如何使用HTML5 canvas標簽顯示視頻

Question

我想使用包含播放暫停功能的畫布顯示視頻，該功能允許用戶通過單擊畫布來切換播放，我還想在視頻暫停時在視頻上方繪制一個疊加層。 如何在javascript中完成？

畫布可用於顯示來自各種來源的視頻。 本示例說明如何將視頻加載為文件資源，顯示它並在屏幕播放/暫停切換上添加簡單的單擊。

只是一張圖片

就畫布而言，視頻只是圖像。 您可以像繪制任何圖像一樣繪制它。 區別在於視頻可以播放並具有聲音。

獲取畫布和基本設置

// It is assumed you know how to add a canvas and correctly size it.
var canvas = document.getElementById("myCanvas"); // get the canvas from the page
var ctx = canvas.getContext("2d");
var videoContainer; // object to hold video and associated info

創建和加載視頻

var video = document.createElement("video"); // create a video element
video.src = "urlOffVideo.webm"; 
// the video will now begin to load.
// As some additional info is needed we will place the video in a
// containing object for convenience
video.autoPlay = false; // ensure that the video does not auto play
video.loop = true; // set the video to loop.
videoContainer = {  // we will add properties as needed
     video : video,
     ready : false,   
};

與圖像元素不同，視頻不必完全加載即可在畫布上顯示。 視頻還提供了許多額外的事件，可用於監視視頻的狀態。

在這種情況下，我們希望知道何時可以播放視頻。 oncanplay意味着已經加載了足夠的視頻以播放其中的一些視頻，但可能還不足以播放到最后。

video.oncanplay = readyToPlayVideo; // set the event to the play function that 
                                  // can be found below

另外，您可以使用oncanplaythrough ，當加載了足夠多的視頻時，它將觸發，從而可以播放到最后。

video.oncanplaythrough = readyToPlayVideo; // set the event to the play function that
                                         // can be found below

只能使用canPlay事件之一，不能同時使用兩者。

可以播放事件（相當於圖片加載）

function readyToPlayVideo(event){ // this is a referance to the video
    // the video may not match the canvas size so find a scale to fit
    videoContainer.scale = Math.min(
                         canvas.width / this.videoWidth, 
                         canvas.height / this.videoHeight); 
    videoContainer.ready = true;
    // the video can be played so hand it off to the display function
    requestAnimationFrame(undateCanvas);
}

---

顯示中

視頻將不會在畫布上播放。 您需要為每個新框架繪制它。 由於很難知道確切的幀速率以及何時發生幀速率，最好的辦法是將視頻顯示為以60fps的速度運行。 如果幀速率較低，則w將同一幀渲染兩次。 如果幀速率較高，則看不到多余的幀，因此我們將其忽略。

video元素只是一個圖像元素，可以像繪制任何圖像一樣進行繪制，您可以縮放，旋轉，平移視頻，對其進行鏡像，淡入淡出，對其進行裁剪和僅顯示部分，並使用全局合成模式第二次繪制兩次添加效果，如變亮，屏幕等。

function updateCanvas(){
    ctx.clearRect(0,0,canvas.width,canvas.height); // Though not always needed 
                                                     // you may get bad pixels from 
                                                     // previous videos so clear to be
                                                     // safe
    // only draw if loaded and ready
    if(videoContainer !== undefined && videoContainer.ready){ 
        // find the top left of the video on the canvas
        var scale = videoContainer.scale;
        var vidH = videoContainer.video.videoHeight;
        var vidW = videoContainer.video.videoWidth;
        var top = canvas.height / 2 - (vidH /2 ) * scale;
        var left = canvas.width / 2 - (vidW /2 ) * scale;
        // now just draw the video the correct size
        ctx.drawImage(videoContainer.video, left, top, vidW * scale, vidH * scale);
        if(videoContainer.video.paused){ // if not playing show the paused screen 
            drawPayIcon();
        }
    }
    // all done for display 
    // request the next frame in 1/60th of a second
    requestAnimationFrame(updateCanvas);

}

基本播放暫停控制

現在，我們已經加載並顯示了視頻，我們需要的只是播放控件。 我們將其作為單擊切換在屏幕上播放。 當視頻正在播放並且用戶單擊時，視頻暫停。 暫停后，點擊繼續播放。 我們將添加使視頻變暗並繪制播放圖標（三角形）的功能

function drawPayIcon(){
     ctx.fillStyle = "black";  // darken display
     ctx.globalAlpha = 0.5;
     ctx.fillRect(0,0,canvas.width,canvas.height);
     ctx.fillStyle = "#DDD"; // colour of play icon
     ctx.globalAlpha = 0.75; // partly transparent
     ctx.beginPath(); // create the path for the icon
     var size = (canvas.height / 2) * 0.5;  // the size of the icon
     ctx.moveTo(canvas.width/2 + size/2, canvas.height / 2); // start at the pointy end
     ctx.lineTo(canvas.width/2 - size/2, canvas.height / 2 + size);
     ctx.lineTo(canvas.width/2 - size/2, canvas.height / 2 - size);
     ctx.closePath();
     ctx.fill();
     ctx.globalAlpha = 1; // restore alpha
}    

現在播放暫停事件

function playPauseClick(){
     if(videoContainer !== undefined && videoContainer.ready){
          if(videoContainer.video.paused){                                 
                videoContainer.video.play();
          }else{
                videoContainer.video.pause();
          }
     }
}
// register the event
canvas.addEventListener("click",playPauseClick);

---

摘要

使用畫布播放視頻非常容易，實時添加效果也很容易。 但是，格式，播放和搜索方式存在一些限制。 MDN HTMLMediaElement是獲得對視頻對象的完整引用的地方。

