node平台截取图片模块——jimp

前几天介绍了一个简单的截图模块——iamges，虽然简单，但是功能还是有很多局限的地方。

jimp的优势：1.简单，2.支持回调方式和ES6（promise）语法也可以链式调用 3. 丰富的api  4.无需安装任何程序（gm和canvas都需要）

jimp的回调用法如下：

var Jimp = require("jimp");

Jimp.read("lenna.png", function (err, lenna) {
    if (err) throw err;
    lenna.resize(256, 256)            
         .quality(60)                
         .greyscale()                 
         .write("lena-small-bw.jpg"); 
});

promise用法：

var Jimp = require("jimp");

Jimp.read("lenna.png").then(function (lenna) {
    lenna.resize(256, 256)          
         .quality(60)                
         .greyscale()              
         .write("lena-small-bw.jpg"); 
}).catch(function (err) {
    console.error(err);
});

因为代码是运行在node平台，个人强烈建议使用promise的写法。

更多的用法可以参考github：https://github.com/oliver-moran/jimp，我把一些api复制下来

/* Resize */
image.contain( w, h[, mode] );    // scale the image to the given width and height, some parts of the image may be letter boxed
image.cover( w, h[, mode] );      // scale the image to the given width and height, some parts of the image may be clipped
image.resize( w, h[, mode] );     // resize the image. Jimp.AUTO can be passed as one of the values.
image.scale( f[, mode] );         // scale the image by the factor f
image.scaleToFit( w, h[, mode] ); // scale the image to the largest size that fits inside the given width and height

// An optional resize mode can be passed with all resize methods.

/* Crop */
image.autocrop();                 // automatically crop same-color borders from image (if any)
image.crop( x, y, w, h );         // crop to the given region

/* Composing */
image.blit( src, x, y[, srcx, srcy, srcw, srch] );
                                  // blit the image with another Jimp image at x, y, optionally cropped.
image.composite( src, x, y );     // composites another Jimp image over this iamge at x, y
image.mask( src, x, y );          // masks the image with another Jimp image at x, y using average pixel value image.rotate) and when writing formats that don't support alpha channels

/* Flip and rotate */
image.flip( horz, vert );         // flip the image horizontally or vertically
image.mirror( horz, vert );       // an alias for flip
image.rotate( deg[, mode] );      // rotate the image clockwise by a number of degrees. Optionally, a resize mode can be passed. If `false` is passed as the second parameter, the image width and height will not be resized.

// JPEG images with EXIF orientation data will be automatically re-orientated as appropriate.

/* Colour */
image.brightness( val );          // adjust the brighness by a value -1 to +1
image.contrast( val );            // adjust the contrast by a value -1 to +1
image.dither565();                // ordered dithering of the image and reduce color space to 16-bits (RGB565)
image.greyscale();                // remove colour from the image
image.invert();                   // invert the image colours
image.normalize();                // normalize the channels in an image

/* Alpha channel */
image.fade( f );                  // an alternative to opacity, fades the image by a factor 0 - 1. 0 will haven no effect. 1 will turn the image
image.opacity( f );               // multiply the alpha channel by each pixel by the factor f, 0 - 1
image.opaque();                   // set the alpha channel on every pixel to fully opaque
image.background( hex );          // set the default new pixel colour (e.g. 0xFFFFFFFF or 0x00000000) for by some operations (e.g. image.contain and 

/* Blurs */
image.gaussian( r );              // Gaussian blur the image by r pixels (VERY slow)
image.blur( r );                  // fast blur the image by r pixels

/* Effects */
image.posterize( n );             // apply a posterization effect with n level
image.sepia();                    // apply a sepia wash to the image

利用这些api可以帮助我们完成大部分功能。下面还有一些特殊的东西。

1.  在图片上书写文字

Jimp.loadFont( path ).then(function (font) { 
    image.print(font, x, y, str); 
});

2. 读取图片字节流

image.getBuffer( buffer , cb )

3. 图片上某一点像素的操作

image.getPixelColor(x, y)  //获取图片上某一点的像素值
image.setPixelColor(hex, x, y);  //设置图片上的某一点像素

4. 工具函数

Jimp.rgbaToInt(r, g, b, a);   //将十进制转化为十六进制
Jimp.intToRGBA(hex);    //将十六进制转化为十进制

5. 判断图片是否为同一张图

Jimp使用的了pHash算法对图片进行了计算，我们可以利用这个特点进行图片的比较，避免服务器保存了相同的图片

海明威码判别 

Jimp.distance(image1, image2);  //返回值为相似程度，0表示两张图完全一样

像素比较法

var diff = Jimp.diff(image1, image2, threshold); 
diff.image;     //一张用来展示两张图片不一样的地方的图片
diff.percent;   //像素不相同的比例        

其中参数中threshold的取值为0-1，代表了比较的严格程度，0表示最严格，实际中可以结合使用两种方式来判断。