还原共享抽帧像素格式并在 BgFilter 消费点合成 alpha

撤掉 extract_video_frames_to_png 里无条件的 -pix_fmt rgb24，共享抽帧不再携带 BgFilter 专属策略
prepare_editor_character_animation_bgfilter_input 解码后先按白底合成 alpha 再转 RGB，补上对应回归测试

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server-rs/crates/api-server/src/character_animation_assets.rs

@@ -4334,8 +4334,6 @@ fn extract_video_frames_to_png(
             format!("[f{frame_index}]"),
             "-frames:v".to_string(),
             "1".to_string(),
-            "-pix_fmt".to_string(),
-            "rgb24".to_string(),
             "-f".to_string(),
             "image2".to_string(),
             output_path.to_string_lossy().into_owned(),
@@ -4471,6 +4469,25 @@ fn run_process_with_timeout(
     }
 }
 
+/// BgFilter 只接受不透明 RGB 输入；直接丢弃 alpha 会让全透明像素下未定义的
+/// RGB 值以杂色进入抠图，这里先按白底合成再转 RGB。
+fn flatten_alpha_onto_white_rgb(image: image::DynamicImage) -> image::RgbImage {
+    if !image.color().has_alpha() {
+        return image.to_rgb8();
+    }
+    let rgba = image.to_rgba8();
+    let mut flattened = image::RgbImage::new(rgba.width(), rgba.height());
+    for (source, target) in rgba.pixels().zip(flattened.pixels_mut()) {
+        let [red, green, blue, alpha] = source.0;
+        let opacity = f32::from(alpha) / 255.0;
+        let blend = |channel: u8| -> u8 {
+            (f32::from(channel) * opacity + 255.0 * (1.0 - opacity)).round() as u8
+        };
+        target.0 = [blend(red), blend(green), blend(blue)];
+    }
+    flattened
+}
+
 fn prepare_editor_character_animation_bgfilter_input(
     source: &[u8],
     mime_type: &str,
@@ -4486,8 +4503,8 @@ fn prepare_editor_character_animation_bgfilter_input(
             "provider": "character-animation",
             "message": format!("解析 BgFilter 输入动作帧图片失败：{error}"),
         }))
-    })?
-    .to_rgb8();
+    })?;
+    let image = flatten_alpha_onto_white_rgb(image);
 
     let (draw_width, draw_height) =
         compute_contain_dimensions(image.width(), image.height(), target_width, target_height);
@@ -6478,6 +6495,38 @@ mod tests {
         );
     }
 
+    #[test]
+    fn editor_character_animation_bgfilter_input_flattens_alpha_onto_white() {
+        let mut source = RgbaImage::from_pixel(8, 8, Rgba([17, 99, 201, 255]));
+        source.put_pixel(0, 0, Rgba([255, 0, 0, 0]));
+        source.put_pixel(1, 0, Rgba([0, 0, 0, 127]));
+        let mut source_png = Vec::new();
+        PngEncoder::new(&mut source_png)
+            .write_image(
+                source.as_raw(),
+                source.width(),
+                source.height(),
+                ColorType::Rgba8.into(),
+            )
+            .expect("source RGBA frame should encode");
+
+        let prepared = prepare_editor_character_animation_bgfilter_input(
+            source_png.as_slice(),
+            "image/png",
+            8,
+            8,
+        )
+        .expect("BgFilter input should be prepared");
+        let decoded =
+            image::load_from_memory_with_format(prepared.bytes.as_slice(), ImageFormat::Png)
+                .expect("prepared BgFilter input should decode")
+                .to_rgb8();
+
+        assert_eq!(decoded.get_pixel(0, 0).0, [255, 255, 255]);
+        assert_eq!(decoded.get_pixel(1, 0).0, [128, 128, 128]);
+        assert_eq!(decoded.get_pixel(2, 0).0, [17, 99, 201]);
+    }
+
     #[test]
     fn editor_character_animation_final_frame_adds_transparent_vertical_padding() {
         let source = RgbaImage::from_pixel(323, 434, Rgba([31, 127, 223, 191]));