如何水平翻转图像

Question

我怎样才能通过反转只绘制两张图像我不知道如何反转。请帮忙。

    var canvas = document.createElement('canvas');
    canvas.width = 16 * scale;
    canvas.height = 32 * scale;
    //we assign the same classname the image has, for CSS purposes
    canvas.setAttribute('class', skinImage.getAttribute('class'));
    canvas.setAttribute('title', skinImage.getAttribute('title'));
    var context = canvas.getContext("2d"),
    s = scale;
    context.imageSmoothingEnabled=!1,
    context.mozImageSmoothingEnabled=!1,
    context.oImageSmoothingEnabled=!1,
    CanvasRenderingContext2D.webkitImageSmoothingEnabled=!1,
    context.clearRect(0,0,canvas.width,canvas.height);

    //back draw the head
    context.drawImage(skinImage, 24,  8,  8, 8,  4*s,  0*s,  8*s, 8*s);
    //back draw the body
    context.drawImage(skinImage, 32, 20, 8, 12, 4*s,  8*s,  8*s, 12*s);
    //back draw the right leg
    context.drawImage(skinImage, 52, 20, 4, 12, 12*s, 8*s,  4*s, 12*s);
    //back draw the right arm
    context.drawImage(skinImage, 12, 20, 4, 12, 8*s,  20*s, 4*s, 12*s);

    //back draw the left arm
    context.drawImage(skinImage, 12, 20, 4, 12, 4*s,  20*s, 4*s, 12*s);
    context.drawImage(skinImage, 52, 20, 4, 12, 0*s,  8*s,  4*s, 12*s);

Answer 1

首先，要删除 2 个不需要的图像，只需清除画布并重新绘制所需的图像。您可以使用context.clearRect(0,0,canvas.width,canvas.height) 清除画布。

水平翻转图像

如何水平翻转图像：

1. 将画布原点移动（平移）到您想要的 X 坐标加上图像宽度：context.translate(x+img.width,y); 添加 img.width 是必要的，因为我们正在抓取图像的左边缘并将其向左翻转。在不添加 img.width 的情况下，img 将绘制在所需 x 坐标的左侧。

2. 使用context.scale(-1,1);水平翻转

3. 绘制图像：`context.drawImage(img,0,0);

4. 通过将转换重置为其默认值进行清理：context.setTransform(1,0,0,1,0,0);

带注释的代码和演示：

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/multple/sillouette2.png";
function start(){

    ctx.fillText('original',10,30);
    ctx.drawImage(img,10,30);

    ctx.fillText('flipped',150,30);
    flipHorizontally(img,150,30);

}

function flipHorizontally(img,x,y){
    // move to x + img's width
    ctx.translate(x+img.width,y);

    // scaleX by -1; this "trick" flips horizontally
    ctx.scale(-1,1);
    
    // draw the img
    // no need for x,y since we've already translated
    ctx.drawImage(img,0,0);
    
    // always clean up -- reset transformations to default
    ctx.setTransform(1,0,0,1,0,0);
}

#canvas{border:1px solid red; margin:0 auto; }

<canvas id="canvas" width=300 height=300></canvas>

[补充：从精灵表中翻转精灵]

要翻转包含在 spritesheet 中的 sprite，您可以使用 drawImage 的裁剪形式。裁剪 drawImage 将通过指定 sprite 的 spriteX, spriteY, spriteWidth & spriteHeight 从 spritesheet 裁剪所需的 sprite。同样的裁剪drawImage也会通过指定想要的画布canvasX, canvasY, spriteWidth & spriteHeight将裁剪后的子图绘制到画布上。

这里是显示如何从 spritesheet 中翻转 sprite 的代码

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/multple/cars.jpg";
function start(){

  var spriteX=92;
  var spriteY=63;
  var spriteW=80;
  var spriteH=40;

  ctx.fillText('original',10,30);
  ctx.drawImage(img,spriteX,spriteY,spriteW,spriteH,10,30,spriteW,spriteH);

  ctx.fillText('flipped',150,30);
  flipSpriteHorizontally(img,150,30, spriteX,spriteY,spriteW,spriteH);

}

function flipSpriteHorizontally(img,x,y,spriteX,spriteY,spriteW,spriteH){
  // move to x + img's width
  // adding img.width is necessary because we're flipping from
  //     the right side of the img so after flipping it's still
  //     at [x,y]
  ctx.translate(x+spriteW,y);

  // scaleX by -1; this "trick" flips horizontally
  ctx.scale(-1,1);

  // draw the img
  // no need for x,y since we've already translated
  ctx.drawImage(img,
                spriteX,spriteY,spriteW,spriteH,0,0,spriteW,spriteH
               );

  // always clean up -- reset transformations to default
  ctx.setTransform(1,0,0,1,0,0);
}

#canvas{border:1px solid red; margin:0 auto; }

<h4>Drawing & horizontally flipping a sprite</h4>
<canvas id="canvas" width=300 height=100></canvas>
<h4>Spritesheet:</h4>
<img src='https://dl.dropboxusercontent.com/u/139992952/multple/cars.jpg'>

Answer 2

镜像图像

这是一个简单的实用函数，可以水平、垂直或同时镜像图像。

// arguments
// ctx : the context on which to draw the mirrored image
// image : the image to mirror
// x,y : the top left of the rendered image
// horizontal : boolean if true mirror along X
// vertical : boolean if true mirror along y
function mirrorImage(ctx, image, x = 0, y = 0, horizontal = false, vertical = false){
    ctx.save();  // save the current canvas state
    ctx.setTransform(
        horizontal ? -1 : 1, 0, // set the direction of x axis
        0, vertical ? -1 : 1,   // set the direction of y axis
        x + horizontal ? image.width : 0, // set the x origin
        y + vertical ? image.height : 0   // set the y origin
    );
    ctx.drawImage(image,0,0);
    ctx.restore(); // restore the state as it was when this function was called
}

用法

mirrorImage(ctx, image, 0, 0, true, false); // horizontal mirror
mirrorImage(ctx, image, 0, 0, false, true); // vertical mirror
mirrorImage(ctx, image, 0, 0, true, true);  // horizontal and vertical mirror

更多镜子

如果您希望能够沿任意线镜像，请参阅答案Mirror along line

Answer 3

要镜像，您可以使用带有负比例的setTransform。

context.setTransform(-1, 0, 0, 1, 0, 0);    //Now all images will be horizontally mirrored
context.drawImage(<myimage>, srcx, srcy, srcw, srch, dstx, dsty, dstw, dsth);

完成后不要忘记恢复变换。

context.setTransform(1, 0, 0, 1, 0, 0);