I’ve run into similar player compatibility headaches before—KMPlayer v3 does have some quirky behavior when parsing metadata from FFmpeg-generated MP4s, especially around aspect ratios. The fact that HandBrake’s output works fine confirms it’s an issue with how FFmpeg is writing metadata, not the player itself. Here are targeted fixes to resolve the stretching:

• Force explicit Display Aspect Ratio (DAR)
  Sometimes just setting scale and setsar isn’t enough—KMPlayer might prioritize the DAR metadata over the sample aspect ratio. Add setdar to your filter chain, or set it directly to match your source video’s aspect ratio:

  cmd = "-i "+in+" -vf scale=480:-2,setsar=1:1,setdar=16:9 "+out;
  # Replace 16:9 with your source's actual aspect ratio (e.g., 4:3)
  

  This explicitly tells the player exactly how the frame should be displayed, leaving no room for misinterpretation.

• Use libx264 encoder options for consistent metadata
  HandBrake uses optimized encoder settings that ensure metadata is properly embedded. You can replicate this by passing explicit aspect ratio parameters directly to the libx264 encoder:

  cmd = "-i "+in+" -vf scale=480:-2,setsar=1:1 -c:v libx264 -profile:v main -level:v 3.1 -x264opts \"sar=1/1:dar=16/9\" "+out;
  

  This bypasses any potential mismatches between the filter chain and encoder metadata handling.

• Validate and fix container metadata
  Sometimes the MP4 container’s aspect ratio metadata doesn’t match the video stream’s, causing KMPlayer to misread it. First, use ffprobe to check your input and output metadata:

  ffprobe -v error -select_streams v:0 -show_entries stream=width,height,sample_aspect_ratio,display_aspect_ratio -of default=noprint_wrappers=1:nokey=1 input.mp4
  

  If you see discrepancies, re-wrap the video to force correct metadata in the container:

  ffmpeg -i input_ffmpeg.mp4 -c copy -bsf:v h264_metadata=sample_aspect_ratio=1/1:display_aspect_ratio=16/9 fixed_output.mp4
  

• Mirror HandBrake’s FFmpeg parameters
  HandBrake is just a frontend for FFmpeg—enable its log output (in settings) to see the exact FFmpeg command it uses. Copy those parameters into your workflow; HandBrake goes out of its way to ensure all metadata (aspect ratio, rotation, etc.) is correctly formatted for broad player compatibility.

These steps should resolve the stretching in KMPlayer v3. The key is making sure both the video stream and container have consistent, explicit aspect ratio metadata that the player can parse reliably.

内容的提问来源于stack exchange，提问作者Mehran