # Play sound repeatedly every 'beats_per_ms'.

running = True
while running:
    dt = clock.tick(60)

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == PLAY_CLICK:
            sound.play()
            display_dot = True
            dot_timer = dot_display_time

    screen.fill((0, 0, 0))

    if display_dot:
        dot_timer -= dt
        if dot_timer <= 0:
            display_dot = False
        pygame.draw.circle(screen, pygame.Color('green'), (200, 200), 20)

    pygame.display.update()

如果您不习惯安排事件，您可以自己计算时间。

import pygame

# This fixes the latency issue with the pygame mixer.
pygame.mixer.pre_init(22050, -16, 2, 1024)
pygame.init()

font = pygame.font.Font('freesansbold.ttf', 15)
screen = pygame.display.set_mode((400, 400))

dot_display_time = 250  # Display the dot for 250 ms (quarter of a second)
dot_timer = 0           # Keeps track on how long the dot has been displayed.
display_dot = False     # Whether to display the dot or not.

bpm = 120

clock = pygame.time.Clock()
sound = pygame.mixer.Sound('bop.wav')

time = 0

running = True
while running:
    dt = clock.tick(60)
    time += dt

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.KEYUP:
            if event.key == pygame.K_LEFT:
                bpm -= 5
            elif event.key == pygame.K_RIGHT:
                bpm += 5

    # Calculate how long to wait based on the bpm.
    beats_per_ms = (bpm / 60) / 1000
    ms_per_beat  = 1 / beats_per_ms  # How many milliseconds each beat takes.

    if time >= ms_per_beat:
        time -= ms_per_beat
        sound.play()
        display_dot = True
        dot_timer = dot_display_time

    screen.fill((0, 0, 0))
    screen.blit(font.render('BPM {}'.format(bpm), True, (255, 255, 255), (0, 0, 0)), (160, 20))

    if display_dot:
        dot_timer -= dt
        if dot_timer <= 0:
            display_dot = False
        pygame.draw.circle(screen, pygame.Color('green'), (200, 200), 20)

    pygame.display.update()

但是请注意，pygame 在特定时间播放声音时似乎有困难。如果您减少缓冲区，效果会更好(如 documentation 中所述)。

pygame.mixer.pre_init(22050, -16, 2, 1024)
pygame.init()