# CRT & VHS Effect Generator

> Give an image CRT scanlines and phosphor masks or VHS tape artefacts. Render a still or a short seeded loop with CRT scanlines, aperture-grille, shadow or slot masks, curvature and bloom, or VHS chroma bleed, tracking noise, head switching and line jitter. Optionally quantise to a retro palette first. Download PNG, animated GIF and APNG.

Part of usefulweb.app (https://usefulweb.app/). Part of [Flipline](https://flipline.usefulweb.app/) (ownerApp: flipline). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://fliplinecrtvhsfilter.usefulweb.app/ (interactive tool at https://fliplinecrtvhsfilter.usefulweb.app/tool)
- JSON API: POST https://fliplinecrtvhsfilter.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://fliplinecrtvhsfilter.usefulweb.app/api/v1/tool)
- OpenAPI: https://fliplinecrtvhsfilter.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://fliplinecrtvhsfilter.usefulweb.app/mcp - tool `flipline_crt_vhs_filter`

## Inputs
- image (file, required): Image (PNG, JPEG or WebP)
- effect (select): Display effect
- resolution (select): Machine resolution
- width (integer): Working width (px)
- fit (select): Image fit for machine resolutions
- scale (integer): Output scale (whole pixels)
- scanlines (integer): Scanline strength (%)
- mask (select): Phosphor mask
- maskStrength (integer): Mask strength (%)
- curvature (integer): Screen curvature (%)
- bloom (integer): Bloom (%)
- bloomRadius (integer): Bloom radius (source pixels)
- vignette (integer): Vignette (%)
- crtNoise (integer): CRT grain (%)
- flicker (integer): Frame flicker (%)
- lumaBlur (integer): Luma softness (%)
- sharpen (integer): Edge ringing (%)
- chromaBleed (integer): Chroma bleed (%)
- chromaShift (number): Chroma delay (px at 640 px width)
- tapeSaturation (integer): Tape colour saturation (%)
- tracking (integer): Tracking error (%)
- trackingPosition (integer): Tracking band position (% of height)
- roll (number): Tracking roll per frame (% of height)
- headSwitch (integer): Head-switching noise (%)
- jitter (integer): Line jitter (%)
- tapeNoise (integer): Tape noise (%)
- seed (integer): Noise seed
- frames (integer): Animation frames
- fps (integer): Animation frame rate (FPS)
- palette (select): Retro palette
- customPalette (textarea): Custom palette (HEX list, GPL or JASC-PAL)
- dither (select): Dithering method
- strength (number): Dither strength (%)
- serpentine (boolean): Serpentine scanning for error diffusion
- match (select): Colour matching
- cells (select): Attribute cells
- brightness (number): Brightness
- contrast (number): Contrast
- gamma (number): Gamma
- saturation (number): Saturation (%)

## Method
- The source is area-resampled to the working grid in linear light, developed, and optionally quantised exactly as in the dither tool. Frame n uses seed + n, so the same inputs always give the same frames.
- CRT: each output pixel maps through a barrel curvature, interpolates the two nearest source lines horizontally and weights them with Gaussian beams whose width grows with channel brightness, normalised so flat areas keep their level. A phosphor mask dims the other channels per subpixel column, with mean-brightness compensation; bloom screen-blends a blurred, squared copy; vignette, grain and flicker follow. Light is mixed in linear light.
- VHS: each line is shifted by seeded jitter, a rolling tracking band and bottom head-switching skew, then converted to YIQ (FCC matrix). Luma is low-passed with an edge-ringing term; chroma is blurred more widely, delayed to the right and averaged with the previous line. Tape noise, dropout streaks and band snow use hashed noise.
- Exports: PNG of frame 1, and for animations an animated GIF (adaptive 256-colour palette, 10 ms timing, at most 1600 px per side) and a lossless APNG. Requests are limited to 4 MP per frame and 12 MP in total.

## Limits
- Stylised effects, not a calibrated simulation of a particular monitor, tape or recorder.