Enum BlendMode 

pub enum BlendMode {
    Clear,
    Src,
    Dst,
    SrcOver,
    DstOver,
    SrcIn,
    DstIn,
    SrcOut,
    DstOut,
    SrcATop,
    DstATop,
    Xor,
    Plus,
    Modulate,
    Multiply,
    Screen,
    Overlay,
    Darken,
    Lighten,
    ColorDodge,
    ColorBurn,
    HardLight,
    SoftLight,
    Difference,
    Exclusion,
    Hue,
    Saturation,
    Color,
    Luminosity,
}

How a draw combines with what a target already holds.

Three families: the Porter-Duff set, which decides which of the source and destination survive and where; the separable modes, which mix the two arithmetically channel by channel; and the non-separable modes, which exchange whole attributes of a color. Only the first works everywhere.

Variants

Clear

Leave nothing behind. Neither source nor destination survives.

Src

Replace the destination outright, alpha included.

Not the same as Self::SrcOver with an opaque source: this overwrites destination alpha too, which decides whether a layer composites correctly when it is itself drawn onto something else.

Dst

Keep the destination and discard the source.

SrcOver

Source over destination: the default for drawing one thing on another.

DstOver

Destination over source, as though the source were drawn underneath.

SrcIn

Source, clipped to where the destination is.

DstIn

Destination, clipped to where the source is.

SrcOut

Source, clipped to where the destination is not.

DstOut

Destination, clipped to where the source is not.

SrcATop

Source drawn on top, but only within the destination’s shape.

DstATop

Destination drawn on top, but only within the source’s shape.

Xor

Whichever of the two is not overlapped by the other.

Plus

The two added together, saturating.

Used for light accumulation, where overlapping contributions should brighten rather than replace.

Modulate

The two multiplied together.

Multiply

Multiply the two, which always darkens.

Screen

The inverse of multiplying the inverses, which always lightens.

Overlay

Multiply or screen depending on the destination, so the destination decides the contrast.

Darken

The darker of the two, per channel.

Lighten

The lighter of the two, per channel.

ColorDodge

Brighten the destination in proportion to the source.

ColorBurn

Darken the destination in proportion to the inverse of the source.

HardLight

Overlay with the roles reversed, so the source decides the contrast.

SoftLight

A gentler hard-light, without the hard transition at the midpoint.

Difference

The absolute difference, which inverts where the two agree.

Exclusion

Like difference, but with a softer response near the midpoint.

Hue

The source’s hue, with the backdrop’s saturation and luminosity.

Saturation

The source’s saturation, with the backdrop’s hue and luminosity.

Color

The source’s hue and saturation, with the backdrop’s luminosity.

Luminosity

The source’s luminosity, with the backdrop’s hue and saturation.

Implementations

impl BlendMode

pub const PORTER_DUFF: &'static [Self]

The modes every device can do, with no extension.

pub const ADVANCED: &'static [Self]

The modes that need advanced blending.

pub const ALL: &'static [Self]

Every mode, for exhaustive tests and reporting.

Deliberately not what a backend iterates to decide what it supports — that is what Self::is_advanced and the capability flag are for.

pub fn is_destructive(self) -> bool

Whether this mode changes the destination where the source is absent.

The modes for which “the source drew nothing here” is not the same as “leave this pixel alone”: DstIn zeroes what the source did not cover, Clear zeroes everything, Src replaces it. A layer composited with one of these has to cover everything it might affect rather than only what it drew, so it is the test for whether a layer’s target may be narrowed to its content.

The same nine upstream lists in Entity::IsBlendModeDestructive, and the reason to match it exactly is that this decides a target’s size: a mode missing here is a layer that silently stops masking what it used to.

pub const fn code(self) -> f32

Whether this mode needs advanced blending.

Callers check crate::Capabilities::advanced_blend before using one, and a backend without it refuses rather than substituting something that looks close: a silently wrong blend mode is a picture nobody can debug from. The number a shader reads to identify this mode.

Spelled out rather than cast from the discriminant, because the discriminant is an implementation detail a reordering would change and this number is written into a uniform buffer that a shader decodes. The two must agree, and only one of them is visible from the shader.

pub const fn is_advanced(self) -> bool

pub const fn factors(self) -> Option<BlendFactors>

The factors this mode blends with, assuming premultiplied color.

None for an advanced mode: those are not expressible as factors at all, which is exactly why they need an extension. Returning an option rather than a plausible pair keeps a backend from silently rendering the wrong thing.

pub const fn reads_destination(self) -> bool

Whether the destination contributes to the result.

A mode that ignores it can skip loading the target on a tiler, which is a bandwidth saving rather than a correctness one.

pub const fn respects_coverage(self) -> bool

Whether a source scaled by coverage composites as though the shape were partly there.

This is what decides whether a shape may be antialiased by computing coverage in the fragment stage rather than by multisampling. Such a shape is drawn on a quad larger than itself and emits a fragment everywhere on it, including where coverage is nothing — so the mode has to leave the destination alone for a fully transparent source, or the quad erases what is behind it in the gap between the two.

Which is exactly the destination factor being One or OneMinusSrcAlpha: the source term vanishes with the alpha whatever its own factor is, so only the destination’s own weight is left. Every advanced mode qualifies as well — their equations carry a (1 - source alpha) on the destination by construction, and reduce to it when the source contributes nothing.

The same condition happens to be what makes the partly covered edge correct rather than merely harmless. Weighting a premultiplied source by coverage and compositing gives the same answer as mixing the unweighted result into the destination by coverage, which is what multisampling would have produced.

pub const fn is_plain_write(self) -> bool

Whether this simply writes the source, so blending can be switched off.

pub const fn name(self) -> &'static str

Trait Implementations

impl Clone for BlendMode

fn clone(&self) -> Self

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Copy for BlendMode

impl Debug for BlendMode

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

Formats the value using the given formatter.

impl Default for BlendMode

fn default() -> Self

Returns the “default value” for a type.

impl Display for BlendMode

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

Formats the value using the given formatter.

impl Eq for BlendMode

impl Hash for BlendMode

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 BlendMode

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

Equality operator ==.

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

Inequality operator !=.

impl StructuralPartialEq for BlendMode