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Practical articles on tools and concepts
Editorial articles about the concepts, formulas and workflows behind J-Kit, always linking to the right tool.
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- How OCR actually works: from image to textOCR does not read an image, it processes pixels through several stages until they become characters. This guide opens the real pipeline behind the site’s own tool (Tesseract, in the browser, no upload), the shift from the classic engine to LSTM neural networks in 2018, and why a skewed photo or handwriting breaks the result.Jul 15, 2026 · 14 min
- The science behind the Pomodoro Technique (and what it doesn't prove)Francesco Cirillo built the Pomodoro method in the 1980s with a tomato-shaped kitchen timer. This guide separates the real history from the legend and shows, with sources, what psychology actually supports about timed breaks.Jul 15, 2026 · 15 min
- The hidden math of Minecraft: XP and portalsBehind the XP bar, the enchanting table and the Nether portal sit exact, documented formulas. This guide walks through the three experience curves, the 15-bookshelf math and the 1:8 ratio between dimensions, with real numbers worked out.Jul 15, 2026 · 12 min
- Numerology: the real origins, the calculation behind the number, and what science saysModern numerology cites Pythagoras as its founder, but the "Life Path" number an app hands you is a much more recent invention. This guide separates the real history from the legend, walks the digit-summing and reduction algorithm with master numbers, and explains, via the research of psychologist Bertram Forer, why the reading feels so personal.Jul 15, 2026 · 16 min
- The real history of tarot: from Italian card game to the 78 arcanaTarot began as a 15th-century Italian card game, not an ancient Egyptian artifact. This guide separates the documented history, Visconti-Sforza, Etteilla, Rider-Waite-Smith, from the myth invented in 1781, and explains the 78 arcana card by card.Jul 15, 2026 · 12 min
- Credit card interest: revolving debt, installments and what the CET really measuresBrazil’s credit card revolving balance is historically one of the country’s most expensive debts, with rates the Central Bank publishes in the hundreds of percent a year. This guide explains the 30-day rule, the Desenrola law’s 100% cap, and why the CET, not the nominal rate, is the honest comparison against installment plans.Jul 15, 2026 · 15 min
- Inside the FIPE table: market research, not a formulaBrazil's FIPE table does not come from a depreciation formula. It is a monthly market survey run by a foundation tied to the University of São Paulo, with a real structure, a code, a structural lag and limits worth knowing before you negotiate a car.Jul 15, 2026 · 15 min
- Dice probability in tabletop RPGs: advantage, disadvantage and exploding diceAdvantage in D&D does not double your odds, it pushes the curve closer to a coin flip. This guide derives the math behind advantage/disadvantage, shows why adding dice (3d6) creates a bell curve, and works out the expected value of an exploding die.Jul 15, 2026 · 14 min
- Inside the poker ranking: the exact probability of every handThe poker hand ranking is not an arbitrary convention: it follows rarity exactly. This guide derives the 2,598,960 five-card combinations from a standard deck and the exact count of every category, from a pair to a royal flush.Jul 15, 2026 · 13 min
- Blackjack basic strategy: the math behind the chartWhy the basic strategy chart says stand on 16 against a weak dealer and hit against a strong one. The expected-value math, the dealer bust probability by up-card, and the chart’s origin with Edward Thorp in 1961.Jul 15, 2026 · 18 min
- Inside Robux: buying, Roblox's cut and the DevEx cash-outRobux carries two very different dollar prices: what you pay to buy it and what a creator actually gets back cashing out via DevEx, currently $0.0038 per Robux. This guide walks the full chain from purchased pack to real cash.Jul 15, 2026 · 14 min
- Big Five vs. MBTI: what psychometrics actually saysWhat separates a psychometric instrument from a magazine quiz: reliability, validity, the problem of slicing continuous dimensions into types, and the Barnum effect. Why the Big Five holds up empirically and the MBTI is unstable, without turning either into a diagnosis.Jul 9, 2026 · 13 min
- How a birth chart is calculatedThe exact astronomy behind a natal chart: Julian Day, sidereal time, the Ascendant formula, ephemerides, house systems and precession, and what controlled tests say about the interpretation.Jul 9, 2026 · 13 min
- Heart rate zones: max HR, Karvonen and the fat-burning zoneWhere "220 − age" came from, why it is off by ±10–12 bpm, what Tanaka and Gulati proposed, what Karvonen actually computes (heart rate reserve) and why the "fat-burning zone" is misread.Jul 9, 2026 · 11 min
