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About this tool
Create CRT scan line, aperture grille and RGB mask overlays as transparent SVG or PNG with copyable CSS.
Scan Line Overlay Generator builds CRT-style stripe overlays — horizontal scan lines, a vertical aperture grille, an RGB shadow mask or interlaced field lines — and exports them as a transparent SVG, PNG or pure CSS. The pattern is described by a period (start of one line to the start of the next) and a thickness, so surface coverage is thickness ÷ period and the average light removed is coverage × opacity. It also matches real display standards: 486 active lines for NTSC, 576 for PAL, 480 for VGA and 200 for CGA.
Open Scan Line Overlay Generator on AltFTool — it loads instantly in your browser.
Provide your input — an image, text, or data.
Let the tool analyze or generate the result.
Review, refine, and reuse the output wherever you need it.
Coverage, average alpha and the resulting brightness change are calculated, not guessed at by eye.
One click sets the period to match NTSC, PAL, VGA or CGA active line counts at your output height.
Transparent SVG, transparent PNG and a repeating-gradient CSS block, all from the same settings.
Match the standard you are imitating: NTSC 525-line systems show 486 active lines, PAL and SECAM 625-line systems show 576, VGA shows 480 and CGA shows 200. On a 1080-pixel-tall canvas that works out to a period of roughly 2.2 px for NTSC and 1.9 px for PAL.
A stripe period under about 4 CSS pixels aliases against the display grid once the overlay is scaled or the browser zoom is not a whole number. Either increase the period, or export at exactly the display size so no resampling happens.
Multiply coverage by opacity. A 2 px line on a 4 px period covers 50% of the surface, so at 0.35 opacity it removes about 17.5% of the light. Compensate by raising the underlying image's brightness before the overlay goes on.
An aperture grille (Trinitron-style) uses continuous vertical stripes, so the overlay is vertical lines. A shadow mask uses discrete red, green and blue phosphor triads, which reads as a tinted three-stripe repeat rather than a plain darkening.