PortfolioSoftware · Simulation · PhotographyBratislava

06 Skies from first principles A twilight renderer that traces sunlight through the atmosphere. 13 Biela Noc The White Night, from 120 m. 08 Trolejbus BA Driving a Škoda 15Tr across Hodžovo námestie.

Inside

Selected work, the people I’ve built it for, a handful of photographs and the fastest way to reach me.

Contents

02Contents
Features Six sections · sixteen pages
  1. 06Terminator A physically based twilight renderer in one HTML file.
  2. 08Trolejbus BA Škoda 14Tr, 15Tr and Trollino 24 on real satellite imagery.
  3. 12Photographs Experiments over Bratislava and Biela Noc, from the air.

Departments

  1. 04LetterSoftware that behaves like the physical world.
  2. 10ClientsCompanies and people I’ve built for.
  3. 14ContactEmail, phone, socials and a vCard.
03Contents

The physics ofordinary things

A rendered sunrise, printed as a black halftone on kraft: the dots thin out into a glow around the sun on the horizon. The sun, 0.3° up

Fig. 0 Sunrise, rendered. Terminator, preset “Sunrise”, 30 m above the ground.

04Letter

I’m Ján Markuš. I write software in Bratislava and I’m the CTO of House of Comms.

Most of my days go into the plumbing that keeps a team’s work online: the sites, the tools and the systems underneath them. It is quiet work when it goes well, which is the point.

In my own time I build small, stubborn simulations. Terminator renders the sky from first principles, scattering sunlight through a spherical atmosphere until twilight looks the way twilight actually looks. Trolejbus BA puts a Škoda trolleybus on satellite imagery of the city and asks you to drive it with tyre physics that don’t forgive.

Both are single HTML files. No build step, no framework, nothing to install. I like it when the hard part is the idea, not the toolchain.

When I’m away from a keyboard I’m usually somewhere with a camera, which is where the photographs near the end come from.

JánEditor and author

05Ján Markuš
A sunset rendered by Terminator: the sun on a flat horizon under an orange to violet sky.
Project 01 Fig. 1 · four renders, one scroll
  1. 01SunsetSun +0.8°
  2. 02AfterglowSun −3.5°
  3. 03Blue hourSun −8.0°
  4. 04The limbFrom 400 km

Terminator

Keep scrolling: the hour changes before the page does.

06Work

Project 01 · WebGL2 · 2026

Physically based skies, at any hour.

A twilight renderer that doesn’t paint gradients. It traces light.

Every pixel is a ray through a spherical atmosphere: Rayleigh scattering in the air, Mie scattering in the haze, ozone absorbing the orange out of the zenith, a stratospheric aerosol layer and the faint green of airglow. The Earth’s shadow climbs the eastern sky at dusk because the geometry says it must.

Sun, moon and stars follow solar time, date and latitude. A film stage adds grain, halation, bloom and split toning, so the result reads like a photograph rather than a shader demo. Export goes up to 16 384 px.

Fig. 2 Rayleigh scattering in Terminator, 380 to 720 nm. Blue light at 440 nm scatters 5.7 times more than red at 680 nm. The colour of the sky starts here.
  • Sunrise
  • Belt of Venus
  • Stratosphere
  • Blue hour
Presets
11
Parameters
39
File
52 KB
Build step
None
Open Terminator
07Ján Markuš

Project 02 · Canvas · Vehicle dynamics

Trolejbus BA

A top-down simulator for Bratislava’s trolleybuses, driven over real satellite imagery from Hodžovo námestie outwards.

Mass, wheelbase and drive layout come from the real vehicles: the articulated 15Tr weighs 15 900 kg empty and 26 100 kg full. Tyres use a simplified Pacejka model with combined slip; articulation joints have stiffness, damping and a hard limit.

Load the bus, wet the road, and the same corner becomes a different problem. You can steer by drawing circles on a touchpad, like a real wheel.

Fig. 3 Lateral force over load against slip angle, the simulator’s own tyre on its four surfaces.
Vehicles
14Tr · 15Tr · 24
Top speed
65 km/h
Wheel lock
42°
15Tr, loaded
26 100 kg
  • W power
  • S brake
  • AD steer
  • Q touchpad wheel
  • M surface
08Work
Top-down satellite view of Hodžovo námestie in Bratislava with an articulated Škoda 15Tr trolleybus drawn at a stop. A · 15Tr at the stop B · Bus lane C · Island
A rigid Škoda 14Tr trolleybus seen from above at a stop on Hodžovo námestie.
Fig. 5 The rigid 14Tr: one driven axle, 16 000 kg loaded.

Fig. 4 A 15Tr waiting at Hodžovo námestie. Imagery © Google.

Drive it
09Ján Markuš

Workedwith

Companies and people I’ve designed, built or kept running things for. The list grows slowly, on purpose.

10Clients
Clients & collaboratorsSelected
  1. 01House of CommsChief technology officer
  2. 02Lancre.skTech lead
  3. 03Linde MHConsultant

More on request: business@janmark.us

11Ján Markuš

Photographs · 8 October 2026

Experiments over Bratislava

Six minutes of one flight at dusk, 18:20 to 18:26, while the sky over the city went from pink to blue.

Bratislava from the air at dusk: red-roofed houses and the city beyond, under pink-lit streaks of cloud. 01
  • A wooded ridge with a TV tower on top, villas and a hospital block below, under a blue evening sky.02
  • The city at blue hour, a railway line cutting through houses toward lit blocks on the horizon.03
  • Bratislava's centre and its high-rises at blue hour, the Danube catching the last light in the distance.04
Flight log, Experiments over Bratislava
No.TimeHeightHeadingShutterISO
0118:20:3355.0 m154° SSE1/30 s100
0218:24:51119.9 m320° NW1/20 s100
0318:25:37120.4 m184° S1/13 s200
0418:25:58119.8 m163° SSE1/15 s200

DJI drone, 24 mm equivalent, f/2.8, −⅓ EV. Heights above the take-off point.

12Photographs
Night over Bratislava from 120 metres: blue laser beams fan out from the lit Kamzík TV tower across the city lights.

Photographs · 3 October 2026

Biela Noc

Bratislava’s White Night, from the air: lasers thrown from the Kamzík TV tower across the city. Two to four seconds a frame, hovering at up to 120 m.

  • The Kamzík tower lit in white and red, a fan of blue beams reaching from it over the dark city.06
  • A band of blue laser light laid flat over the lights of Bratislava, the forest dark below.07
  • Blue and green beams cross from the tower over the city, the hillside forest lit gold beneath.08
Flight log, Biela Noc
No.TimeHeightHeadingShutterISO
0519:43:14119.6 m185° S4 s800
0619:44:3351.4 m182° S3 s1600
0719:59:31119.0 m184° S2.5 s1600
0820:04:20120.0 m192° SSW2 s1600
13Ján Markuš

Letters tothe editor.

14Contact

Write, call, or send a pigeon

jan@janmark.us
Business
business@janmark.us
Phone
+421 949 610 110
Address book
Download vCard
Based in
Bratislava, Slovakia · CET
15Ján Markuš
The Belt of Venus at dusk, rendered by Terminator and printed as an orange halftone disc.

janmark.us

Colophon

Designed and built by Ján Markuš. Set in Switzer and Source Serif 4. Every illustration is drawn by code from a seed at page load. Smooth scrolling by Lenis. Skies rendered with Terminator; satellite imagery © Google; aerial photographs by the author. No paper was harmed.

janmark.us