带有汽车痕迹的 Html5 画布错误答案

【问题标题】：Html5 canvas bug with car traces带有汽车痕迹的 Html5 画布错误
【发布时间】：2015-08-05 15:04:36
【问题描述】：

这是我点击启动车时留下的代码。这个怎么去掉。我需要在 javascript 中进行更改。我认为可能是函数 drawCar(); 的问题。或移动汽车（）；或者可能是其他功能，但我不知道。

HTML

 <body onload='draw();'></body> 
    <canvas id="motion" width="1000" height="200"></canvas>             
    <div style="position: absolute; top: 7px; left: 7px;" >
     <input id="loop" onclick="javascript:initCars(); startLoop();" type="button" class="prev" value="Start">   

                      </div>

CSS

.prev{
    background-color: rgba(0, 0, 0, 0.7);
    width:1000px; 
    height:200px;
    font-size:40px;
    color:#ffffff;
}
.prev:focus {     
    background-color:transparent; 
    font-size:0px;
    color:#ffffff;
    width:1000px; 
    height:200px;  
}

/*jslint plusplus: true, sloppy: true, indent: 4 */
(function () {
    "use strict";
    // this function is strict...
}());

// RequestAnimFrame: a browser API for getting smooth animations
window.requestAnimFrame = (function() {
    return window.requestAnimationFrame ||
        window.webkitRequestAnimationFrame ||
        window.mozRequestAnimationFrame ||
        window.oRequestAnimationFrame ||
        window.msRequestAnimationFrame ||
    function(callback) {
        window.setTimeout(callback, 1000 / 60);
    };
})();

// Globals
var canvas = null,
    ctx = null,
    background = null,
    car_sprite = null,
    game_data = null,
    CAR_WIDTH = 100,
    CAR_HEIGHT = 37,
    STEP_COUNT_MILLISECONDS =  1000 / 30,
    RACE_LENGTH = 20,
    RACE_FINISH_LINE_X = 770,
    iTime = 0,
    iFinishPlace = 1,
    random_graph;

function clearCanvas() {

    // clear canvas
    ctx.clearRect(0, 0, canvas.height, canvas.width);
}

function drawBackground() {

    clearCanvas();
    ctx.drawImage(background, 0, -400);

    loadCarSprite();
}

function loadBackground() {

    // Load the timer
    background = new Image();
    background.src = 'http://www.upslike.net/imgdb/race-scence-d9894d.png';
    background.onload = drawBackground;
}

function setupGameData() {

    var json =
    {
        cars:
        [
            {
                "colour": 'blue',
                "x": 0,
                "y": 50,
                "spritex": 0,
                "spritey": 0,
                "graph": null,
                "step": 77,
                "position": null,
                "speed": 10,
                "speed_late": 100
            },
            {
                "colour": 'green',
                "x": 0,
                "y": 70,
                "spritex": 0,
                "spritey": 37,
                "graph": null,
                "step": 65,
                "position": null,
                "speed": 15,
                "speed_late": 50
            },
            {
                "colour": 'red',
                "x": 0,
                "y": 90,
                "spritex": 0,
                "spritey": 74,
                "graph": null,
                "step": 53,
                "position": null,
                "speed": 15,
                "speed_late": 15
            },
            {
                "colour": 'green',
                "x": 0,
                "y": 110,
                "spritex": 0,
                "spritey": 111,
                "graph": null,
                "step": 39,
                "position": null,
                "speed": 20,
                "speed_late": 20
            }
        ],
        graphs:
        [
            [0,5,10,20,40,60,70],
            [0,10,20,30,40,50,60],
            [0,20,39,40,50,55,58],
            [0,10,20,30,40,50,55],
            [0,25,45,47,49,50,52],
            [0,10,20,29,38,45,50],
            [0,15,20,25,30,40,45],
            [0,2,4,8,20,30,40],
            [0,5,10,15,20,25,30],
            [0,1,3,14,15,22,30],
            [0,5,11,14,17,22,25],
            [0,20,30,44,67,72,90],
            [0,2,7,24,47,52,65],
            [0,2,9,20,40,52,70]
        ]
    };

    random_graph = Math.floor( Math.random() * json.graphs.length );

    return json;    
}

function drawCar(car) {

    // Draw the car onto the canvas

    ctx.drawImage(car_sprite,
        car.spritex, car.spritey,
        CAR_WIDTH, CAR_HEIGHT,
        car.x + car.step, car.y,
        CAR_WIDTH, CAR_HEIGHT);

     drawText(car);
}

function drawCars() {

    var iCarCounter;

    for(iCarCounter = 0; iCarCounter < game_data.cars.length; iCarCounter++) {

        drawCar(game_data.cars[iCarCounter]);
    }
}

function initCar(current_car) {

    current_car.graph = random_graph;

}

function initGameState() {

    var iCarCounter;

    for(iCarCounter = 0; iCarCounter < game_data.cars.length; iCarCounter++) {

        initCar(game_data.cars[iCarCounter]);

