Fix FlashVSR quality: match naxci1 reference preprocessing

- Remove front dummy frames (not used by reference implementation)
- Use centered reflect padding instead of right/bottom replicate
- Crop output from center matching padding offsets
- Simplify temporal padding to 8k+1 alignment
- Update progress bar estimation to match new formula

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

inference.py

@@ -727,73 +727,70 @@ class FlashVSRModel:
         return sw, sh, tw, th
 
     @staticmethod
-    def _align_frames(n):
-        """Round n to the form 8k+1 (pipeline requirement)."""
-        return 0 if n < 1 else ((n - 1) // 8) * 8 + 1
-
-    @staticmethod
-    def _restore_video_sequence(result, original_count):
-        """Strip 2 front warmup frames and trim to original count."""
-        # Strip the first 2 frames (pipeline warmup from front dummy padding)
-        if result.shape[0] > 2:
-            result = result[2:]
-        # Trim or pad to exact expected count
-        if result.shape[0] > original_count:
-            result = result[:original_count]
-        elif result.shape[0] < original_count:
-            pad = result[-1:].expand(original_count - result.shape[0], *result.shape[1:])
+    def _restore_video_sequence(result, expected):
+        """Trim pipeline output to the expected frame count."""
+        if result.shape[0] > expected:
+            result = result[:expected]
+        elif result.shape[0] < expected:
+            pad = result[-1:].expand(expected - result.shape[0], *result.shape[1:])
             result = torch.cat([result, pad], dim=0)
         return result
 
     def _prepare_video(self, frames, scale):
-        """Convert [F, H, W, C] [0,1] frames to padded [1, C, F_aligned, H, W] [-1,1].
+        """Convert [F, H, W, C] [0,1] frames to padded [1, C, F_padded, H, W] [-1,1].
 
-        Matches the 1038lab reference preprocessing:
-        1. Add 2 tail copies of last frame
-        2. Add 2 front dummy copies of first frame + back padding to align_frames(N+4)
-        3. Bicubic-upscale each frame to target resolution
-        4. Normalize to [-1, 1]
+        Matches naxci1/ComfyUI-FlashVSR_Stable preprocessing:
+        1. Bicubic-upscale each frame to target resolution
+        2. Centered symmetric padding to 128-pixel alignment (reflect mode)
+        3. Normalize to [-1, 1]
+        4. Temporal padding: repeat last frame to reach 8k+1 count
 
-        The 2 front dummies are required by the pipeline's LQ conditioning
-        indexing. The pipeline outputs corresponding warmup frames that are
-        stripped by _restore_video_sequence.
+        No front dummy frames — the pipeline handles LQ indexing correctly
+        starting from frame 0.
+
+        Returns:
+            video: [1, C, F_padded, H, W] tensor
+            th, tw: padded spatial dimensions
+            nf: padded frame count
+            sh, sw: actual (unpadded) spatial dimensions
+            pad_top, pad_left: spatial padding offsets for output cropping
         """
         N, H, W, C = frames.shape
         sw, sh, tw, th = self._compute_dims(W, H, scale)
 
-        # Pad sequence: 2 tail copies + alignment (matches _pad_video_sequence)
-        padded_n = N + 2
-        added_alignment = 0
-        remainder = (padded_n - 5) % 8
-        if remainder != 0:
-            added_alignment = 8 - remainder
-            padded_n += added_alignment
-
-        # Total with 2 front dummies + back padding (matches prepare_video)
-        aligned = self._align_frames(padded_n + 4)
+        # Centered spatial padding offsets
+        pad_top = (th - sh) // 2
+        pad_bottom = th - sh - pad_top
+        pad_left = (tw - sw) // 2
+        pad_right = tw - sw - pad_left
 
