Question

I’ve read Does HTML5/Canvas Support Double Buffering? and related questions and I use double buffering but it’s doesent help some times. Actually I use another way for double buffering but it have same issue.

function drawLoop() {
    var scr_context = scr.getContext('2d'); // main canvas
    var units_context = units_buffer.getContext('2d'); // units buffer
    var unitcolors_context = unitcolors_buffer.getContext('2d'); // units color buffer
    var i;

    // background
    scr_context.fillStyle = "#FFFFEE";
    scr_context.fillRect(0, 0, scr.width, scr.height);

    units_context.clearRect(0, 0, units_buffer.width, units_buffer.height);
    unitcolors_context.clearRect(0, 0, unitcolors_buffer.width, unitcolors_buffer.height);

    for (i = 0; i < units_list.length; i++) {
        units_list[i].draw(units_context, camera); // flicker some times
        units_list[i].drawColor(unitcolors_context, camera); // never flicker
    }

    scr_context.drawImage(unitcolors_buffer, 0, 0);
    scr_context.drawImage(units_buffer, 0, 0);

    drawTimer = setTimeout(drawLoop, 1000 / FPS);
}

Draw methods:

this.draw = function(context, camera) {
    var sprite;
    var drawCoordinates;
    sprite = this.sloader.getSpriteByName(this.spritePos.spriteName);
    drawCoordinates = sprite.getDrawCoordinatesByXY(this.spritePos.x, this.spritePos.y, camera);
    context.drawImage(sprite.getImage(), drawCoordinates.x, drawCoordinates.y);
    return;
}
this.drawColor = function(context, camera) {
    var sprite;
    var drawCoordinates;
    sprite = this.sloader.getSpriteByName(this.spritePos.spriteName);
    drawCoordinates = sprite.getDrawCoordinatesByXY(this.spritePos.x, this.spritePos.y, camera);
    context.putImageData(this.unitMarkBackground, drawCoordinates.x + 21, drawCoordinates.y + 17);
    return;
}

Have no idea how flickering happen or never happen in almost same code, same buffered. I’ve test it in Chrome 20 and FireFox 13, both hase same issue.

Answer 1

You don’t need doublebuffering, it’s kinda already implemented by the browser’s engine. Just draw everything on scr_context and keep scr_context.fillRect() on top of drawLoop() before any other drawing. Are you sure getDrawCoordinatesByXY() returns always on screen values?