New composable glitch-related filters toolset

[KaliedaRik]

Overview

The existing GLITCH filter will be deprecated - though we can keep the existing filter under a new name eg legacy-glitch.

Instead, Scrawl-canvas will provide a set of small, composable glitch primitives that can be mixed and matched to create modern “glitch” looks (VHS, scanline, streaming errors, codec corruption, etc.).

This approach:

• Improves clarity and composability (each operator does one thing well)
• Matches the modular architecture of the current filter engine
• Keeps filters performant (O(n) row/column or 32-bit array operations)
• Makes it easier to define presets

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What’s Already Possible (No New Filters Needed)

Channel Split / RGB Drift

Use: OFFSET (per-channel dx/dy) + optional GAUSSIAN_BLUR
Notes:

• OFFSET already supports offsetRedX/Y, offsetGreenX/Y, offsetBlueX/Y, etc.
• Apply small-radius blur with includeAlpha:false to simulate chroma bleed.

Quantize / Posterize / Banding

Use:

• STEP_CHANNELS (bit-depth style stepping)
• LOCK_CHANNELS_TO_LEVELS (explicit level locking)
• REDUCE_PALETTE (palette quantization with optional dithering)

Noise Overlays (Grain, RF Snow)

Use: RANDOM_NOISE

• noiseType: 'random' | 'ordered' | 'bluenoise'
• Combine with subtle vertical blur for analog “hum”.

Color-Space “Oops” (Range Crush / Chroma Smear)

Use: TINT_CHANNELS, MODULATE_CHANNELS, or OKLab filters

• Range crush via MODULATE_CHANNELS (amplify/clamp)
• Chroma smear via channel-only blur:

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Partially Covered (Workable via Chaining)

Row/Column Displace (Band “Tears”)

Current options:

• Deprecated GLITCH has row shift logic
• DISPLACE with a banded displacement map in lineMix
  Recommendation: add a clean, seedable ROW_COL_DISPLACE primitive for performance and ergonomics.

Slice Repeat / Dropout

Current status: can fake dropouts via THRESHOLD + COMPOSE.
Missing: true “duplicate this slice” and “zero this span” operations.
Recommendation: implement a dedicated SLICE_REPEAT_DROPOUT primitive.

Scanline Mod (Sinusoidal Luminance)

Current status: not achievable with current per-grayweight filters.
Recommendation: add a cheap per-row modulation primitive.

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New Filters to Add

1) ROW_COL_DISPLACE

Purpose: Shift rows/columns in random bands (VHS/scanline effect).

Arguments:

• direction: 'x' | 'y' | 'both'
• bands: { chance, heightMin, heightMax }
• offset: { xMin, xMax, yMin, yMax }
• edgeMode: 'wrap' | 'clamp' | 'transparent'
• seed

Implementation notes:

• Bulk Uint32Array subarray copies per band for speed; O(n)

2) SLICE_REPEAT_DROPOUT

Purpose: Duplicate or zero random horizontal slices (streaming tear).

Arguments:

• repeat: { chance, widthMin, widthMax, offsetMin, offsetMax }
• dropout: { chance, widthMin, widthMax }
• wrapMode: 'row' | 'image'
• seed

Implementation notes:

• Row-local set and fill operations; O(n)

3) SCANLINE_MOD

Purpose: Apply per-row brightness modulation (sin wave or hum).

Arguments:

• amp, freq, phase, bias = 0
• includeAlpha = false

Implementation notes:

• Fast multiply/add scalar per row on R/G/B; O(n)