canvas系列--图像处理(三) 图像缩放

àì夳堔傛蜴生んèń 2021-09-11 08:24 1016阅读 0赞

![20170301142939997][]

图像缩放

一.首先分析原理

![20170301143913221][]

如图所示：每一次都显示整个图像，只不过显示整个图像时候，无论是放大还是缩小，显示的目标图像的左上角坐标不一样，而此时的sx,sy就是目标图像的左上角的坐标。当图像放大时（scale>1），sx,sy都是负数，目标图像将从canvas的外部显示，不过在canvas画布外面是不显示的，会被自动裁；当图像缩小时（scale<1）,sx,sy都正数，目标图像就会在canvas画布里面显示

sx = canvas.width /2 - imageWidth / 2;  
sy = canvas.height / 2 - imageHeight / 2;

二.所有要用知识点

1.[滑块控件(滑竿)-html5新对象][-html5]

step是每一步滑动的进度值

2.拖动滑块控件的获取值

slider.onmousemove = function()\{  
                    scale = slider.value;  
        \}

3.图片的加载:

var image = new Image();

image.src = "timg.jpg";  
 image.onload = function()\{

context.drawImage( image , dx ,dy , imageWidth , imageHeight );

4.当图像充满画布时，又要重新绘制图像，要记得清空画布

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

三.代码

<!DOCTYPE html>
    <html>
    <head lang="en">
        <meta charset="UTF-8">
        <title></title>
        <style type="text/css">
            body{
                background: black;
            }
            #scale-range{
                display:block;
                margin:20px auto;
                width:800px;
            }
            #canvas{
                display:block;
                margin:0 auto;
                border:1px solid #aaa;
            }
        </style>
    </head>
    <body>
        <canvas id="canvas">您的浏览器尚不支持canvas </canvas>
        <input type="range" id="scale-range" min="0.5" max="3.0" step="0.01" value="1.0"/>
        <script>
            var canvas = document.getElementById("canvas");
            var context = canvas.getContext("2d");
            var slider = document.getElementById("scale-range");
            var image = new Image();
            var imgW=1024;
            var imgH=750;
            canvas.width = 1024;
            canvas.height = 750;
    
            window.onload = function(){
                var scale = slider.value;
                image.src = "timg.jpg";
                image.onload = function(){
                    drawImageByScale( scale );
                    slider.onmousemove = function(){
                        scale = slider.value;
                        drawImageByScale( scale );
                    }
                }
    
            }
            function drawImageByScale( scale ){
                var imageWidth = imgW * scale; //imgW，imgH是图片的大小
                var imageHeight =imgH * scale;
    
                dx = canvas.width /2 - imageWidth / 2;//目标图像的坐标
                dy = canvas.height / 2 - imageHeight / 2;//目标图像的坐标
    
                context.clearRect( 0 , 0 , canvas.width , canvas.height );
                context.drawImage( image , dx ,dy , imageWidth , imageHeight );
            }
    
    
        </script>
    </body>
    </html>

[20170301142939997]: /images/20210813/83bf01917ee7413faf2f9407d6c34241.png
[20170301143913221]: /images/20210813/fc2b760a475b40dc8d8432d26f92cd23.png
[-html5]: http://www.w3school.com.cn/jsref/dom_obj_range.asp