- Brazilian sentencing: the three-phase method, step by stepHow a Brazilian prison sentence is actually built: the three-phase method of art. 68 of the Penal Code, the eight circumstances of art. 59, the 1/8 case-law fraction, STJ Precedent 231, and a worked theft case from art. 59 to the initial regime and to the substitution for alternative penalties.Jul 9, 2026 · 15 min
- Civil procedure deadlines in Brazil: business days, start point and the exceptionsHow to count a deadline in Brazilian civil procedure: business days (art. 219), the start point in art. 231, the winter-recess suspension (art. 220), doubled terms and the contrast with criminal, labor and small-claims procedure.Jul 9, 2026 · 14 min
- Mouse sensitivity: cm/360, DPI and why eDPI misleadseDPI only compares within one game; cm/360 is the only physical quantity. How yaw ties sensitivity to distance, how to convert between linear FPS games, and why R6 and Fortnite have no exact conversion.Jul 9, 2026 · 11 min
- Loudness, LUFS and streaming audio normalizationWhy peak dBFS says nothing about how loud something sounds, what LUFS actually measures in ITU-R BS.1770, the broadcast targets (−23 LUFS in EBU R128, −24 LKFS in ATSC A/85) and why streaming normalization ended the loudness war.Jul 9, 2026 · 15 min
- CSS clamp() and fluid typography: the math (and the accessibility trap)clamp() is linear interpolation in disguise. This guide derives the formula from scratch, slope, intercept, preferred value, walks two complete numeric examples and explains why a font-size in pure vw breaks WCAG 1.4.4 and how the rem term hands zoom back to the user.Jul 9, 2026 · 13 min
- EXIF metadata: what your photos revealEvery photo travels with a dossier: EXIF carries camera, date, GPS and even the serial number. See what tags ride along, what platforms strip (and what they do not) and how to erase it for real, without mistaking that for anonymity.Jul 9, 2026 · 12 min
- WebP, AVIF, JPEG, PNG: which format to useA decision criterion, not a list. What each format really does, lossy vs lossless, the size gains Google and AOMedia publish (with the caveats), when AVIF loses to WebP, and why "convert everything to AVIF" is bad advice.Jul 9, 2026 · 14 min
- Cron expressions explained (and the OR trap)The five cron fields, the * , - / operators, the @daily/@reboot shortcuts, the non-POSIX Quartz extensions and the rule almost everyone gets wrong: when day-of-month and day-of-week are an OR, not an AND.Jul 9, 2026 · 14 min
- JWT: anatomy, signature and securityA JWT is not encrypted, it is signed and readable by anyone. This guide opens the header, payload and signature, explains the registered claims, the classic attacks (alg:none, RS256/HS256 confusion, kid injection) and an honest rule for when NOT to use JWT.Jul 9, 2026 · 12 min
- Time zones, UTC and daylight saving: why dates break in productionUTC is not GMT, and an offset is not a time zone. This guide explains the time scale, the IANA database, times that do not exist and that happen twice, the 2038 problem and the golden rule for storing dates without getting burned in production.Jul 9, 2026 · 12 min
- Correlation is not causation: what r and R² actually tell youWhat Pearson’s r measures, why r = 0 is not independence, why R² is so widely misread, and how Anscombe’s quartet proves that identical statistics hide opposite data.Jul 9, 2026 · 12 min
- The math of pity: why 0.6% is not the rate you feelBase rate, soft pity, hard pity, consolidated rate and the 50/50 system in Genshin, Honkai: Star Rail, Zenless Zone Zero and Wuthering Waves, with the expected-pulls math to guarantee the featured character.Jul 9, 2026 · 13 min
- Lottery probability: where 1 in 50 million comes fromThe honest math of the Mega-Sena: where 1 in 50,063,860 comes from, why betting more numbers gets combinatorially expensive, what a wheel really guarantees and why the expected value stays negative even with giant rollover jackpots.Jul 9, 2026 · 13 min
- Percentages: increase, discount and the right formulasThe percentage operations you use every day, X% of Y, what % X is of Y, increase, discount and change, with the exact formula and the four errors that cost money: increase and discount do not cancel, a percentage point is not a percent, discounts do not add up and averaging percentages misleads.Jul 6, 2026 · 15 min
- Unit conversion: why temperature is not like distance and weightDistance and mass are ratio scales: converting is multiplying by an exact factor. Temperature in °C and °F is an interval scale with an arbitrary zero, 40 °C is not "twice" 20 °C. The right formulas, the factors fixed by international agreement, and the care that significant figures demand.Jul 6, 2026 · 16 min
