How do I get the best PNG compression with gulp-imagemin + plugins?

Question

I am trying to get the best setup possible for compressing pngs.

I have found there are some plugins available here for imagemin: https://www.npmjs.com/browse/keyword/imageminplugin

I have tried all the PNG options but the compression rates appear to be pretty bad.

• example image original size - 1.4MB
• TinyPNG.com - 413KB
• pngquant - 541KB
• pngout - can't get it to work - see https://github.com/imagemin/imagemin-pngout/issues/2
• pngcrush - 992KB
• optipng (default options) - 906KB
• advpng - 906KB

pngquant appears to have the highest compression rate, (obviously the worst quality) similar quality to tinypng, but could still get nearer the tinypng figure.

Essentially I have 3 questions:

Question 1: Changing the options for both advpng & optipng doesn't seem to alter the filesize, here is an example, am I using this correctly? I am guessing that maybe it isn't using my settings at all and falling back to the default settings? If you notice the filesizes of these two are identical!:

.pipe(imagemin(
    imageminOptipng({
        optimizationLevel: 4
    })
))

Question 2: Am I using "pngout" in correctly? Is there a method to use it that I do not know about? The examples from their page seem not to work & neither does this method:

.pipe(imagemin([
    imageminPngout({
        strategy: 1
    })
]))

Question 3: Is there a better method to deal with png compression that I haven't yet found? Ideally I would somehow like to get a method that has the rate of pngquant, but the quality is a bit better.

Answer 1

After a long process of trials and errors, I've managed to get the imagemin-pngquant plugin to produce the image of the similarly small size as TinyPNG.com. Still a little bigger, but some parts of the image were looking better than the TinyPNG.com result.

Input image size: 1.1MB
TinyPNG.com: 223kb
imagemin-pngquant: 251kb

I used the next plugin settings:

{
    quality: '70-90', // When used more then 70 the image wasn't saved
    speed: 1, // The lowest speed of optimization with the highest quality
    floyd: 1 // Controls level of dithering (0 = none, 1 = full).
}

Also, it may be a good idea to generate .webp images from .png and serve it to the browsers that support this format with the .png as a backup. More info about .webp image format: Google Developer Section

After generating .webp from the optimized image (with the minimal quality loss) the image size was 62kB. You can use the imagemin-webp plugin with gulp-rename to stream images to the dist folder with the same base name, but different extension. Example gulp task:

const config = require('./src/gulp/config.json');

const gulp = require('gulp');
const plugins = require('gulp-load-plugins')();
const imageminWebp = require('imagemin-webp');

gulp.task('images', function () {
    return gulp.src(config.src.images)
        pipe(plugins.imagemin([imageminWebp({
            method: 6,
        })]))
        .pipe(plugins.rename({ extname: '.webp' }))
        .pipe(gulp.dest(config.dist.images));
});