qqlrs patch: debug pass to show progress of layout through flow line groups

layout-order.patch

diff --git a/src/art.rs b/src/art.rs
index 78e28bf..970b0e0 100644
--- a/src/art.rs
+++ b/src/art.rs
@@ -813,6 +813,7 @@ impl MarginChecker {
 
 #[derive(Debug, Clone)]
 pub struct Point {
+    pub group_idx: u32,
     pub position: (f64, f64),
     pub scale: f64,
     pub primary_color: Hsb,
@@ -821,6 +822,7 @@ pub struct Point {
 }
 #[derive(Debug, Copy, Clone, PartialEq)]
 pub struct Hsb(pub f64, pub f64, pub f64);
+#[derive(Clone)]
 pub struct Points(pub Vec<Point>);
 
 impl Hsb {
@@ -883,7 +885,7 @@ impl Points {
         let margin_checker = MarginChecker::from_traits(traits);
 
         let mut all_points = Vec::new();
-        for group in grouped_flow_lines.0 {
+        for (i, group) in grouped_flow_lines.0.into_iter().enumerate() {
             if rng.odds(color_change_odds.group) {
                 primary_color_idx =
                     pick_next_color(&color_scheme.primary_seq, primary_color_idx, rng);
@@ -893,6 +895,7 @@ impl Points {
             }
             Self::build_group(
                 &mut all_points,
+                i as u32,
                 color_db,
                 group,
                 color_scheme,
@@ -915,6 +918,7 @@ impl Points {
     #[allow(clippy::too_many_arguments)]
     fn build_group(
         dest: &mut Vec<Point>,
+        group_idx: u32,
         color_db: &ColorDb,
         group: FlowLineGroup,
         color_scheme: &ColorScheme,
@@ -975,6 +979,7 @@ impl Points {
                 primary_color = perturb_color(primary_color, primary_color_spec, rng);
                 secondary_color = perturb_color(secondary_color, secondary_color_spec, rng);
                 dest.push(Point {
+                    group_idx,
                     position: (x, y),
                     scale,
                     primary_color,
@@ -1103,12 +1108,36 @@ impl PaintCtx {
     }
 }
 
+#[derive(Debug, Copy, Clone)]
+enum PaintSpec {
+    All,
+    OnlyBg,
+    OnlyGroup(u32),
+}
+impl PaintSpec {
+    fn paint_bg(&self) -> bool {
+        match self {
+            PaintSpec::All => true,
+            PaintSpec::OnlyBg => true,
+            PaintSpec::OnlyGroup(_) => false,
+        }
+    }
+    fn prune_points(&self, points: &mut Points) {
+        match self {
+            PaintSpec::All => (),
+            PaintSpec::OnlyBg => points.0.clear(),
+            PaintSpec::OnlyGroup(desired) => points.0.retain(|p| p.group_idx == *desired),
+        }
+    }
+}
+
 fn paint(
     canvas_width: i32,
     traits: &Traits,
     color_db: &ColorDb,
     config: &Config,
     mut points: Points,
+    paint_spec: PaintSpec,
     color_scheme: &ColorScheme,
     colors_used: &mut HashSet<ColorKey>,
     rng: &mut Rng,
@@ -1126,6 +1155,7 @@ fn paint(
             .expect("invalid background");
         &Hsb(spec.hue, spec.sat, spec.bright).to_rgb().to_source()
     };
+    paint_spec.prune_points(&mut points);
 
     let hsteps: u32 = config.chunks.w.into();
     let vsteps: u32 = config.chunks.h.into();
@@ -1176,14 +1206,16 @@ fn paint(
                         f64::from(top_px) * height_ratio + full_fvp.top(),
                     );
                     let mut pctx = PaintCtx::new(config, &fvp, canvas_width);
-                    pctx.dt.fill_rect(
-                        0.0,
-                        0.0,
-                        pctx.dt.width() as f32,
-                        pctx.dt.height() as f32,
-                        background_color,
-                        &DrawOptions::new(),
-                    );
+                    if paint_spec.paint_bg() {
+                        pctx.dt.fill_rect(
+                            0.0,
+                            0.0,
+                            pctx.dt.width() as f32,
+                            pctx.dt.height() as f32,
+                            background_color,
+                            &DrawOptions::new(),
+                        );
+                    }
                     let mut colors_used = HashSet::new();
                     paint_onto_ctx(
                         &mut pctx,
@@ -1536,19 +1568,48 @@ pub fn draw(seed: &[u8; 32], color_db: &ColorDb, config: &Config, canvas_width:
     );
     eprintln!("laid out points");
     let num_points = points.0.len() as u64;
-    let dt = paint(
+    let mut acc = paint(
         canvas_width,
         &traits,
         color_db,
         &config,
-        points,
+        points.clone(),
+        PaintSpec::OnlyBg,
         &color_scheme,
         &mut colors_used,
-        &mut rng,
+        &mut rng.clone(),
     );
+    acc.write_png(format!("out-{:04}.png", 0))
+        .expect("writing background");
+    for group_idx in 0..points.0.last().map_or(0, |p| p.group_idx + 1) {
+        let points = points.clone();
+        let dt = paint(
+            canvas_width,
+            &traits,
+            color_db,
+            &config,
+            points,
+            PaintSpec::OnlyGroup(group_idx),
+            &color_scheme,
+            &mut colors_used,
+            &mut rng,
+        );
+        acc.draw_image_at(
+            0.0,
+            0.0,
+            &raqote::Image {
+                width: dt.width(),
+                height: dt.height(),
+                data: dt.get_data(),
+            },
+            &DrawOptions::new(),
+        );
+        acc.write_png(format!("out-{:04}.png", group_idx + 1))
+            .expect(&format!("writing group {}", group_idx));
+    }
     eprintln!("drew points");
     Render {
-        dt,
+        dt: acc,
         num_points,
         colors_used,
     }

zzz-create-webm.sh

ffmpeg -framerate 3 -f image2 -i out-%04d.png out.webm