Question

我有一个已解码的jpeg图像，我用putImageData将它写入画布。那么可以移动这个图像吗？我找不到任何文件。

我的想法是，我将使用dirtyX和dirtyY裁剪解码图像的某个部分，现在我想裁剪图像的另一部分。我使用像spritesheet这样的解码图像。

Answer 1

是和否:-) 由于画布以立即模式绘制，因此不知道您在画布上绘制了图像。因此，您不能在画布上一次绘制它并移动绘制的图像。您必须在新位置手动重绘每个动画帧中的完整图像。你可以使用requestAnimationFrame()做到这一点，正如Jonas Grumann解释的那样。

Answer 2

使用剪裁版drawImage将您的个人精灵绘制到画布

（不要使用getImageData＆amp; putImageData进行剪裁，因为drawImage更容易，更快。）

如果您想将解码后的图像用作spritesheet，那么您可以使用画布的剪裁版本context.drawImage来剪切精灵图片的特定子部分并将其绘制到画布上。

以下是关于drawImage剪辑版本的SO帖子。

HTML / Java Script Canvas - how to draw an image between source and destination points?

这是示例代码和演示：

请注意，在画布上“移动”精灵的方法是清除画布并在其新位置重绘所需的精灵。（您不能“移动”画布上的现有图形。）

$(window).load(function(){

  // canvas related variables       
  var canvas=document.getElementById("canvas");
  var ctx=canvas.getContext("2d");

  // animation related variables
  var lastFlap,lastMove;

  // define a bird object
  // x,y are the position of the bird on the canvas
  // spriteX,spriteY is the position of the first desired
  //      sprite image on the spritesheet
  // width,height is the size of 1 sprite image
  // currentFrame is the index of which of the sprite images to display
  // currentDirection.  The sprite plays forward and then backward to
  //      accomplish 1 flap.  This determines if the next frame index will
  //      be increased (play forward) or decreased (play backward)
  var bird={
    x:30,
    y:30,
    spriteX:0,
    spriteY:52,
    width:51,
    height:51,
    frames:4,
    currentFrame:0,
    currentDirection:1
  }

  // load the spritesheet and start the animation
  var spritesheet=new Image();
  spritesheet.onload=start;
  spritesheet.src="https://dl.dropboxusercontent.com/u/139992952/multple/birdSpritesheet.png";
  function start(){
    requestAnimationFrame(animate);
  }

  function animate(time){

    // request another animation frame

    if(bird.x<canvas.width){
      requestAnimationFrame(animate);
    }

    // if the lastFlap or lastMove times don't aren't set, then set them

    if(!lastFlap){lastFlap=time;}
    if(!lastMove){lastMove=time;}

    // calculate the elapsed times since the last flap and the last move

    var elapsedFlap=time-lastFlap;
    var elapsedMove=time-lastMove;

    // if 50ms have elapsed, advance to the next image in this sprite

    if(elapsedFlap>50){

      // advance to next sprite on the spritesheet (flap)

      bird.currentFrame+=bird.currentDirection;

      // clamp bird.currentFrame between 0-3  (0,1,2,3)
      // (because there are 4 images that make up the whole bird sprite)

      if(bird.currentFrame<0 || bird.currentFrame>bird.frames-1){
        bird.currentDirection*=-1;
        bird.currentFrame+=bird.currentDirection;
      }

      // reset the flap timer

      lastFlap=time;
    }

    // locate the current sprite from the spritesheet

    var sx=bird.spriteX+bird.currentFrame*bird.width;
    var sy=bird.spriteY;

    // if 100ms have elapsed, move the bird across the canvas

    if(elapsedMove>100){
      bird.x+=3;
      lastMove=time;
    }

    // clear the whole canvas

    ctx.clearRect(0,0,canvas.width,canvas.height);

    // draw the current part of the bird sprite at the current bird.x

    ctx.drawImage(spritesheet,
                  sx,sy,bird.width,bird.height,
                  bird.x,bird.y,bird.width,bird.height
                 );

  }

}); // end $(function(){});

body{ background-color: white; }
canvas{border:1px solid red;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<h4>The canvas animating a clipped sprite</h4>
<canvas id="canvas" width=300 height=100></canvas>
<br>
<h4>The spritesheet</h4>
<img id=spritesheet src='https://dl.dropboxusercontent.com/u/139992952/multple/birdSpritesheet.png'>

动画/移动由putImageData放置的画布图像

2 个答案: