Skip to content

Place moving pictures to a fraction of a pixel, so zooms and slides don't jitter #6

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
calvin-archastro merged 1 commit into from
Oct 22, 2026
Merged

Place moving pictures to a fraction of a pixel, so zooms and slides don't jitter #6

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
89 changes: 70 additions & 19 deletions crates/splice-core/src/export.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -283,12 +283,6 @@ fn build(
Source::Generated(_) => (natural, (0.0, 0.0)),
};
let pose = look.pose_expr(clip.duration);
// Local time inside the clip, in each filter's own clock.
let local = if offset > 0 {
format!("(t+{})", fmt(offset))
} else {
"t".into()
};
let mut chain = vec![format!("fps={fps}")];
if matches!(source, Source::Media(_))
&& let Some(pixels) = look.pixel_filters()
Expand All @@ -304,13 +298,21 @@ fn build(
if opacity < 1.0 {
chain.push(format!("colorchannelmixer=aa={:.4}", opacity.max(0.0)));
}
let animated =
pose.x.constant().is_none() || pose.y.constant().is_none() || pose.scale.constant().is_none();
// A moving picture keeps one size and is placed anew each
// frame to a fraction of a pixel. Resizing it every frame
// rounded its size to whole pixels and moved its centre with
// the rounding, so a slow zoom or slide jittered.
let (pw, ph) = (even(nw.round()), even(nh.round()));
let (fw, fh) = (pw.max(w), ph.max(h));
match pose.scale.constant() {
Some(k) => chain.push(format!("scale={}:{}", even((nw * k).round()), even((nh * k).round()))),
None => {
let k = pose.scale.ffmpeg(&local);
chain.push(format!(
"scale=w='max(2,trunc({nw:.3}*{k}/2)*2)':h='max(2,trunc({nh:.3}*{k}/2)*2)':eval=frame"
));
Some(k) if !animated => {
chain.push(format!("scale={}:{}", even((nw * k).round()), even((nh * k).round())))
}
_ => {
chain.push(format!("scale={pw}:{ph}"));
chain.push(format!("pad={fw}:{fh}:(ow-iw)/2:(oh-ih)/2:color=black@0"));
}
}
for (motion, inward) in [(look.enter, true), (look.exit, false)] {
Expand All @@ -324,6 +326,32 @@ fn build(
if inward { "in" } else { "out" }
));
}
if animated {
// perspective has no clock of its own; its frame count is one.
let t = if offset > 0 {
format!("(in/{fps}+{})", fmt(offset))
} else {
format!("(in/{fps})")
};
let k = format!("max(0.0001,{})", pose.scale.ffmpeg(&t));
let along = |by: f64| {
if by.abs() < 1e-6 {
String::new()
} else {
format!("+{by:.3}*{k}")
}
};
// The picture's centre, in the frame it moves in.
let cx_at = format!("({fw}/2+{}*{w}{})", pose.x.ffmpeg(&t), along(cx));
let cy_at = format!("({fh}/2+{}*{h}{})", pose.y.ffmpeg(&t), along(cy));
let (hx, hy) = (format!("{}*{k}", fw as f64 / 2.0), format!("{}*{k}", fh as f64 / 2.0));
chain.push("format=yuva444p".into());
chain.push(format!(
"perspective=sense=destination:interpolation=cubic:eval=frame:\
x0='{cx_at}-{hx}':y0='{cy_at}-{hy}':x1='{cx_at}+{hx}':y1='{cy_at}-{hy}':\
x2='{cx_at}-{hx}':y2='{cy_at}+{hy}':x3='{cx_at}+{hx}':y3='{cy_at}+{hy}'"
));
}
// Over a see-through canvas the overlay negotiates its format
// from its inputs: a picture that changes size frame to frame
// only stays right as RGBA (as yuva420p it froze at its first size).
Expand All @@ -344,8 +372,6 @@ fn build(
} else {
since
};
let animated =
pose.x.constant().is_none() || pose.y.constant().is_none() || pose.scale.constant().is_none();
// A cropped picture's remains stay where they were, scaled with it.
let shift = |by: f64| {
if by.abs() < 1e-6 {
Expand All @@ -354,12 +380,18 @@ fn build(
format!("+{by:.3}*{}", pose.scale.ffmpeg(&since))
}
};
let x = format!("(W-w)/2+{}*W{}", pose.x.ffmpeg(&since), shift(cx));
let y = format!("(H-h)/2+{}*H{}", pose.y.ffmpeg(&since), shift(cy));
// A moving picture has already been placed in its frame.
let (x, y) = if animated {
(format!("-{}", (fw - w) / 2), format!("-{}", (fh - h) / 2))
} else {
(
format!("(W-w)/2+{}*W{}", pose.x.ffmpeg(&since), shift(cx)),
format!("(H-h)/2+{}*H{}", pose.y.ffmpeg(&since), shift(cy)),
)
};
filters.push(format!(
"[{picture}][v{tag}]overlay=x='{x}':y='{y}':{}{}eof_action=pass:\
"[{picture}][v{tag}]overlay=x='{x}':y='{y}':{}eof_action=pass:\
enable='between(t,{start},{end})'[p{tag}]",
if animated { "eval=frame:" } else { "" },
// Over a see-through canvas, keep what is see-through.
if layer { "format=auto:" } else { "" }
));
Expand Down Expand Up @@ -483,7 +515,7 @@ fn strings(items: &[&str]) -> Vec<String> {
#[cfg(test)]
mod tests {
use super::*;
use crate::look::{Key, Mask};
use crate::look::{Drift, Key, Mask};
use crate::model::{Generator, MediaId, Title};
use crate::time::SECOND;

Expand Down Expand Up @@ -584,6 +616,25 @@ mod tests {
assert_eq!(graph.matches("amix=inputs=2").count(), 1, "titles make no sound");
}

#[test]
fn a_moving_picture_is_placed_to_a_fraction_of_a_pixel() {
let mut p = Project::default();
let v = video(&mut p, "a", 4);
let clip = p.append_clip(v).unwrap();
p.set_look(clip, |look| {
look.drift = Drift::ZoomIn;
look.x = 0.1;
});
let plan = plan(&p, &Fake, Path::new("/o.mp4"), Quality::Draft).unwrap();
let graph = graph(&plan);
// Resizing every frame rounds to whole pixels, and a slow zoom jitters.
assert!(!graph.contains("trunc("), "no per-frame resize: {graph}");
assert!(graph.contains("scale=1920:1080,pad=1920:1080:"), "{graph}");
assert!(graph.contains("perspective=sense=destination:interpolation=cubic:eval=frame"));
assert!(graph.contains("(in/30)"), "the zoom follows the frame count");
assert!(graph.contains("overlay=x='-0':y='-0'"), "{graph}");
}

#[test]
fn still_shows_what_is_under_the_time() {
let mut p = Project::default();
Expand Down
Loading