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fix(render): preserve alpha on .webm overlays by forcing libvpx-vp9 decoder #59

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fix(render): preserve alpha on .webm overlays by forcing libvpx-vp9 decoder #59

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9 changes: 8 additions & 1 deletion helpers/render.py
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Original file line number Diff line number Diff line change
Expand Up @@ -515,7 +515,14 @@ def build_final_composite(
inputs: list[str] = ["-i", str(base_path)]
for ov in overlays:
ov_path = resolve_path(ov["file"], edit_dir)
inputs += ["-i", str(ov_path)]
# VP9/WebM overlays carry alpha as a side-data plane. ffmpeg's native vp9
# decoder drops it (decodes to yuv420p → opaque), which turns a full-frame
# transparent overlay into an opaque black layer. Force the alpha-aware
# libvpx-vp9 decoder so overlay alpha is honored.
if ov_path.suffix.lower() == ".webm":

@cubic-dev-ai cubic-dev-ai Bot Jun 6, 2026 •
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P1: Extension-based decoder selection for .webm files is incorrect — WebM is a container that can hold VP8, VP9, or AV1 video. Forcing libvpx-vp9 (a VP9-only decoder) for all .webm files will cause ffmpeg decode failure and abort the render when the overlay uses VP8 or AV1.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At helpers/render.py, line 522:

<comment>Extension-based decoder selection for .webm files is incorrect — WebM is a container that can hold VP8, VP9, or AV1 video. Forcing `libvpx-vp9` (a VP9-only decoder) for all `.webm` files will cause ffmpeg decode failure and abort the render when the overlay uses VP8 or AV1.</comment>

<file context>
@@ -515,7 +515,14 @@ def build_final_composite(
+        # decoder drops it (decodes to yuv420p → opaque), which turns a full-frame
+        # transparent overlay into an opaque black layer. Force the alpha-aware
+        # libvpx-vp9 decoder so overlay alpha is honored.
+        if ov_path.suffix.lower() == ".webm":
+            inputs += ["-c:v", "libvpx-vp9", "-i", str(ov_path)]
+        else:
</file context>
Fix with cubic

inputs += ["-c:v", "libvpx-vp9", "-i", str(ov_path)]
else:
inputs += ["-i", str(ov_path)]

filter_parts: list[str] = []
# PTS-shift every overlay so its frame 0 lands at start_in_output
Expand Down