         processed = []
-        for i in range(aligned):
-            if i < 2:
-                idx = 0              # 2 front dummy frames (copy of first)
-            elif i > padded_n + 1:
-                idx = N - 1          # back padding (copy of last)
-            else:
-                src = i - 2          # shifted by 2 for the front dummies
-                idx = min(src, N - 1)  # clamp to original range
-
-            frame = frames[idx].permute(2, 0, 1).unsqueeze(0)  # [1, C, H, W]
+        for i in range(N):
+            frame = frames[i].permute(2, 0, 1).unsqueeze(0)  # [1, C, H, W]
             upscaled = F.interpolate(frame, size=(sh, sw), mode='bicubic', align_corners=False)
-            pad_h, pad_w = th - sh, tw - sw
-            if pad_h > 0 or pad_w > 0:
-                upscaled = F.pad(upscaled, (0, pad_w, 0, pad_h), mode='replicate')
+            if pad_top > 0 or pad_bottom > 0 or pad_left > 0 or pad_right > 0:
+                # Centered reflect padding (matches naxci1 reference)
+                try:
+                    upscaled = F.pad(upscaled, (pad_left, pad_right, pad_top, pad_bottom), mode='reflect')
+                except RuntimeError:
+                    # Reflect requires pad < input size; fall back to replicate
+                    upscaled = F.pad(upscaled, (pad_left, pad_right, pad_top, pad_bottom), mode='replicate')
             normalized = upscaled * 2.0 - 1.0
             processed.append(normalized.squeeze(0).cpu().to(self.dtype))
 
         video = torch.stack(processed, 0).permute(1, 0, 2, 3).unsqueeze(0)
-        nf = video.shape[2]  # = aligned
 
-        return video, th, tw, nf, sh, sw, padded_n
+        # Temporal padding: repeat last frame to reach 8k+1 (pipeline requirement)
+        target = max(N, 25)  # minimum 25 for streaming loop (P >= 1)
+        remainder = (target - 1) % 8
+        if remainder != 0:
+            target += 8 - remainder
+        if target > N:
+            pad = video[:, :, -1:].repeat(1, 1, target - N, 1, 1)
+            video = torch.cat([video, pad], dim=2)
+        nf = video.shape[2]
+
+        return video, th, tw, nf, sh, sw, pad_top, pad_left
 
     @staticmethod
     def _to_frames(video):
@@ -836,8 +833,8 @@ class FlashVSRModel:
 
         original_count = frames.shape[0]
 
-        # Prepare video tensor (bicubic upscale + pad with 2 front dummies)
-        video, th, tw, nf, sh, sw, padded_n = self._prepare_video(frames, scale)
+        # Prepare video tensor (bicubic upscale + centered pad)
+        video, th, tw, nf, sh, sw, pad_top, pad_left = self._prepare_video(frames, scale)
 
         # Move LQ video to compute device (except for "long" mode which streams)
         if "long" not in self.pipe.__class__.__name__.lower():
@@ -857,10 +854,11 @@ class FlashVSRModel:
             color_fix=color_fix, unload_dit=unload_dit,
         )
 
-        # Convert to ComfyUI format: crop to padded_n frames + spatial padding
-        result = self._to_frames(out).cpu()[:padded_n, :sh, :sw, :]
+        # Convert to ComfyUI format with centered spatial crop
+        result = self._to_frames(out).cpu()
+        result = result[:, pad_top:pad_top + sh, pad_left:pad_left + sw, :]
 
-        # Strip 2 front warmup frames and trim to original count
+        # Trim to original frame count
         result = self._restore_video_sequence(result, original_count)
 
         return result

nodes.py

@@ -1731,16 +1731,15 @@ class FlashVSRUpscale:
                 chunks.append((prev_start, last_end))
 
         # Estimate total pipeline steps for progress bar
-        # Mirrors _prepare_video: N+2 tail, align (n-5)%8, then align_frames(padded+4)
+        # Mirrors _prepare_video: target = max(N, 25), round up to 8k+1
         total_steps = 0
         for cs, ce in chunks:
             n = ce - cs
-            padded = n + 2
-            remainder = (padded - 5) % 8
+            target = max(n, 25)
+            remainder = (target - 1) % 8
             if remainder != 0:
-                padded += 8 - remainder
-            aligned = FlashVSRModel._align_frames(padded + 4)
-            total_steps += max(1, (aligned - 1) // 8 - 2)
+                target += 8 - remainder
+            total_steps += max(1, (target - 1) // 8 - 2)
 
         pbar = ProgressBar(total_steps)
         step_ref = [0]