Enum PresentMode 

pub enum PresentMode {
    Immediate,
    Vsync,
}

When a queued flip actually reaches the panel.

A real trade, not a quality setting. Which way it should go depends on what is on the screen, and the default here is chosen for the kind of thing Denise is built for rather than for the kind of thing a compositor is built for.

Variants

Immediate

Flip immediately, part-way through a scan-out if necessary. Tears.

The default, and measured on a Pi 3 A+ driving 1920x1080 the reason is plain: waiting for vblank costs about 17 ms of latency and was described by the person operating it as lagging several milliseconds behind the whole time. Flipping immediately removes that wait entirely.

The cost is a horizontal seam where the panel switched buffers mid-frame. With damage tracking most updates are a few thousand pixels, so the seam is small, brief and in practice invisible — a control panel redrawing a button is nothing like a compositor scrolling a window.

This used to say “reconsider for signage or anything with large fast-moving content, where a tear crosses something worth looking at”. A scrolling viewport turned out to be exactly that, and the flicker was reported from a Pi within a day of the gallery gaining one. So the mode is no longer a promise about every frame: a frame whose damage covers a quarter of the screen’s rows or more flips at vblank anyway, and this asks for async flips on the frames where the seam is short and the latency is felt. See flip_flags_for, including why it counts rows rather than pixels, and why it counts the rows covered rather than the rows spanned.

Requires DRM_CAP_ASYNC_PAGE_FLIP; drivers without it fall back to PresentMode::Vsync, and DrmSurface::present_mode reports what was actually obtained.

This mode paces the caller only on its large frames. See DrmSurface.

Vsync

Flip at the next vblank. Never tears.

A flip queued just after a vblank cannot land before the next one, so this costs on the order of one refresh period — about 17 ms at 60 Hz. Every tear-free system pays it.

In exchange, Surface::acquire blocks until the flip retires, so the display paces the render loop for free and an application needs no frame timing of its own.

Trait Implementations

impl Clone for PresentMode

fn clone(&self) -> PresentMode

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Copy for PresentMode

impl Debug for PresentMode

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

Formats the value using the given formatter.

impl Default for PresentMode

fn default() -> PresentMode

Returns the “default value” for a type.

impl Eq for PresentMode

impl PartialEq for PresentMode

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

Equality operator ==.

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

Inequality operator !=.

impl StructuralPartialEq for PresentMode