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。