Optimize point clouds (#4932)

### What
Some optimizations I found while playing around with large point clouds.

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/4932/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/4932/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/4932/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG

- [PR Build Summary](https://build.rerun.io/pr/4932)
- [Docs
preview](https://rerun.io/preview/772cd4195531d6731463ca5b74e3f61eddb38600/docs)
<!--DOCS-PREVIEW-->
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preview](https://rerun.io/preview/772cd4195531d6731463ca5b74e3f61eddb38600/examples)
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Author: emilk

crates/re_renderer/src/allocator/cpu_write_gpu_read_belt.rs

@@ -119,21 +119,30 @@ where
         elements: impl Iterator<Item = T>,
     ) -> Result<usize, CpuWriteGpuReadError> {
         re_tracing::profile_function!();
-        let num_written_before = self.num_written();
 
-        for element in elements {
-            if self.unwritten_element_range.start >= self.unwritten_element_range.end {
-                return Err(CpuWriteGpuReadError::BufferFull {
-                    buffer_element_capacity: self.capacity(),
-                });
+        // TODO(emilk): optimize the extend function.
+        // Right now it is 3-4x faster to collect to a vec first, which is crazy.
+        if true {
+            let vec = elements.collect::<Vec<_>>();
+            self.extend_from_slice(&vec)?;
+            Ok(vec.len())
+        } else {
+            let num_written_before = self.num_written();
+
+            for element in elements {
+                if self.unwritten_element_range.start >= self.unwritten_element_range.end {
+                    return Err(CpuWriteGpuReadError::BufferFull {
+                        buffer_element_capacity: self.capacity(),
+                    });
+                }
+
+                self.as_mut_byte_slice()[..std::mem::size_of::<T>()]
+                    .copy_from_slice(bytemuck::bytes_of(&element));
+                self.unwritten_element_range.start += 1;
             }
 
-            self.as_mut_byte_slice()[..std::mem::size_of::<T>()]
-                .copy_from_slice(bytemuck::bytes_of(&element));
-            self.unwritten_element_range.start += 1;
+            Ok(self.num_written() - num_written_before)
         }
-
-        Ok(self.num_written() - num_written_before)
     }
 
     /// Fills the buffer with n instances of an element.

crates/re_space_view_spatial/src/visualizers/mod.rs

@@ -127,21 +127,50 @@ pub fn process_colors<'a, A: Archetype>(
 /// Process [`Color`] components using annotations and default colors.
 pub fn process_color_slice<'a>(
     colors: Option<&'a [Option<Color>]>,
-    default_len: usize,
     ent_path: &'a EntityPath,
     annotation_infos: &'a ResolvedAnnotationInfos,
-) -> impl Iterator<Item = egui::Color32> + 'a {
+) -> Vec<egui::Color32> {
+    // This can be rather slow for colors with transparency, since we need to pre-multiply the alpha.
     re_tracing::profile_function!();
+    use rayon::prelude::*;
+
     let default_color = DefaultColor::EntityPath(ent_path);
 
-    let colors = colors.as_ref().map_or(
-        itertools::Either::Left(std::iter::repeat(&None).take(default_len)),
-        |data| itertools::Either::Right(data.iter()),
-    );
+    match (colors, annotation_infos) {
+        (None, ResolvedAnnotationInfos::Same(count, annotation_info)) => {
+            re_tracing::profile_scope!("no colors, same annotation");
+            let color = annotation_info.color(None, default_color);
+            vec![color; *count]
+        }
 
-    itertools::izip!(annotation_infos.iter(), colors).map(move |(annotation_info, color)| {
-        annotation_info.color(color.map(|c| c.to_array()), default_color)
-    })
+        (None, ResolvedAnnotationInfos::Many(annotation_infos)) => {
+            re_tracing::profile_scope!("no-colors, many annotations");
+            annotation_infos
+                .par_iter()
+                .map(|annotation_info| annotation_info.color(None, default_color))
+                .collect()
+        }
+
+        (Some(colors), ResolvedAnnotationInfos::Same(count, annotation_info)) => {
+            re_tracing::profile_scope!("many-colors, same annotation");
+            debug_assert_eq!(colors.len(), *count);
+            colors
+                .par_iter()
+                .map(|color| annotation_info.color(color.map(|c| c.to_array()), default_color))
+                .collect()
+        }
+
+        (Some(colors), ResolvedAnnotationInfos::Many(annotation_infos)) => {
+            re_tracing::profile_scope!("many-colors, many annotations");
+            colors
+                .par_iter()
+                .zip(annotation_infos.par_iter())
+                .map(move |(color, annotation_info)| {
+                    annotation_info.color(color.map(|c| c.to_array()), default_color)
+                })
+                .collect()
+        }
+    }
 }
 
 /// Process [`Text`] components using annotations.
@@ -174,20 +203,25 @@ pub fn process_radii<'a, A: Archetype>(
 
