Proposed improvement: 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

---

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:

---

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.

---

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)

[KaliedaRik]

My gut feeling is that this work, while interesting, offers no concrete benefits to the filters system, beyond niche usage. I'll leave the discussion open for now, but any work in this area is likely to be very low priority.