offscreen: Take damage into account when rendering

Does not yet signal the damage outside, but does skip rerendering if there was
no damage.

src/render_helpers/offscreen.rs

@@ -2,15 +2,16 @@ use std::cell::RefCell;
 
 use anyhow::Context as _;
 use smithay::backend::allocator::Fourcc;
+use smithay::backend::renderer::damage::OutputDamageTracker;
 use smithay::backend::renderer::element::utils::{Relocate, RelocateRenderElement};
 use smithay::backend::renderer::element::RenderElement;
 use smithay::backend::renderer::gles::{GlesRenderer, GlesTexture};
 use smithay::backend::renderer::sync::SyncPoint;
-use smithay::backend::renderer::{Bind as _, Offscreen as _, Texture as _};
+use smithay::backend::renderer::{Bind as _, Color32F, Offscreen as _, Texture as _};
 use smithay::utils::{Logical, Point, Scale, Transform};
 
+use super::encompassing_geo;
 use super::texture::TextureBuffer;
-use super::{encompassing_geo, render_elements};
 
 /// Buffer for offscreen rendering.
 #[derive(Debug)]
@@ -18,7 +19,13 @@ pub struct OffscreenBuffer {
     /// The cached texture buffer.
     ///
     /// Lazily created when `render` is called. Recreated when necessary.
-    buffer: RefCell<Option<TextureBuffer<GlesTexture>>>,
+    inner: RefCell<Option<Inner>>,
+}
+
+#[derive(Debug)]
+struct Inner {
+    buffer: TextureBuffer<GlesTexture>,
+    damage: OutputDamageTracker,
 }
 
 impl OffscreenBuffer {
@@ -31,20 +38,20 @@ impl OffscreenBuffer {
         let _span = tracy_client::span!("OffscreenBuffer::render");
 
         let geo = encompassing_geo(scale, elements.iter());
-        let elements = elements.iter().rev().map(|ele| {
+        let elements = Vec::from_iter(elements.iter().map(|ele| {
             RelocateRenderElement::from_element(ele, geo.loc.upscale(-1), Relocate::Relative)
-        });
+        }));
 
         let buffer_size = geo.size.to_logical(1).to_buffer(1, Transform::Normal);
         let offset = geo.loc.to_f64().to_logical(scale);
 
-        let mut buffer = self.buffer.borrow_mut();
+        let mut inner = self.inner.borrow_mut();
 
         // Check if we need to create or recreate the texture.
         let size_string;
         let mut reason = "";
-        if let Some(buf) = buffer.as_mut() {
-            let old_size = buf.texture().size();
+        if let Some(Inner { buffer, .. }) = inner.as_mut() {
+            let old_size = buffer.texture().size();
             if old_size != buffer_size {
                 size_string = format!(
                     "size changed from {} × {} to {} × {}",
@@ -52,18 +59,18 @@ impl OffscreenBuffer {
                 );
                 reason = &size_string;
 
-                *buffer = None;
-            } else if !buf.is_texture_reference_unique() {
+                *inner = None;
+            } else if !buffer.is_texture_reference_unique() {
                 reason = "not unique";
 
-                *buffer = None;
+                *inner = None;
             }
         } else {
             reason = "first render";
         }
 
-        let buffer = if let Some(buffer) = buffer.as_mut() {
-            buffer
+        let inner = if let Some(inner) = inner.as_mut() {
+            inner
         } else {
             trace!("creating new texture: {reason}");
             let span = tracy_client::span!("creating offscreen buffer");
@@ -73,44 +80,52 @@ impl OffscreenBuffer {
                 .create_buffer(Fourcc::Abgr8888, buffer_size)
                 .context("error creating texture")?;
 
-            buffer.insert(TextureBuffer::from_texture(
+            let buffer = TextureBuffer::from_texture(
                 renderer,
                 texture,
                 scale,
                 Transform::Normal,
                 Vec::new(),
-            ))
+            );
+            let damage = OutputDamageTracker::new(geo.size, scale, Transform::Normal);
+
+            inner.insert(Inner { buffer, damage })
         };
 
-        // Update the texture scale.
-        buffer.set_texture_scale(scale);
+        // Recreate the damage tracker if the scale changes. We already recreate it for buffer size
+        // changes, and transform is always Normal.
+        if inner.buffer.texture_scale() != scale {
+            inner.buffer.set_texture_scale(scale);
 
-        // Increment the commit counter since we're rendering new contents to the buffer.
-        buffer.increment_commit_counter();
+            trace!("recreating damage tracker due to scale change");
+            inner.damage = OutputDamageTracker::new(geo.size, scale, Transform::Normal);
+        }
 
-        // Render to the buffer.
-        let mut texture = buffer.texture().clone();
-        let mut target = renderer
-            .bind(&mut texture)
-            .context("error binding texture")?;
+        let res = {
+            let mut texture = inner.buffer.texture().clone();
+            let mut target = renderer.bind(&mut texture)?;
+            inner.damage.render_output(
+                renderer,
+                &mut target,
+                1,
+                &elements,
+                Color32F::TRANSPARENT,
+            )?
+        };
 
-        let sync_point = render_elements(
-            renderer,
-            &mut target,
-            geo.size,
-            scale,
-            Transform::Normal,
-            elements,
-        )?;
+        if res.damage.is_some() {
+            // Increment the commit counter if some contents updated.
+            inner.buffer.increment_commit_counter();
+        }
 
-        Ok((buffer.clone(), sync_point, offset))
+        Ok((inner.buffer.clone(), res.sync, offset))
     }
 }
 
 impl Default for OffscreenBuffer {
     fn default() -> Self {
         OffscreenBuffer {
-            buffer: RefCell::new(None),
+            inner: RefCell::new(None),
         }
     }
 }