rt(mtl): fix history buffer off-by-one by buffering the history read.

librashader-runtime-mtl/src/filter_chain.rs

@@ -66,6 +66,14 @@ pub struct FilterChainMetal {
     output_framebuffers: Box<[OwnedTexture]>,
     feedback_framebuffers: Box<[OwnedTexture]>,
     history_framebuffers: VecDeque<OwnedTexture>,
+    /// Metal does not allow us to push the input texture to history
+    /// before recording framebuffers, so we double-buffer it.
+    ///
+    /// First we swap OriginalHistory1 with the contents of this buffer (which were written to
+    /// in the previous frame)
+    ///
+    /// Then we blit the original to the buffer.
+    prev_frame_history_buffer: OwnedTexture,
     disable_mipmaps: bool,
     default_options: FrameOptionsMetal,
 }
@@ -206,7 +214,18 @@ impl FilterChainMetal {
         cmd: &ProtocolObject<dyn MTLCommandBuffer>,
         input: &ProtocolObject<dyn MTLTexture>,
     ) -> error::Result<()> {
-        if let Some(mut back) = self.history_framebuffers.pop_back() {
+        // If there's no history, there's no need to do any of this.
+        let Some(mut back) = self.history_framebuffers.pop_back() else {
+            return Ok(());
+        };
+
+        // Push the previous frame as OriginalHistory1
+        std::mem::swap(&mut back, &mut self.prev_frame_history_buffer);
+        self.history_framebuffers.push_front(back);
+
+        // Copy the current frame into prev_frame_history_buffer, which will be
+        // pushed to OriginalHistory1 in the next frame.
+        let back = &mut self.prev_frame_history_buffer;
         let mipmapper = cmd
             .blitCommandEncoder()
             .ok_or(FilterChainError::FailedToCreateCommandBuffer)?;
@@ -220,15 +239,13 @@ impl FilterChainMetal {
             };
 
             let _old_back = std::mem::replace(
-                    &mut back,
+                back,
                 OwnedTexture::new(&self.common.device, size, 1, input.pixelFormat())?,
             );
         }
 
         back.copy_from(&mipmapper, input)?;
         mipmapper.endEncoding();
-            self.history_framebuffers.push_front(back);
-        }
         Ok(())
     }
 
@@ -290,6 +307,8 @@ impl FilterChainMetal {
         // initialize history
         let (history_framebuffers, history_textures) = framebuffer_init.init_history()?;
 
+        let history_buffer = framebuffer_gen()?;
+
         let draw_quad = DrawQuad::new(&device)?;
         Ok(FilterChainMetal {
             common: FilterCommon {
@@ -306,6 +325,7 @@ impl FilterChainMetal {
             output_framebuffers,
             feedback_framebuffers,
             history_framebuffers,
+            prev_frame_history_buffer: history_buffer,
             disable_mipmaps: options.map(|f| f.force_no_mipmaps).unwrap_or(false),
             default_options: Default::default(),
         })