CarrierWave: How to remove GIF animation

When accepting GIF images, you will also accept animated GIFs. Resizing them can be a time-consuming task and will block a Rails worker until the image is processed.

Save yourself that trouble, and simply tell ImageMagick to drop any frames but the first one.

Add the following to your uploader class:

process :remove_animation


def remove_animation
  if content_type == 'image/gif'
    manipulate! { |image| image.collapse! }

You may also define that process for specific versions only (e....


How to make your application assets cachable in Rails

Note: Modern Rails has two build pipelines, the asset pipeline (or "Sprockets") and Webpacker. The principles below apply for both, but the examples shown are for Sprockets.

Every page in your application uses many assets, such as images, javascripts and stylesheets. Without your intervention, the browser will request these assets again and again on every request. There is no magic in Rails that gives you automatic caching for assets. In fact, if you haven't been paying attention to this, your application is probabl...


CSS variables aka CSS Custom Properties

CSS variables are very different from preprocessor variables. While preprocessors use variables to compile a static piece of CSS, CSS custom properties are a reactive (i.e. live) part of the styles. Think of them like usual CSS properties that cascade, but have:

  • a special syntax: CSS variables always start with a double-dash (--color)
  • no inherent meaning: Defining a CSS variable will not change any styles in itself
  • a special functionality: CSS variables can be used within the values of other properties, including CSS variables...

Carrierwave: How to migrate to another folder structure

A flat folder structure can be cool if you have only a few folders but can be painful for huge amounts. We recently had this issue in a project with more than 100.000 attachments, where we used a structure like this /attachments/123456789/file.pdf.

Even the ls command lasted several minutes to show us the content of the attachments folder.

So we decided to use a more hierarchical structure with a limited maximum of folder per layer. As our attachment folder will grow very fast we choosed to use three layers, but that's up to you. Here...


Selenium: How to debug the communication with the Webdriver API

Selenium allows you to log all requests to the Webdriver API. Therefore add the following line to e.g. features/support/selenium.rb:

Selenium::WebDriver.logger.level = :debug

If you want to see the output of the driver itself, here is an example on how to enable Chromedriver logging.

Example output

When you run a command like bundle exec cucumber --format=pretty features/some.feature you will see the API communication before the step is printed (here you see the log for the step And I press "Save").



Optimizing images for web

For webpages to load fast it's recommended to optimize images. Ideally an image's file size should be as small as possible while still being of decent quality. This card demonstrates two command line tools for image optimization

Use identify to fetch information about pictures. convert can change size/quality and strip meta information. Both commands are supplied by ImageMagick.

$ identify in.jpg
in.jpg JPEG 294x440 294x440+0+0 8-bit sRGB 92.8KB 0.000u 0:00.000
$ convert in.jpg -resize x220 -strip -set profile sRGB2014.icc -qua...

The HTML5 video element

# Basic HTML example
<video poster="preview_image.png" controls>
  <source src="or_here.webm" type="video/webm" />
  <source src="alternative_if_browser_cant_pay_first_source.mp4" type="video/mp4" />
  <track src="optional_subtitles.vtt" kind="subtitles" srclang="de" label="Deutsch" default>

# Javascript API (notable methods and properties)
video = document.querySelector('video')
video.load() // Reset to the beginning and select the best available source
video.currentSrc // The selected source

Do not use transparent PNGs for iOS favicons

Safari on iOS accepts an apple-touch-icon favicon that is used for stuff like desktop bookmarks. Always define a solid background color for them.

If you use PNGs with a transparent background, Safari will use just set a black background on your pretty icon. This is almost never what you want.
You can fix that by applying a white background via ImageMagick like this:

convert a...

CarrierWave: How to generate versions with different file extensions

We use CarrierWave in many of our projects to store and serve files of various formats - mostly images. A common use case of CarrierWave's DSL is to "process" the original file in order to create multiple "versions", for example different resolutions of the same image.

Now we could go one step further: What if we want to create versions that have a different file extension than the original file? For example, let's assume we'd like to create a ve...


Manipulate color with Sass functions

Sass comes with many built-in functions to manipulate color. Some of the more interesting functions include:

adjust-hue($color, $degrees)
Changes the hue of a color.

lighten($color, $amount)
Makes a color lighter.

darken($color, $amount)
Makes a color darker.

saturate($color, $amount)
Makes a color more saturated.

desaturate($color, $amount)
Makes a color less saturated.

Converts a color to grayscale.

Returns the compleme...

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Lazy-loading images

Since images are magnitudes larger in file size than text (HTML, CSS, Javascript) is, loading the images of a large web page takes a significant amount of the total load time. When your internet connection is good, this is usually not an issue. However, users with limited bandwidth (i.e. on mobile) need to mine their data budget better.

One popular strategy to improve the website performance is to not load images until they enter the viewport – aka "lazy-loading images".

General Issues

  • Crawlers do not execute JavaScript (generally sp...

How to grep through the DOM using the Capybara API

When your Cucumber feature needs to browse the page HTML, and you are not sure how to express your query as a clever CSS or XPath expression, there is another way: You can use all and find to grep through the DOM and then perform your search in plain Ruby.

Here is an example for this technique:

Then /^I should see an image with the filename...

Async control flow in JavaScript: Promises, Microtasks, async/await

Slides for Henning's talk on Sep 21st 2017.

Understanding sync vs. async control flow

Talking to synchronous (or "blocking") API

print('script start')
html = get('/foo')
print('script end')

Script outputs 'script start', (long delay), '<html>...</html>', 'script end'.

Talking to asynchronous (or "evented") API

print('script start')
get('foo', done: function(html) {
print('script end')

Script outputs 'script start', `'...

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Printing background color of elements

Browsers' printing methods usually don't print background colors. In most cases this is the desired behavior, because you don't want to spent tons of ink printing the background of a web page. But in some cases you want to print the background color of elements, e.g. bars of a chart. For those elements you need to set the following css styles:

-webkit-print-color-adjust: exact; /* Chrome and Safari */
color-adjust: exact; /* Firefox */

Note that if you're using autoprefixer, you don't need...

Video transcoding: Web and native playback overview (April 2020)


Embedding videos on a website is very easy, add a <video> tag to your source code and it just works. Most of the time.

The thing is: Both the operating system and Browser of your client must support the container and codecs of your video. To ensure playback on every device, you have to transcode your videos to one or more versions of which they are supported by every device out there.

In this card, I'll explore the available audio and video standards we have right now. The goal is to built a pipeline that...

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Colorful output for several linux command line tools: grc

Because colors improve readability so much.
On Ubuntu 18.04 you can install it with sudo apt install grc

From github:

For the impatient - try following commands:

grc netstat
grc ping hostname
grc tail /var/log/syslog
grc ps aux

Carrierwave: Deleting files outside of forms

TL;DR Use the #remove_<mounted_attribute>! method to delete attachments. Also, save the record after removing the file!

As you know, Carrierwave file attachments work by mounting an Uploader class to an attribute of the model. Though the database field holds the file name as string, calling the attribute will always return the uploader, no matter if a file is attached or not. (Side note: use #present? on the uploader to check if the file exists.)

class User < ApplicationRecord
  mount :avatar, AvatarUploader


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HTML/CSS: "transparent" is not a color

Heads up: transparent is not a real color, but black with 0% opacity.

In Safari, this makes e.g. gradients between transparent and white have some gray tones in the middle.

/* Unexpected in Safari */
linear-gradient(to right, white 0%, transparent 100%)

/* Correct */
linear-gradient(to right, white 0%, rgba(255,255,255,0) 100%)
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