Plain-English explainers behind the free tools at polyatic.com. https://polyatic.com/guides/feed.xml 2026-07-10T00:00:00Z Polyatic https://polyatic.com/guides/aqi-pm25-explained/ 2026-07-10T00:00:00Z

A first-principles guide to the US EPA Air Quality Index: what AQI is, the piecewise-linear breakpoint formula Ip=((IHi-ILo)/(BPHi-BPLo))*(Cp-BPLo)+ILo, the full PM2.5 breakpoint table, a PM2.5-to-AQI example computed step by step, the six category bands with honest EPA health guidance, PM2.5 vs PM10 vs ozone, and the honest limit that AQI only reflects the concentration you measure.

https://polyatic.com/guides/base64-encoding-explained/ 2026-07-10T00:00:00Z

A plain-English guide to Base64: the problem it solves (moving binary safely through text-only channels), the 64-character alphabet, how 3 bytes map to 4 characters bit by bit, why = padding appears, the URL-safe variant, the ~33% size cost, data: URIs, and the honest truth that Base64 is encoding — not encryption. With a worked byte-by-byte example.

https://polyatic.com/guides/cidr-subnetting-explained/ 2026-07-10T00:00:00Z

A plain-English guide to CIDR subnetting: what an IPv4 address and /prefix mean, how /24, /26 and /30 partition a block, and how to work out the network, broadcast and usable-host count by hand — with worked examples and a subnet-size table.

https://polyatic.com/guides/color-models-explained/ 2026-07-10T00:00:00Z

A first-principles guide to how screen color works: RGB as additive light on 0–255 channels, HEX as those channels written in base-16, HSL's intuitive hue/saturation/lightness, alpha for transparency, one color converted by hand across HEX ↔ RGB ↔ HSL, and honest notes on sRGB and wide-gamut displays.

https://polyatic.com/guides/cron-expressions-explained/ 2026-07-10T00:00:00Z

A plain-English guide to cron expressions: what the five fields and their ranges mean, how *, ranges, lists and steps (*/15) work, the day-of-month vs day-of-week OR trap explained honestly, a table of ten real schedules decoded field by field, and where standard cron ends and Quartz, @reboot and @macro shortcuts begin.

https://polyatic.com/guides/exif-metadata-explained/ 2026-07-10T00:00:00Z

A first-principles guide to EXIF and image metadata: what a photo secretly records (camera make/model, exposure, orientation, capture time and GPS geotags), where that block lives in a JPEG (APP1) and HEIC, the real privacy risk of sharing geotagged photos, how stripping works by re-encoding the pixels, and when to keep metadata versus remove it.

https://polyatic.com/guides/great-circle-distance-explained/ 2026-07-10T00:00:00Z

Why a straight-line lat/long distance is wrong on a sphere, what the great-circle (orthodromic) shortest path is, the haversine formula worked step by step between London and Paris, great-circle vs constant-bearing rhumb lines, computing an initial bearing, and the honest spherical-vs-WGS-84 accuracy limit — using the IUGG mean radius 6371008.771 m.

https://polyatic.com/guides/hashing-explained/ 2026-07-10T00:00:00Z

A first-principles guide to cryptographic hashing: deterministic fixed-length digests, one-way preimage resistance, the avalanche effect, the honest difference between hashing, encryption and encoding, why MD5 and SHA-1 are broken while SHA-256 is the default, and what hashing is genuinely for — integrity checks, deduplication and salted-KDF password storage. With a worked one-character-change example.

https://polyatic.com/guides/json-explained/ 2026-07-10T00:00:00Z

A first-principles guide to JSON: what it is and why it beat XML as the web's data format, the six value types (object, array, string, number, boolean, null) with a worked example, the strictness gotchas people actually hit (double quotes only, no trailing commas, no comments), how JSON differs from JavaScript object literals, pretty-print vs minify, and what each parse error really means.

https://polyatic.com/guides/jwt-structure-explained/ 2026-07-10T00:00:00Z

A first-principles guide to the JSON Web Token: the three dot-separated parts (header.payload.signature), how header and payload are Base64URL-encoded JSON anyone can read, the registered claims (iss, sub, aud, exp, iat, nbf) shown in a worked example, signed (JWS) vs the myth that a JWT is encrypted, the alg:none footgun, and the honest rule — never trust an unverified token.

https://polyatic.com/guides/number-bases-explained/ 2026-07-10T00:00:00Z

A first-principles guide to number bases: what positional notation and a radix really mean, why computers use binary, why programmers read bytes in hex (1 hex digit = 4 bits), the number 156 converted by hand across all four bases, how hex maps to #RRGGBB colors and byte dumps, and two's-complement for signed integers in plain terms.

https://polyatic.com/guides/qr-codes-explained/ 2026-07-10T00:00:00Z

A first-principles guide to how QR codes actually work: the finder, alignment and timing patterns; the mandatory quiet zone; the four encoding modes (numeric, alphanumeric, byte, kanji) and why the mode sets capacity; versions from 21×21 up; the L/M/Q/H error-correction levels and their ~7/15/25/30% recovery; and why Reed–Solomon lets a damaged or logo-covered code still scan. With worked examples.

https://polyatic.com/guides/regex-quick-reference/ 2026-07-10T00:00:00Z

An example-dense regex reference: character classes and their negations, anchors (^ $ \b), quantifiers with greedy vs lazy shown side by side, capturing vs non-capturing groups, alternation and backreferences, all four lookarounds worked through, and a table of real patterns (email-ish, ISO date, hex colour, US phone) each with an honest note on what it does NOT catch.

https://polyatic.com/guides/unix-timestamp-explained/ 2026-07-10T00:00:00Z

A first-principles guide to Unix timestamps: what the 1970 epoch is, why timestamps are always UTC and carry no timezone, how to tell seconds from milliseconds by digit count, the Year 2038 32-bit overflow and its 64-bit fix, ISO 8601 versus epoch, and a worked conversion in both directions.

https://polyatic.com/guides/url-encoding-explained/ 2026-07-10T00:00:00Z

A first-principles guide to URL percent-encoding: why URLs are ASCII-only, reserved vs unreserved characters, how %XX hex escapes work, the %20-vs-+ space question, encodeURIComponent vs encodeURI, UTF-8 multi-byte encoding for non-ASCII, and the double-encoding bugs that bite in production. With worked examples.

https://polyatic.com/guides/uuid-explained/ 2026-07-10T00:00:00Z

A first-principles guide to UUIDs: the 128-bit identifier behind the 8-4-4-4-12 hex string, how the version nibble and variant bits are encoded, a worked field-by-field dissection of one real UUID, v4 (opaque random) vs v7 (time-sortable), collision probability with honest birthday-bound math, and UUID vs auto-increment integer keys — when to pick which.

https://polyatic.com/guides/wcag-contrast-explained/ 2026-07-10T00:00:00Z

A first-principles guide to WCAG color contrast: what the contrast ratio measures, how relative luminance is computed from linearized sRGB channels weighted 0.2126R/0.7152G/0.0722B, the 4.5:1 / 3:1 / 7:1 thresholds and what 'large text' means — with #767676 on #FFFFFF worked out by hand.