在畫布上繪制圖像后，您可以使用ctx.getImageData訪問其中包含的像素。 或者，您可以使用canvas.toDataURL捕捉靜止圖像並下載。 （僅當視頻來自受信任的來源並且沒有污染畫布時）。

---

請注意，如果視頻有聲音，則播放時也會播放聲音。

祝您拍攝愉快。

Answer 1

使用畫布顯示視頻

顯示視頻與顯示圖像幾乎相同。 較小的差異與onload事件有關，而事實是您需要每幀渲染視頻，否則您只會看到一個幀而不是動畫幀。

下面的演示與示例有一些細微的差異。 靜音功能（在視頻下，單擊靜音/打開聲音以切換聲音），並進行一些錯誤檢查以捕獲IE9 +和Edge（如果它們沒有正確的驅動程序）。 我本來會使用IE支持的另一種格式，但找不到屬於公共領域的格式。

注意IE9 +和Edge的用戶。 您可能無法播放視頻格式WebM，因為它需要其他驅動程序來播放視頻。 可以在tools.google.com上找到它們， 下載IE9 + WebM支持

 // This code is from the example document on stackoverflow documentation. See HTML for link to the example. // This code is almost identical to the example. Mute has been added and a media source. Also added some error handling incase the media load fails and a link to fix IE9+ and Edge suport. // Code by Blindman67. var mediaSource = "http://video.webmfiles.org/big-buck-bunny_trailer.webm"; var muted = true; var canvas = document.getElementById("myCanvas"); // get the canvas from the page var ctx = canvas.getContext("2d"); var videoContainer; // object to hold video and associated info var video = document.createElement("video"); // create a video element video.src = mediaSource; // the video will now begin to load. // As some additional info is needed we will place the video in a // containing object for convenience video.autoPlay = false; // ensure that the video does not auto play video.loop = true; // set the video to loop. video.muted = muted; videoContainer = { // we will add properties as needed video : video, ready : false, }; // To handle errors. This is not part of the example at the moment. Just fixing for Edge that did not like the ogv format video video.onerror = function(e){ document.body.removeChild(canvas); document.body.innerHTML += "<h2>There is a problem loading the video</h2><br>"; document.body.innerHTML += "Users of IE9+ , the browser does not support WebM videos used by this demo"; document.body.innerHTML += "<br><a href='https://tools.google.com/dlpage/webmmf/'> Download IE9+ WebM support</a> from tools.google.com<br> this includes Edge and Windows 10"; } video.oncanplay = readyToPlayVideo; // set the event to the play function that // can be found below function readyToPlayVideo(event){ // this is a referance to the video // the video may not match the canvas size so find a scale to fit videoContainer.scale = Math.min( canvas.width / this.videoWidth, canvas.height / this.videoHeight); videoContainer.ready = true; // the video can be played so hand it off to the display function requestAnimationFrame(updateCanvas); // add instruction document.getElementById("playPause").textContent = "Click video to play/pause."; document.querySelector(".mute").textContent = "Mute"; } function updateCanvas(){ ctx.clearRect(0,0,canvas.width,canvas.height); // only draw if loaded and ready if(videoContainer !== undefined && videoContainer.ready){ // find the top left of the video on the canvas video.muted = muted; var scale = videoContainer.scale; var vidH = videoContainer.video.videoHeight; var vidW = videoContainer.video.videoWidth; var top = canvas.height / 2 - (vidH /2 ) * scale; var left = canvas.width / 2 - (vidW /2 ) * scale; // now just draw the video the correct size ctx.drawImage(videoContainer.video, left, top, vidW * scale, vidH * scale); if(videoContainer.video.paused){ // if not playing show the paused screen drawPayIcon(); } } // all done for display // request the next frame in 1/60th of a second requestAnimationFrame(updateCanvas); } function drawPayIcon(){ ctx.fillStyle = "black"; // darken display ctx.globalAlpha = 0.5; ctx.fillRect(0,0,canvas.width,canvas.height); ctx.fillStyle = "#DDD"; // colour of play icon ctx.globalAlpha = 0.75; // partly transparent ctx.beginPath(); // create the path for the icon var size = (canvas.height / 2) * 0.5; // the size of the icon ctx.moveTo(canvas.width/2 + size/2, canvas.height / 2); // start at the pointy end ctx.lineTo(canvas.width/2 - size/2, canvas.height / 2 + size); ctx.lineTo(canvas.width/2 - size/2, canvas.height / 2 - size); ctx.closePath(); ctx.fill(); ctx.globalAlpha = 1; // restore alpha } function playPauseClick(){ if(videoContainer !== undefined && videoContainer.ready){ if(videoContainer.video.paused){ videoContainer.video.play(); }else{ videoContainer.video.pause(); } } } function videoMute(){ muted = !muted; if(muted){ document.querySelector(".mute").textContent = "Mute"; }else{ document.querySelector(".mute").textContent= "Sound on"; } } // register the event canvas.addEventListener("click",playPauseClick); document.querySelector(".mute").addEventListener("click",videoMute) 

 body { font :14px arial; text-align : center; background : #36A; } h2 { color : white; } canvas { border : 10px white solid; cursor : pointer; } a { color : #F93; } .mute { cursor : pointer; display: initial; } 

 <h2>Basic Video & canvas example</h2> <p>Code example from Stackoverflow Documentation HTML5-Canvas<br> <a href="https://stackoverflow.com/documentation/html5-canvas/3689/media-types-and-the-canvas/14974/basic-loading-and-playing-a-video-on-the-canvas#t=201607271638099201116">Basic loading and playing a video on the canvas</a></p> <canvas id="myCanvas" width = "532" height ="300" ></canvas><br> <h3><div id = "playPause">Loading content.</div></h3> <div class="mute"></div><br>