javascript - HTML5 镜像网络摄像头 Canvas

Question

我正在尝试使用网络摄像头供稿 -(横向格式)，切掉中间位(纵向格式)并将其渲染到 Canvas 上，以便它以 1080px x 1920px 的比例填充屏幕纵向(为此我缩放位我削减了 3.8)。然后我需要翻转这个 Canvas 以便镜像图像。我已经成功地切掉了中间部分，并将其渲染到 Canvas 上......我只是不知道如何翻转它。

编辑

感谢所有指点我 context.scale(-1, 1) 的人 - 我的问题是，我已经在使用 scale...我认为我的问题与保存上下文有关，但是我尝试的一切都失败了？

<script>
       // Put event listeners into place
        window.addEventListener("DOMContentLoaded", function() {
            // Grab elements, create settings, etc.
            var canvas = document.getElementById("canvas"),
                context = canvas.getContext("2d"),
                video = document.getElementById("video"),
                videoObj = { 
                    video: {
                        mandatory: {
                            minWidth: 1280,
                            minHeight: 720,
                            /*Added by Chad*/
                            maxWidth: 1280,
                            maxHeight: 720
                        }
                    }
                },
                errBack = function(error) {
                    console.log("Video capture error: ", error.code); 
                };

            // Put video listeners into place
            if(navigator.getUserMedia) { // Standard
                navigator.getUserMedia(videoObj, function(stream) {
                    video.src = stream;
                    video.play();
                }, errBack);
            } else if(navigator.webkitGetUserMedia) { // WebKit-prefixed
                navigator.webkitGetUserMedia(videoObj, function(stream){
                    video.src = window.URL.createObjectURL(stream);
                    video.play();
                }, errBack);
            } else if(navigator.mozGetUserMedia) { // WebKit-prefixed
                navigator.mozGetUserMedia(videoObj, function(stream){
                    video.src = window.URL.createObjectURL(stream);
                    video.play();
                }, errBack);
            }

            /*
                video : video source tag
                320,0 : the shift coords
                320,180 : the canvas size
                0,0 : no shift in the canvas
                640,360 : important ! it’s the native resolution of video source
            */
            context.scale(3.8,3.8);

            function loop(){
               context.drawImage(video, 450, 0, 1080, 1920, 0, 0, 720, 1280);
               setTimeout(loop, 1000 / 30);
            }

            loop();


        }, false);
</script> 


    <video id="video" height="1080" width="1920" autoplay></video>
    <canvas id="canvas" height="1920" width="1080"></canvas>


// Put event listeners into place
window.addEventListener("DOMContentLoaded", function() {
    // Grab elements, create settings, etc.
    var canvas = document.getElementById("canvas"),
        context = canvas.getContext("2d"),
        video = document.getElementById("video"),
        videoObj = { 
            video: {
                mandatory: {
                    minWidth: 1280,
                    minHeight: 720,
                    /*Added by Chad*/
                    maxWidth: 1280,
                    maxHeight: 720
                }
            }
        },
        errBack = function(error) {
            console.log("Video capture error: ", error.code); 
        };

    // Put video listeners into place
    if(navigator.getUserMedia) { // Standard
        navigator.getUserMedia(videoObj, function(stream) {
            video.src = stream;
            video.play();
        }, errBack);
    } else if(navigator.webkitGetUserMedia) { // WebKit-prefixed
        navigator.webkitGetUserMedia(videoObj, function(stream){
            video.src = window.URL.createObjectURL(stream);
            video.play();
        }, errBack);
    } else if(navigator.mozGetUserMedia) { // WebKit-prefixed
        navigator.mozGetUserMedia(videoObj, function(stream){
            video.src = window.URL.createObjectURL(stream);
            video.play();
        }, errBack);
    }

    /*
        video : video source tag
        320,0 : the shift coords
        320,180 : the canvas size
        0,0 : no shift in the canvas
        640,360 : important ! it’s the native resolution of video source
    */




    context.scale(-3.8,3.8);
    context.translate(-720,0);
    function loop(){
       context.drawImage(video, 450, 0, 1080, 1920, 0, 0, 720, 1280);
       setTimeout(loop, 1000 / 30);
    }

    loop();


}, false);

Answer 1

您不应该使用 ctx.scale 和 ctx.translate 方法进行裁剪。

相反，在加载视频时计算裁剪位置，然后在绘图循环的调用中应用这些计算出的位置。

完成后，很容易应用@Mahout 提议的context.scale(-1, 1);。
请注意，在应用 scale() 之前，您还需要 context.translate(canvas.width, 0);。

我重构了你的代码，因为你请求视频强制的方式是 out of date(chrome 也是如此)。

我也更改了您的循环，以便仅在视频加载时调用它，无需尝试绘制任何尚不存在的内容，并且我将您的 setTimeout 更改为 requestAnimationFrame方法，大约以 30fps 的速度触发。

// Put event listeners into place
window.addEventListener("DOMContentLoaded", function() {
    // Grab elements, create settings, etc.
    var canvas = document.getElementById("canvas"),
        context = canvas.getContext("2d"),
        // we don't need to append the video to the document
        video = document.createElement("video"),
        videoObj = 
        navigator.getUserMedia || navigator.mozGetUserMedia ? // our browser is up to date with specs ?
        { 
        video: {
            width: { min: 1280,  max: 1280 },
            height: { min: 720,  max: 720 },
            require: ['width', 'height']
            }
        }:
        {
        video: {
            mandatory: {
                minWidth: 1280,
                minHeight: 720,
                maxWidth: 1280,
                maxHeight: 720
            }
        }
    },
    errBack = function(error) {
        console.log("Video capture error: ", error.code); 
    };
    // create a crop object that will be calculated on load of the video
    var crop;
    // create a variable that will enable us to stop the loop.
    var raf;
    
    navigator.getUserMedia = navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia;
    // Put video listeners into place
        navigator.getUserMedia(videoObj, function(stream) {
            video.srcObject = stream;
            video.onplaying = function(){
                var croppedWidth = ( Math.min(video.videoHeight, canvas.height) / Math.max(video.videoHeight,canvas.height)) * Math.min(video.videoWidth, canvas.width),
                croppedX = ( video.videoWidth - croppedWidth) / 2;
                crop = {w:croppedWidth, h:video.videoHeight, x:croppedX, y:0};
                // call our loop only when the video is playing
                raf = requestAnimationFrame(loop);
                };
            video.onpause = function(){
                // stop the loop
                cancelAnimationFrame(raf);
                }
            video.play();
        }, errBack);

    function loop(){
       context.drawImage(video, crop.x, crop.y, crop.w, crop.h, 0, 0, canvas.width, canvas.height);
       raf = requestAnimationFrame(loop);
    }
// now that our video is drawn correctly, we can do...
context.translate(canvas.width, 0);
context.scale(-1,1);

}, false);

body,html{margin:0}
canvas{ border:1px solid;}

<canvas id="canvas" height="1920" width="1080"></canvas>

jsfiddle for chrome