 /// Process [`re_types::components::Radius`] components to [`re_renderer::Size`] using auto size
 /// where no radius is specified.
-pub fn process_radius_slice<'a>(
-    radii: Option<&'a [Option<re_types::components::Radius>]>,
+pub fn process_radius_slice(
+    radii: Option<&[Option<re_types::components::Radius>]>,
     default_len: usize,
     ent_path: &EntityPath,
-) -> impl Iterator<Item = re_renderer::Size> + 'a {
+) -> Vec<re_renderer::Size> {
     re_tracing::profile_function!();
-    let ent_path = ent_path.clone();
+    use rayon::prelude::*;
 
-    let radii = radii.as_ref().map_or(
-        itertools::Either::Left(std::iter::repeat(&None).take(default_len)),
-        |data| itertools::Either::Right(data.iter()),
-    );
+    let ent_path = ent_path.clone();
 
-    radii.map(move |radius| process_radius(&ent_path, radius))
+    match radii {
+        None => {
+            vec![re_renderer::Size::AUTO; default_len]
+        }
+        Some(radii) => radii
+            .par_iter()
+            .map(|radius| process_radius(&ent_path, radius))
+            .collect(),
+    }
 }
 
 fn process_radius(

crates/re_space_view_spatial/src/visualizers/points2d.rs

@@ -143,13 +143,7 @@ impl Points2DVisualizer {
             re_tracing::profile_scope!("labels");
 
             // Max labels is small enough that we can afford iterating on the colors again.
-            let colors = process_color_slice(
-                data.colors,
-                data.positions.len(),
-                ent_path,
-                &annotation_infos,
-            )
-            .collect::<Vec<_>>();
+            let colors = process_color_slice(data.colors, ent_path, &annotation_infos);
 
             let instance_path_hashes_for_picking = {
                 re_tracing::profile_scope!("instance_hashes");
@@ -190,33 +184,16 @@ impl Points2DVisualizer {
         }: &Points2DComponentData<'_>,
         ent_path: &EntityPath,
     ) -> Vec<re_renderer::Size> {
-        re_tracing::profile_function!();
-        let radii = crate::visualizers::process_radius_slice(radii, positions.len(), ent_path);
-        {
-            re_tracing::profile_scope!("collect");
-            radii.collect()
-        }
+        crate::visualizers::process_radius_slice(radii, positions.len(), ent_path)
     }
 
     #[inline]
     pub fn load_colors(
-        &Points2DComponentData {
-            positions, colors, ..
-        }: &Points2DComponentData<'_>,
+        &Points2DComponentData { colors, .. }: &Points2DComponentData<'_>,
         ent_path: &EntityPath,
         annotation_infos: &ResolvedAnnotationInfos,
     ) -> Vec<re_renderer::Color32> {
-        re_tracing::profile_function!();
-        let colors = crate::visualizers::process_color_slice(
-            colors,
-            positions.len(),
-            ent_path,
-            annotation_infos,
-        );
-        {
-            re_tracing::profile_scope!("collect");
-            colors.collect()
-        }
+        crate::visualizers::process_color_slice(colors, ent_path, annotation_infos)
     }
 
     #[inline]

crates/re_space_view_spatial/src/visualizers/points3d.rs

@@ -134,13 +134,7 @@ impl Points3DVisualizer {
             re_tracing::profile_scope!("labels");
 
             // Max labels is small enough that we can afford iterating on the colors again.
-            let colors = process_color_slice(
-                data.colors,
-                data.positions.len(),
-                ent_path,
-                &annotation_infos,
-            )
-            .collect::<Vec<_>>();
+            let colors = process_color_slice(data.colors, ent_path, &annotation_infos);
 
             let instance_path_hashes_for_picking = {
                 re_tracing::profile_scope!("instance_hashes");
@@ -325,33 +319,16 @@ impl LoadedPoints {
         }: &Points3DComponentData<'_>,
         ent_path: &EntityPath,
     ) -> Vec<re_renderer::Size> {
-        re_tracing::profile_function!();
-        let radii = crate::visualizers::process_radius_slice(radii, positions.len(), ent_path);
-        {
-            re_tracing::profile_scope!("collect");
-            radii.collect()
-        }
+        crate::visualizers::process_radius_slice(radii, positions.len(), ent_path)
     }
 
     #[inline]
     pub fn load_colors(
-        &Points3DComponentData {
-            positions, colors, ..
-        }: &Points3DComponentData<'_>,
+        &Points3DComponentData { colors, .. }: &Points3DComponentData<'_>,
         ent_path: &EntityPath,
         annotation_infos: &ResolvedAnnotationInfos,
     ) -> Vec<re_renderer::Color32> {
-        re_tracing::profile_function!();
-        let colors = crate::visualizers::process_color_slice(
-            colors,
-            positions.len(),
-            ent_path,
-            annotation_infos,
-        );
-        {
-            re_tracing::profile_scope!("collect");
-            colors.collect()
-        }
+        crate::visualizers::process_color_slice(colors, ent_path, annotation_infos)
     }
 
     #[inline]