javascript - 如何动画网球从水平表面反弹
问题描述
我有一个网球的图像:
有必要制作一个球落下的动画,随后从固体表面反弹。
我得到了这种动画,但它看起来并不真实:
要开始动画,请单击图像:
<svg id="svg1" version="1.1" xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
width="200" height="200" viewBox="0 0 200 200" preserveAspectRatio="xMinYMin meet" style="border:1px solid" >
<image xlink:href="https://i.stack.imgur.com/hXyA5.png" x="82" width="25px" height="25px" >
<animateTransform attributeName="transform" type="translate" dur="1s" begin="svg1.click" values="0,0;0,168;0" repeatCount="3" />
</image>
<polyline points="5,190 190,190" stroke="silver" stroke-width="4" />
</svg>
第一次反弹小于球落下的高度,第二次反弹小于第一次反弹的高度,第三次反弹小于第二次。
你如何做到这一点?解决方案可能在 SMIL SVG、CSS、JS 上
首选 SMIL SVG 解决方案。
解决方案
最现实的方法是用 JS 模拟物理。
像这样的东西:
let ballElem = document.getElementById("ball");
let GRAVITY = 40; // Acceleration due to gravity (pixels / sec /sec)
let FLOOR_Y = 200 - 25; // Y coord of floor. The 25 here is because ball.y is the top of the ball.
let BOUNCINESS = 0.8; // Velocity retained after a bounce
let LIMIT = 0.1; // Minimum velocity required to keep animation running
let ball = {};
let lastFrameTime = null;
ballElem.addEventListener("click", startAnim);
function startAnim()
{
ball = {x: 82, y: 0, dx: 0, dy: 0};
lastFrameTime = null;
requestAnimationFrame(animStep);
}
function animStep(timestamp)
{
if (lastFrameTime === null)
lastFrameTime = timestamp;
// Milliseconds elapsed since last step
const elapsed = timestamp - lastFrameTime;
lastFrameTime = timestamp;
ball.dy += GRAVITY * elapsed / 1000;
ball.y += ball.dy;
ball.x += ball.dx; // not really used in this example
if (ball.y > FLOOR_Y) {
// Step has taken us below the floor, so we need to rebound the ball.
ball.y -= (ball.y - FLOOR_Y);
ball.dy = -ball.dy * BOUNCINESS;
}
// Update the <image> element x and y
ballElem.x.baseVal.value = ball.x;
ballElem.y.baseVal.value = ball.y;
// Request another animation step
if (Math.abs(ball.y - FLOOR_Y) > LIMIT || // Not on ground
Math.abs(ball.dy) > LIMIT || // or still moving
Math.abs(ball.dx) > LIMIT) {
requestAnimationFrame(animStep);
}
}
<svg id="svg1"
width="200" height="200" viewBox="0 0 200 200" preserveAspectRatio="xMinYMin meet" style="border:1px solid" >
<image id="ball" xlink:href="https://i.stack.imgur.com/hXyA5.png" x="82" width="25px" height="25px"/>
</svg>
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