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Image Filters - image-filters

feature idstatusdescription
image-filtersdraftstandard image filters modeling & implementation details

Master Comparison Table​

Name (UI)Technical termMath backend (per pixel)GridaCSSFigmaApple PhotosPhotoshop / ResolveSkia implOrder sensitivePriorityDifficultyNotes / Usage
ExposureExposurelinear multiply (C * 2^E)plannedfilter: brightness()✓✓✓color matrixNoHighLowBasic linear brightness adjustment
ContrastContrastlinear scale around 0.5plannedfilter: contrast()✓✓✓color matrixNoHighLowLinear contrast scaling
SaturationSaturationlinear interpolationplannedfilter: saturate()✓✓✓color matrixNoHighLowAdjusts color intensity
TemperatureTemperaturecolor temperature shiftplannedno direct CSS✓✓✓color matrixNoMediumMediumWhite balance adjustment
TintTintcolor tint shiftplannedno direct CSS✓✓✓color matrixNoMediumMediumColor cast adjustment
HueHue rotationhue rotation matrix-filter: hue-rotate()✗✗✓color matrixNoHighLowHue rotation
GrayscaleGrayscaledesaturation matrix-filter: grayscale()✗✓✓color matrixNoMediumLowConverts to grayscale
SepiaSepiasepia color matrix-filter: sepia()✗✗✓color matrixNoMediumLowSepia tone effect
InvertInvertinvert colors-filter: invert()✗✗✓color matrixNoMediumLowInverts colors
HighlightsHighlightsnon-linear curve/LUT-no direct CSS✗✓✓LUT / runtime shaderYesMediumHighAdjusts bright areas
ShadowsShadowsnon-linear curve/LUT-no direct CSS✗✓✓LUT / runtime shaderYesMediumHighAdjusts dark areas
Brightness (Apple)Brightnesslinear add-filter: brightness()✗✓✓color matrixNoMediumLowApple-specific brightness
Black PointBlack Pointnon-linear curve/LUT-no direct CSS✗✓✓LUT / runtime shaderYesLowHighAdjusts black level
VibranceVibrancenon-linear saturation curve-no direct CSS✗✓✓LUT / runtime shaderYesMediumHighSelective saturation boost
BrillianceBrilliancenon-linear curve/LUT-no direct CSS✗✓✓LUT / runtime shaderYesLowHighMidtone brightness enhancement

Purpose​

Our objective is to define a consistent, performant, and portable color adjustments model that feels familiar to designers (Figma/Apple/Adobe semantics) while being technically simple enough to run in real‑time via Skia. This document establishes shared terminology, a master comparison across platforms, and a minimal, high‑impact set of adjustments we will standardize first.

Scope (for this spec): color/tone adjustments only (a.k.a. “color filters” / adjustments). Spatial effects (blur, sharpen, shadows) are out of scope except where noted in comparisons.

Challenges​

  • Cross‑app inconsistency: The same slider name (e.g., “Contrast”) can use different math (pivot, gamma, space), leading to visually different outcomes across CSS, Figma, Apple Photos, Photoshop, and Resolve.
  • Color space & gamma: Linear‑light vs sRGB operations change results. Web/CSS typically operates in sRGB for compatibility; pro tools often prefer linear/log spaces for accuracy.
  • Order sensitivity: Linear/affine ops commute, but non‑linear (Shadows/Highlights, Vibrance) and spatial (Blur/Sharpen) do not. A fixed pipeline avoids user confusion.
  • Performance & fusion: Multiple passes cost GPU time. We should fuse all linear ops into a single color matrix and apply non‑linear curves only once.
  • Naming vs math: UX names (Exposure, Temperature, Tint) must map cleanly to stable math so results are predictable and testable.

Focus & Priorities​

We will prioritize straightforward, non‑conflicting, commonly available controls that designers expect and that map 1:1 to a single Skia color matrix (order‑insensitive):

  1. Exposure — linear brightness (scalar gain)
  2. Contrast — linear scale around mid‑gray pivot
  3. Saturation — chroma scale preserving luma
  4. Temperature — R/B channel ratio (warm ↔ cool)
  5. Tint — G ↔ magenta balance

These five cover the majority of day‑to‑day needs in a Figma‑like workflow and are easy to combine/fuse. Non‑linear adjustments (Highlights/Shadows, Vibrance, Black Point, Brilliance) are phase‑two items due to order sensitivity and implementation complexity (curves/LUTs).

Pipeline Order (static)​

For determinism and performance:

Linear group (fused matrix) → Non‑linear curves (optional) → Spatial effects (optional)
i.e., Exposure → Contrast → Temperature → Tint → Saturation → (Highlights/Shadows) → (Blur)

Top 5 Filters​

Below are the top five we will implement first. Each is widely exposed as a top‑level adjustment in other apps (not buried as a sub‑control), useful, and mathematically simple.

1) Exposure​

  • Why: Universal “make brighter/darker” control; first stop for quick fixes.
  • Math: RGB' = RGB * k (often parameterized as k = 2^E, where E is EV‑like). Clamp to [0,1].
  • Skia: Single color matrix multiply with uniform scale; can be fused with others.
  • UX: Default k = 1. Suggested range: 0.5…1.5 (or EV -1…+1). Slider shows percentage or EV.
  • Cross‑app: CSS brightness(), Figma Exposure, Apple Exposure, Photoshop/Resolve global exposure.

2) Contrast​

  • Why: Controls overall “pop”; improves readability/perceived sharpness.
  • Math: c' = (c − p) * k + p with pivot p = 0.5 (sRGB). Clamp.
  • Skia: Implement via color matrix with bias term; fuses with Exposure.
  • UX: Default k = 1. Range 0…2 (0 = flat gray, 1 = neutral, 2 = strong). Consider gentle curve for >1 to avoid clipping.
  • Cross‑app: CSS contrast(), Figma Contrast, Apple Contrast, Photoshop/Resolve Contrast.

3) Saturation​

  • Why: Brand alignment and visual punch; desaturate for disabled states or muted looks.
  • Math: color' = lerp(luma, color, k), luma = dot(RGB, [0.2126, 0.7152, 0.0722]).
  • Skia: Color matrix based on luma coefficients; fuses with other linear ops.
  • UX: Default k = 1. Range 0…2 (0 = grayscale, 2 = vivid). Provide preview safe‑guards to avoid clipping.
  • Cross‑app: CSS saturate(), Figma Saturation, Apple Saturation, Photoshop/Resolve Saturation.

4) Temperature​

  • Why: Quick white‑balance fix (cool daylight ↔ warm tungsten) for consistency across assets.
  • Math: Scale R vs B channels: R' = R * rK, B' = B * bK, typically rK = 1 + t, bK = 1 − t (small t). Keep luma roughly stable.
  • Skia: Color matrix scaling on R/B with optional normalization to preserve luma.
  • UX: Default t = 0. Range −1…+1 (or UI label “Cool ↔ Warm”). Show neutral midpoint.
  • Cross‑app: Figma Temperature, Apple Warmth, Photoshop/Resolve Temperature. (No direct CSS filter.)

5) Tint​

  • Why: Complements Temperature to fix green/magenta casts (fluorescent, mixed lighting).
  • Math: Adjust G relative to R+B: G' = G * gK (or add small bias) with luma preservation.
  • Skia: Color matrix scaling for G with optional normalization.
  • UX: Default gK = 1. Range ~0.9…1.1 (or slider −100…+100). Label “Green ↔ Magenta”.
  • Cross‑app: Figma Tint, Apple Tint, Photoshop/Resolve Tint. (No direct CSS filter.)

References​