Is my thumbnail clickable?

No. The analysis runs entirely in your browser using a canvas element, and the image never leaves your device. There is no upload, no server call and nothing stored, which is why it works offline once the page has loaded.

Measurable visual properties, not taste: luminance spread, brightness band, colour saturation, edge density and resolution. These correlate with whether a thumbnail reads at small sizes, which is the bar a thumbnail has to clear before anything else matters.

Treat it as a floor check rather than a target. A low score reliably indicates a real problem such as low contrast or a frame too busy to parse at phone size; a high score only means nothing obvious is wrong, not that the thumbnail will win.

No. Measured clicks beat any heuristic, including this one. Use the score to catch problems before you publish, then test variants on the live video — our A/B previewer shows a thumbnail at real YouTube sizes, which catches a different class of problem.