Enum ColorPriority 

pub enum ColorPriority {
    PreferIcc,
    PreferCicp,
}

Controls which transfer function metadata the CMS trusts when building a transform.

ICC profiles store transfer response curves (TRC) as curv or para tags — lookup tables or parametric curves baked into the profile. Modern container formats (JPEG XL, HEIF/AVIF, AV1) also carry CICP transfer characteristics — an integer code that names an exact mathematical transfer function (sRGB, PQ, HLG, etc.).

When both are present, they should agree — but in practice, the ICC TRC may be a reduced-precision approximation of the CICP function (limited by curv table size or para parameter quantization). The question is which source of truth to prefer.

Which priority to use

• Standard ICC workflows (JPEG, PNG, TIFF, WebP): use PreferIcc. These formats don’t carry CICP metadata; the ICC profile is the sole authority.

• CICP-native formats (JPEG XL, HEIF, AVIF): use PreferCicp. The CICP code is the authoritative description; the ICC profile exists for backwards compatibility with older software.

Bugs and pitfalls

• CICP ≠ ICC is a real bug. Some encoders embed a generic sRGB ICC profile alongside a PQ or HLG CICP code. Using PreferCicp is correct here — the ICC profile is wrong (or at best, a tone-mapped fallback). Using PreferIcc would silently apply the wrong transfer function.

• PreferIcc for CICP-native formats loses precision. If the ICC profile’s curv table is a 1024-entry LUT approximating the sRGB function, you get quantization steps in dark tones. The CICP code gives the exact closed-form function — no quantization, no table interpolation error.

• PreferCicp for pure-ICC formats is harmless but pointless. If the profile has no embedded CICP metadata, the CMS ignores this flag and falls back to the TRC. No wrong output, just a wasted branch.

• Advisory vs. authoritative. The ICC Votable Proposal on CICP metadata in ICC profiles designates the CICP fields as advisory. The profile’s actual TRC tags remain the normative description. PreferIcc follows this interpretation. PreferCicp overrides it for formats where the container’s CICP is known to be authoritative.

Variants

PreferIcc

Prefer the ICC profile’s own curv/para TRC curves. Ignore any embedded CICP transfer characteristics.

Correct for standard ICC workflows (JPEG, PNG, TIFF, WebP) and any situation where the ICC profile is the sole color authority.

PreferCicp

Allow the CMS to use CICP transfer characteristics when available.

Faster (closed-form math vs. LUT interpolation) and more precise (no table quantization error). Correct only for formats where CICP is the authoritative color description: JPEG XL, HEIF, AVIF.

Trait Implementations

impl Clone for ColorPriority

fn clone(&self) -> ColorPriority

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for ColorPriority

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for ColorPriority

fn default() -> ColorPriority

Returns the “default value” for a type.

impl Hash for ColorPriority

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for ColorPriority

fn eq(&self, other: &ColorPriority) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for ColorPriority

impl Eq for ColorPriority

impl StructuralPartialEq for ColorPriority