- Inside the QR code: Reed–Solomon, Pix, vCard and Wi-FiA QR code is two layers: a matrix with Reed–Solomon error correction (versions 1–40, levels L/M/Q/H) and a standardized text on top. This guide opens both, including the Pix BR Code field by field and the CRC-16 calculation.Jul 6, 2026 · 15 min
- Attorney fees in Brazil: the three kinds and how the OAB table appliesContractual, succumbence and court-arbitrated fees are three different things, each with its own nature and destination. Understand what the OAB table really is (an ethical floor per state bar, not a national ceiling) and how CPC art. 85 calculates succumbence fees, bracket by bracket.Jul 6, 2026 · 17 min
- Monetary correction and interest: the official indices (SELIC, IPCA, IGP-M, Legal Rate)Monetary correction is not default interest. Understand IPCA, IPCA-E, INPC, IGP-M, SELIC and the Legal Rate of art. 406, which index fits each subject, why SELIC is not added to another index, and how Law 14.905/2024 changed civil claims.Jul 6, 2026 · 16 min
- Pregnancy weeks and estimated due date: how it is calculatedGestational age is counted from the last menstrual period (LMP), not from conception. See Naegele’s rule and its cycle correction, when ultrasound replaces the LMP under ACOG’s redating rules, and why the due date is always a median, not a deadline.Jul 6, 2026 · 15 min
- Calories burned by activity: how the estimate is madeEvery exercise-calorie calculator uses the MET model. See the formula derived step by step, the MET table, why 3.5 mL/kg/min overestimates rest for many people, the difference between gross and net expenditure, and worked examples.Jul 6, 2026 · 15 min
- BMI vs. body-fat percentage: what BMI cannot seeBMI is a weight/height² ratio that cannot tell muscle from fat or where the fat sits. See the accuracy hierarchy of methods (DXA down to BMI), the Deurenberg formula, the U.S. Navy method, the ranges by sex, and the waist-to-height ratio from the NICE guideline.Jul 6, 2026 · 16 min
- Meta tags Google and social networks understandTwo different pipelines read your head: the search engine (title, description, canonical, robots, hreflang) and the social scraper (Open Graph, Twitter/X cards). Distinct caches, distinct failures, including the robots.txt trap that keeps pages indexed.Jul 6, 2026 · 18 min
- Color spaces: HEX, RGB, HSL and why HSL lies about brightnessHEX, RGB and HSL describe the same sRGB color, but HSL’s "L" is not real brightness. See why yellow and blue at the same L look so different, how real luminance is computed, and why CSS Color 4 brought OKLCH.Jul 6, 2026 · 15 min
- Color contrast and accessibility: what WCAG requiresHow the contrast ratio is derived from relative luminance (from 1:1 to 21:1), WCAG 2.2 AA and AAA thresholds, the formula’s known defects, and why APCA is still not normative.Jul 6, 2026 · 15 min
- UUID, ULID and IDs: when to use eachThe eight UUID versions of RFC 9562 (v1 to v8), the name-based ids (v3/v5), the sortable ULID and its monotonicity rule, the real collision probability of v4, how to read the date out of a sortable id, and alternatives like Snowflake, KSUID and NanoID, with the criterion to pick the right key.Jul 6, 2026 · 17 min
- JSON and YAML: the three traps that corrupt data on conversionA country code "NO" that turns into false, a 64-bit id that loses precision, a date that converts itself. This guide dissects the silent traps of JSON, YAML and TOML, confirms every claim against the primary source, and shows how to convert without wrecking your data.Jul 6, 2026 · 15 min
- Regex from scratch: anchors, classes and quantifiersA practical regular-expressions guide for people who code: anchors, classes, greedy and lazy quantifiers, named groups, lookahead and lookbehind, Unicode with \p{…}, and the trap that took down 10% of the internet, catastrophic backtracking (ReDoS).Jul 6, 2026 · 17 min
- How to choose and test a strong passwordEntropy is the only honest measure of strength, and it measures how the password was generated, not the string. See why the complexity rule backfires, why NIST changed its mind, and how to test and store passwords well.Jul 6, 2026 · 17 min
- Hashing, encryption and encoding: the differencesThree operations that scramble text and that people constantly mix up. What separates encoding, hashing and encryption is reversibility and the key, and getting it wrong is how passwords become MD5, how collisions broke SHA-1 and why HMAC exists.Jul 6, 2026 · 16 min
- HTTP status codes: the decisions each number carriesNot a list of codes, a guide to the choices they represent: 301 vs 308, 302 vs 307, 401 vs 403, 404 vs 410 and the soft 404, the 304 cache cycle with ETag and the 429 with Retry-After. All anchored in RFC 9110.Jul 6, 2026 · 17 min