    }
}

function getPositionAtTime(graph_index, percentageElapsed, current_car) {

    var graph = game_data.graphs[graph_index],
        iNumberOfGraphPoints = graph.length,
        iGraphPosition = null,
        iFloor = null,
        iCeil = null,
        p = null;
        position = null;

    graph = graph.map( function( val, i ) {

        if ( i === 0 ) {

            return val;

        }

        var car_speed = undefined === current_car.speed ? 1 : current_car.speed,
            car_speed_late = undefined === current_car.speed_late ? car_speed : current_car.speed_late;

        return ( i < Math.floor( graph.length / 2 ) ) ? car_speed : car_speed_late;

    });

    iGraphPosition = (iNumberOfGraphPoints / 100) * percentageElapsed;

    iFloor = Math.floor(iGraphPosition);
    iCeil = Math.ceil(iGraphPosition);

    if(iGraphPosition === iFloor) {

        position = graph[iFloor];

    } else if(iGraphPosition === iCeil) {

        position = graph[iCeil];

    } else {

        p = (graph[iCeil] - graph[iFloor]) / 100;

        position = ((iGraphPosition - iFloor) * 100) * p + graph[iFloor];

    }

    return position;

}

function redrawRoadSection() {

    ctx.drawImage(background, 0, 400, 1000, 200, 0, 400, 1000, 200);   
}

function graphPosToScreenPos() {

    return (900 / 100)  * (position / 60 * 100);   
}

function updateDebugWindow() {

  // Debug window
    var time = document.getElementById('time');

    if(time !== null)  {

       time.value = iTime / 1000;
    }

}


function drawText(current_car) {

    if(current_car.position !== null) {

        ctx.strokeStyle = "black";
        ctx.font = "normal 12px Facebook Letter Faces";
        ctx.strokeText(current_car.position, RACE_FINISH_LINE_X + current_car.step + 110, current_car.y + 25);

    }

}

function moveCar(iCarCounter) {

    var current_car =  game_data.cars[iCarCounter],
        seconds = iTime / 1000,
        percentageElapsed = (seconds / RACE_LENGTH) * 100,
        a = 20,
        velocity = 2,
        position = getPositionAtTime(current_car.graph, percentageElapsed,current_car);

    if(current_car.x < RACE_FINISH_LINE_X) {

        current_car.x =  graphPosToScreenPos(position) + (velocity * seconds) + (1/2 * a * Math.pow(seconds, 2));

    }
    else {

        current_car.x = RACE_FINISH_LINE_X;

        if(current_car.position === null) {

            current_car.position = iFinishPlace++;
        }
    }

    drawCar(current_car);
}

function initCars() {

    game_data = setupGameData();

    initGameState();
    drawCars();
}

function stopLoop() {

    iTime = 0;
    iFinishPlace = 1;
}


function startRace() {

    var iCarCounter;

    redrawRoadSection();

    for(iCarCounter = 0; iCarCounter < game_data.cars.length; iCarCounter++) {

        moveCar(iCarCounter);

    }

    updateDebugWindow();

    if(iFinishPlace > 4) {

        stopLoop();

    } else {

        iTime += STEP_COUNT_MILLISECONDS;

        requestAnimFrame(startRace);
    }
}

function startLoop() {

    stopLoop();

    requestAnimFrame(startRace);
}

function loadCarSprite() {

    // Load the timer
    car_sprite = new Image();
    car_sprite.src = 'http://www.upslike.net/imgdb/car-sprite-f0d812.gif';
    car_sprite.onload = initCars;
}

function draw() {

        // Main entry point got the motion canvas example
        canvas = document.getElementById('motion');

        // Canvas supported?
        if (canvas.getContext) {

                ctx = canvas.getContext('2d');

        loadBackground();

        } else {
                alert("Canvas not supported!");
        }
}

CodePen

【问题讨论】：

如果您想要好的答案，请制作 js fiddler
codepen.io/anon/pen/aOXWeK 这里是codepen
codepen.io/anon/pen/bdzRxR
@user2072826 你能把车停到底并画出位置（1、2、3或4）

标签： javascript html canvas html5-canvas

【解决方案1】：

您只需清除画布并绘制一次背景...这是一个解决轨迹问题的示例。 http://codepen.io/anon/pen/xGMYKg

这里最大的变化是让画布在每一帧上都清晰，并重新绘制背景。

function startRace() {
   clearCanvas();
   ctx.drawImage(background, 0, -400);
   ...

您的 clearRect 参数略有偏差。您交换了宽度和高度。试试这个：

 ctx.clearRect(0, 0, canvas.width, canvas.height);

你的循环有点难以理解，但这是你以后可以担心的事情。

【讨论】：

ctx.drawImage(背景, 0, 400, 1000, 200, 0, 400, 1000, 200);问题出在函数 redrawRoadSection()