增量时间和低估（我不了解增量时间）

Question

嘿，你们在理解这些增量时间问题时遇到了问题。它在 Pong-3 当然是 CS50 - 游戏开发简介。我不明白为什么 dt 在那里。 dt 是所有代码中都没有提到的变量。 BUt 在语言 LUA 的渲染过程中被调用。它就像一个局部变量，但不是预定义的或引入该变量的另一种方式。我会给出代码并等待......一些答案。

    --[[
    GD50 2018
    Pong Remake

    pong-3
    "The Paddle Update"

    -- Main Program --

    Author: Colton Ogden
    cogden@cs50.harvard.edu

    Originally programmed by Atari in 1972. Features two
    paddles, controlled by players, with the goal of getting
    the ball past your opponent's edge. First to 10 points wins.

    This version is built to more closely resemble the NES than
    the original Pong machines or the Atari 2600 in terms of
    resolution, though in widescreen (16:9) so it looks nicer on 
    modern systems.
]]

-- push is a library that will allow us to draw our game at a virtual
-- resolution, instead of however large our window is; used to provide
-- a more retro aesthetic
--
-- https://github.com/Ulydev/push
push = require 'push'

WINDOW_WIDTH = 1280
WINDOW_HEIGHT = 720

VIRTUAL_WIDTH = 432
VIRTUAL_HEIGHT = 243

-- speed at which we will move our paddle; multiplied by dt in update
PADDLE_SPEED = 200

--[[
    Runs when the game first starts up, only once; used to initialize the game.
]]
function love.load()
    love.graphics.setDefaultFilter('nearest', 'nearest')

    -- more "retro-looking" font object we can use for any text
    smallFont = love.graphics.newFont('font.ttf', 8)

    -- larger font for drawing the score on the screen
    scoreFont = love.graphics.newFont('font.ttf', 32)

    -- set LÖVE2D's active font to the smallFont obect
    love.graphics.setFont(smallFont)

    -- initialize window with virtual resolution
    push:setupScreen(VIRTUAL_WIDTH, VIRTUAL_HEIGHT, WINDOW_WIDTH, WINDOW_HEIGHT, {
        fullscreen = false,
        resizable = false,
        vsync = true
    })

    -- initialize score variables, used for rendering on the screen and keeping
    -- track of the winner
    player1Score = 0
    player2Score = 0

    -- paddle positions on the Y axis (they can only move up or down)
    player1Y = 30
    player2Y = VIRTUAL_HEIGHT - 50
end

--[[
    Runs every frame, with "dt" passed in, our delta in seconds 
    since the last frame, which LÖVE2D supplies us.
]]
function love.update(dt)
    -- player 1 movement
    if love.keyboard.isDown('w') then
        -- add negative paddle speed to current Y scaled by deltaTime
        player1Y = player1Y + -PADDLE_SPEED * dt
    elseif love.keyboard.isDown('s') then
        -- add positive paddle speed to current Y scaled by deltaTime
        player1Y = player1Y + PADDLE_SPEED * dt
    end

    -- player 2 movement
    if love.keyboard.isDown('up') then
        -- add negative paddle speed to current Y scaled by deltaTime
        player2Y = player2Y + -PADDLE_SPEED * dt
    elseif love.keyboard.isDown('down') then
        -- add positive paddle speed to current Y scaled by deltaTime
        player2Y = player2Y + PADDLE_SPEED * dt
    end
end

--[[
    Keyboard handling, called by LÖVE2D each frame; 
    passes in the key we pressed so we can access.
]]
function love.keypressed(key)
    -- keys can be accessed by string name
    if key == 'escape' then
        -- function LÖVE gives us to terminate application
        love.event.quit()
    end
end

--[[
    Called after update by LÖVE2D, used to draw anything to the screen, 
    updated or otherwise.
]]
function love.draw()
    -- begin rendering at virtual resolution
    push:apply('start')

    -- clear the screen with a specific color; in this case, a color similar
    -- to some versions of the original Pong
    love.graphics.clear(40/255, 45/255, 52/255, 255/255)

    -- draw welcome text toward the top of the screen
    love.graphics.setFont(smallFont)
    love.graphics.printf('Hello Pong!', 0, 20, VIRTUAL_WIDTH, 'center')

    -- draw score on the left and right center of the screen
    -- need to switch font to draw before actually printing
    love.graphics.setFont(scoreFont)
    love.graphics.print(tostring(player1Score), VIRTUAL_WIDTH / 2 - 50, 
        VIRTUAL_HEIGHT / 3)
    love.graphics.print(tostring(player2Score), VIRTUAL_WIDTH / 2 + 30,
        VIRTUAL_HEIGHT / 3)

    -- render first paddle (left side), now using the players' Y variable
    love.graphics.rectangle('fill', 10, player1Y, 5, 20)

    -- render second paddle (right side)
    love.graphics.rectangle('fill', VIRTUAL_WIDTH - 10, player2Y, 5, 20)

    -- render ball (center)
    love.graphics.rectangle('fill', VIRTUAL_WIDTH / 2 - 2, VIRTUAL_HEIGHT / 2 - 2, 4, 4)

    -- end rendering at virtual resolution
    push:apply('end')
end


                          

Answer 1

The Love update docs 简要提及增量时间。

dt（增量时间）保存自上次调用 update 函数以来的时间（以秒为单位）。在正常情况下，这应该是几分之一秒。作为游戏循环的一部分，Love 框架会反复为您调用 update 函数。它还在框架内的某处定义并跟踪dt 变量，这就是为什么您看不到它在任何地方定义或引入的原因。您只能从更新函数中访问它（除非您将它传递到其他地方）。

为什么需要增量时间？
因为您不能保证update 将始终以一致的速率被调用。假设您有一个玩家角色，每次调用 update 时您移动 1 个像素。如果您的游戏以 60fps 运行，那么您的角色在 1 秒内移动 60px。但是你的游戏速度下降到 59、50 或 12fps，那么你的角色也会变慢，这通常不是你想要的。

您可以使用增量时间来为您的角色保持一致的速度。将所需的每秒速度 (60px) 乘以 dt 将为您获得移动每一帧的确切量，无论 update 被调用的频率如何。

这就是您的代码中在以下位置发生的情况：
player2Y = player2Y + PADDLE_SPEED * dt

PADDLE_SPEED 是移动量每秒乘以自上次更新以来的秒数。结果被添加到玩家的位置。