| # PolyCAT File Formats
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| ## Gaze TSV (`data/gaze/*.tsv`)
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| Tab-separated sample-level gaze data. One file per session, named `{participant_id}_part{part}_{session_id}.tsv`.
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| - **Sampling rate:** 500 Hz per eye (binocular recording; hardware rate 1000 Hz split between eyes)
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| - **Scope:** Only samples during stimulus presentation (4.0 s per trial) are included
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| - **Eyes:** One row per sample per eye (binocular sessions produce 2 rows per timestamp)
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| ### Validity codes
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| | Code | Meaning |
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| |------|---------|
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| | 0 | Valid gaze sample (within screen bounds 0-3840 x 0-2160) |
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| | 1 | Missing or out-of-range (blink, track loss, or coordinates outside screen) |
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| ---
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| ## Fixations CSV (`data/fixations/fixations_all.csv`)
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| Comma-separated fixation events. Fixations detected by EyeLink's built-in algorithm with:
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| - Saccade velocity threshold: 30 deg/s
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| - Saccade acceleration threshold: 8000 deg/s^2
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| Only fixations starting during stimulus presentation are included. Fixation `end_time_ms` is clamped to stimulus offset.
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| ### Per-participant files
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| Individual files at `data/fixations/per_participant/{participant_id}_fixations.csv` with the same schema.
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| ---
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| ## Saliency Maps
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| ### NumPy format (`data/saliency_maps/by_polygon/{polygon_id}/{image_id}_fixmap.npy`)
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| - 2D float32 array at stimulus resolution (3840 x 2160)
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| - Gaussian kernel sigma = 1.0 degrees visual angle (~78.5 pixels)
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| - Values outside polygon aperture set to 0
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| - Normalized to sum to 1 (probability distribution)
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| - Only generated for polygon-image combinations with >= 3 observers
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| ### PNG format (`data/saliency_maps/by_polygon/{polygon_id}/{image_id}_fixmap.png`)
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| - Grayscale visualization of the saliency map
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| - 8-bit, linearly scaled from 0 (min) to 255 (max)
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| ---
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| ## Scanpaths (`data/scanpaths/{participant_id}/{trial_uid}.csv`)
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| One CSV per trial, containing the ordered fixation sequence.
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| Columns: `fixation_index`, `start_time_ms`, `end_time_ms`, `x_px`, `y_px`, `duration_ms`
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| ---
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| ## MATLAB Export (`data/fixations/fixations.mat`)
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| CAT2000-compatible MATLAB struct:
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| ```matlab
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| fixations.x % double vector: x positions (pixels)
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| fixations.y % double vector: y positions (pixels)
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| fixations.dur % double vector: durations (ms)
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| fixations.subj % cell array: participant IDs
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| fixations.img % cell array: image IDs
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| fixations.polygon % cell array: polygon IDs (PolyCAT extension)
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| ```
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| ---
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| ## Coordinate System
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| **Screen pixels:** 3840 x 2160, origin top-left. `x_px` increases rightward, `y_px` increases downward.
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| **Degrees:** Relative to screen center (1920, 1080 px).
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| Conversion formula:
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| ```
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| x_deg = (x_px - 1920) / ppd
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| y_deg = (y_px - 1080) / ppd
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| ```
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| Where `ppd` (pixels per degree) is computed from:
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| - Screen diagonal: 27 inches = 68.58 cm
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| - Resolution: 3840 x 2160 px
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| - Screen width: 59.77 cm, height: 33.62 cm (from diagonal + 16:9 aspect ratio)
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| - Viewing distance: 70 cm
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| - Pixel pitch = screen_width / horizontal_pixels = 59.77 cm / 3840 ≈ 0.01557 cm/px
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| - 1° at 70 cm = 70 × tan(1°) ≈ 1.2228 cm
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| - `ppd = 1.2228 / 0.01557` ≈ **78.5 px/deg**
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| ---
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| ## Time Reference
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| | Context | Reference point | Units |
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| |---------|----------------|-------|
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| | `timestamp_rel_ms` (gaze TSV) | Stimulus onset for that trial | milliseconds |
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| | `start_time_ms` / `end_time_ms` (fixations) | Stimulus onset for that trial | milliseconds |
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| | `ts_*` fields (trials.csv) | Session start (experiment clock) | seconds |
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