- SPF, DKIM and DMARC: how to protect your domain’s emailThe three layers of email authentication and the concept that ties them together: identity alignment. Why an email can pass SPF and still fail DMARC, why SPF breaks on forwarding, and how to roll it out without dropping legitimate mail.Jul 6, 2026 · 18 min
- Public vs private IP: NAT, CGNAT and what your IP revealsThe RFC 1918 private ranges, how NAT really translates port and state, why CGNAT makes your public IP not yours, and why IP geolocation points at the provider, not your home.Jul 6, 2026 · 16 min
- VLSM in practice: allocating and aggregating subnetsThe real work of IPv4 addressing: how to split a /24 among differently sized departments with VLSM, address by address, and how to aggregate contiguous subnets into a single route (summarization), with the binary in hand.Jul 6, 2026 · 16 min
- How to read and audit a Brazilian payslip, line by lineA line-by-line audit manual for a Brazilian payslip: the legal basis of each item (CLT, Constitution, eSocial), the formula and the common error. Includes overtime with its paid-rest reflex and the night-shift premium with the reduced hour, worked step by step.Jul 6, 2026 · 16 min
- Simples Nacional vs Presumed Profit for a service companyTwo formulas decide almost everything: the Simples effective rate and the Fator R. This guide shows why paying yourself more can cut the tax bill, compares Simples and Presumed Profit with real numbers, and covers the dividend taxation that changed in 2026.Jul 6, 2026 · 17 min
- Brazilian severance: the termination-type × entitlement matrixThe type of termination defines every entitlement. A matrix of the five exits (without cause, just cause, resignation, mutual agreement under art. 484-A and indirect termination) against notice, 13th, vacation, the FGTS penalty and withdrawal, and unemployment insurance, with two worked comparisons.Jul 6, 2026 · 17 min
- Brazil’s 13th salary: how it is calculated, when it lands, and why the second half is smallerThe Brazilian 13th salary earned month by month, its two installments and legal deadlines, and the trap of the second installment, which alone carries the INSS and income tax on the whole bonus. Two worked examples and the statute, article by article.Jul 6, 2026 · 17 min
- IRRF withholding: taxable base, the deductible portion and deductionsIRRF is not charged on gross salary. This guide derives the deductible portion step by step, shows the 2026 monthly table, the R$ 5,000 exemption of Law 15,270/2025, the simplified discount, RRA and the 13th salary, with the arithmetic in full.Jul 6, 2026 · 14 min
- INSS 2026: how the progressive contribution rates and the effective rate workThe 14% rate does not apply to your whole salary. See how Brazil’s INSS contribution is charged by brackets in 2026, why the effective rate is always lower, and why above the ceiling the deduction turns regressive, with worked examples, a formula and a chart.Jul 6, 2026 · 13 min
- BMI, BMR and TDEE: the error that piles up at each link of the chainBMI is a population index applied to one person; BMR is a regression fitted on a sample; TDEE multiplies it all by an activity factor that is a categorical guess. This guide quantifies the accumulated error with real numbers.May 9, 2026 · 17 min
- CPF and CNPJ: the check digit, validation and the alphanumeric CNPJThe CPF/CNPJ check digit is a mod-11 checksum, not magic. This guide derives it step by step, works two examples from scratch, explains what validation does NOT prove, and details the 2026 change: the alphanumeric CNPJ.May 9, 2026 · 15 min
- CIDR, IPv4 subnet and IPv6: what the mask does in binaryA subnet mask is a bit-by-bit operation, not a memorized table. This guide opens the binary AND that produces the network address, shows why CIDR replaced Class A/B/C and why IPv6 drops broadcast and NAT.May 9, 2026 · 14 min
- Compound interest, contributions and inflation: the exponential your intuition reads as a lineCompound interest is an exponential, and nearly every gut-feel error comes from treating it as a straight line. This guide derives compounding, the annuity formula and the real return net of tax and inflation (the Fisher effect), with a chart, the tax table and two hand-worked examples.May 9, 2026 · 16 min
- SAC or PRICE: the two formulas, the crossover point and the CETBoth systems charge the same rate on the same balance. What changes is HOW amortization is spread over time. We derive both formulas from scratch, compute the month where the SAC installment drops below PRICE, and show why the effective cost (CET), not the nominal rate, is the number that decides.May 9, 2026 · 16 min
- How to calculate net salary in Brazil: the order of operationsFrom gross to net without getting the order wrong: progressive INSS by brackets, the IRRF base (gross − INSS − dependents − alimony), IRRF with its deductible portion and the exemption up to R$ 5,000. 2026 tables, formulas and two worked examples.May 9, 2026 · 14 min