使用视频循环绘制画布

Question

I loop over a part of a HTML video and simultaneously draw a Canvas with the current frame of the video. When the Videos starts over, there is always 1 gray frame at the canvas. If the loop region is long, its not a big problem, but for my needs these regions are maybe 0.5 seconds and then the canvas starts to flicker if you loop over and over again.

When drawing the canvas, I also tried different video properties (ended, loop, networkState, readyState) - doesnt help

I provided a jsfiddle to show you my problem. (just press play at the video) https://jsfiddle.net/Lz17fnf3/2/

$('#v').on('timeupdate', function () {

    if ($('#v')[0].currentTime > 2) {//Loop for one second
        $('#v')[0].currentTime = 1;
    }

    var $this = $('#v')[0]; //cache
    (function loop() {
        if (!$this.paused && !$this.ended) {
            drawCanvas();
            setTimeout(loop, 1000 / 25); // drawing at 25fps
        }
    })();
});


function drawCanvas() {
    var elem = document.getElementById('c');
    var c = elem.getContext('2d');
    var v = $('#v')[0];
    $('#c').attr('width', v.videoWidth);
    $('#c').attr('height', v.videoHeight);
    if (v.readyState == 4) {
        c.drawImage(v, 0, 0, v.videoWidth, v.videoHeight, 0, 0, v.videoWidth, v.videoHeight);
    }
}

Answer 1

The reason it flickers is because when you assign the width or height to a canvas element, this action resets the entire context of the canvas, most likely that is causing the blank frame. Try moving all the canvas/context definitions outside the drawCanvas .

Something like:

var elem = document.getElementById('c');
var c = elem.getContext('2d');
var v = $('#v')[0];

// In order to set the canvas width & height, we need to wait for the video to load.
function init() {
    if (v.readyState == 4) {
        $('#c').attr('width', v.videoWidth);
        $('#c').attr('height', v.videoHeight);
    } else {
        requestAnimationFrame(init);
    }
}

init();

$('#v').on('timeupdate', function () {
    if ($('#v')[0].currentTime > 2) { //Loop for one second
        $('#v')[0].currentTime = 1;
    }

    var $this = $('#v')[0]; //cache
    (function loop() {
        if (!$this.paused && !$this.ended) {
            drawCanvas();
            setTimeout(loop, 1000 / 25); // drawing at 25fps
        }
    })();
});

function drawCanvas() {
    c.drawImage(v, 0, 0, v.videoWidth, v.videoHeight, 0, 0, v.videoWidth, v.videoHeight);
}