The visual experience of modern productivity tools is usually uniform. Dark mode offers charcoal grays and soft whites, while light mode sticks to standard sRGB values. However, a recent phenomenon has disrupted this balance: "Eye-Searing" HDR emojis. These custom reactions appear to emit actual light, glowing with a luminance that far exceeds the maximum white of the Slack user interface.

This is not a native Slack feature, nor is it a simple color trick using neon palettes. It is a sophisticated exploitation of how modern browsers and Electron-based applications handle High Dynamic Range (HDR) metadata and color profiles. By embedding specific high-luminance data into standard image formats like PNG, users are effectively "hacking" their coworkers' displays to push individual pixels to their hardware limits.

The Science of Luminance and Dynamic Range in Digital Displays

To understand how a Slack emoji can "glow," one must first understand the limitations of Standard Dynamic Range (SDR). For decades, digital content has been mastered for sRGB, a color space designed for cathode-ray tube (CRT) monitors. In this system, "white" is defined as the maximum value a pixel can reach, typically calibrated to around 100 nits of brightness. Even if you turn your monitor's brightness up to 500 nits, the software still treats that "maximum white" as the ceiling for all content.

SDR vs. HDR: The Battle for the White Point

High Dynamic Range (HDR) changes this paradigm. Instead of a fixed ceiling, HDR allows for a much higher peak brightness—often reaching 1,000 to 1,600 nits on high-end displays like the Apple Pro Display XDR or modern MacBook Pro screens.

When a standard application like Slack runs in an SDR environment, every element (text, backgrounds, emojis) competes for space within that 0–100% brightness range. However, when an HDR-capable application encounters an image with HDR metadata, it tells the display: "This specific set of pixels should ignore the standard white point and utilize the monitor's peak luminance."

The result is a visual anomaly. The Slack background remains at its standard SDR level, while the emoji's pixels are rendered using the display's extended headroom. This creates the "glow" effect, as the contrast ratio between the emoji and the surrounding text becomes massive.

How Nits Define Your Slack Experience

Luminance is measured in nits (candelas per square meter). A typical office monitor might peak at 300 nits. A MacBook Pro with an XDR display can hit 1,600 nits for HDR content. When you post a standard :thumbsup: emoji, it might be rendered at 200 nits. When you post a "hacked" HDR version, it can theoretically command the hardware to output 1,000+ nits. The human eye perceives this as a physical light source rather than just a digital color.

The Technical Foundation of the Slack HDR Hack

The "hack" relies on manipulating the ICC (International Color Consortium) profile and metadata of a PNG file. While PNG is traditionally an SDR format, modern rendering engines (specifically Chromium, which powers the Slack desktop app) have begun supporting HDR metadata within these files to facilitate professional photography and video workflows.

The Role of ICC Profiles and Color Gamuts

An ICC profile is a set of data that characterizes a color input or output device. The secret to the glowing emoji is the Rec.2020 (or BT.2020) color gamut combined with a PQ (Perceptual Quantizer) transfer function.

Most web images use the Rec.709 (sRGB) gamut. Rec.2020, however, covers a much wider range of colors and is the standard for Ultra HD 4K and 8K television. By tagging a small 128x128 pixel PNG with a Rec.2020 profile and stretching the luminance values beyond the 1.0 (or 100%) threshold, you trick the browser's color management system.

Why BT.2020 and Rec.2100 are the Secret Ingredients

The specific profile used in these viral emojis is often derived from BT.2100. This standard introduces the Perceptual Quantizer (PQ) curve, which is a mathematical function that maps digital values to absolute brightness levels. Unlike the gamma curves used in SDR, which are relative, PQ is absolute.

If a PNG's metadata tells the renderer that a pixel should be 1,000 nits according to the PQ curve, and the hardware is capable of it, the renderer will bypass the standard UI brightness scaling and output that exact intensity.

Why Slack and Chrome are the Perfect Breeding Ground

Not every application is susceptible to this hack. The reason it works so effectively in Slack is due to its underlying architecture.

The Electron Framework and Chromium’s Rendering Pipeline

Slack is built on Electron, a framework that allows developers to create desktop applications using web technologies (HTML, CSS, and JavaScript). Electron is essentially a specialized version of the Chromium browser engine.

Chromium has been at the forefront of implementing HDR support for the web. To support HDR video streaming (like YouTube) and HDR images, Chromium includes a sophisticated color management engine that can handle multiple color spaces simultaneously on a single page. When Chromium renders a Slack channel, it treats each emoji as an individual image asset. If one of those assets carries an HDR tag, the engine allocates the display's HDR headroom to that specific rectangle of pixels.

This is also why the effect works in Google Chrome but often fails in Safari or Firefox. Safari uses WebKit, which handles HDR differently, often tonemapping the entire page or ignoring HDR tags in small image assets to preserve battery life and visual consistency.

Step-by-Step Tutorial for Creating Custom HDR Emojis

Creating these emojis requires moving beyond standard image editors like Photoshop or Canva, which often strip out "illegal" color data during the export process. Instead, you need a tool that allows for granular control over image metadata. The industry standard for this is ImageMagick.

Preparing the Environment with Homebrew and ImageMagick

To begin, you will need a Unix-based environment (macOS or Linux). Windows users can use WSL (Windows Subsystem for Linux).

  1. Install Homebrew: If you haven't already, install the macOS package manager.
  2. Install ImageMagick: Run the following command in your terminal: brew install imagemagick
  3. Acquire the ICC Profile: You will need a specific HDR color profile, such as 2020_profile.icc or a BT.2100 PQ profile. These are often shared in developer circles or can be extracted from HDR-capable image processing software.

Executing the Multi-Stage Conversion Command

Once your tools are ready, you can convert a standard PNG into an HDR version. The goal is to take a standard image, boost its internal luminance values, and then wrap it in the Rec.2020 container.

Use the following command structure: