Research Article
The Role of Visual Features in Text-Based CAPTCHAs: An fNIRS Study for Usable Security
Table 1
Selected features to be analyzed with difficulty levels.
| Features | # of difficulties | Security features defined for each level |
| Line | 3 levels | No line | Line thin | Line thick | Waving | 3 levels | No wave | Wave light | Wave high | Rotation | 2 levels | No rotation | Rotation {−20, −15, 15, 20} | — |
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