Pad Video

How to fit a 16:9 video into a 4:3 frame by sizing and padding the original picture.

Source Video

For a source video we use the MP4 file from the TED talk video What’s the next window into our universe? by Andrew Connolly. The original video format is Wide 480p or 16:9, 854 x 480.

Complete Program

This code takes a 16:9 480p (854x480) video, and squeezes it into a 4:3 640x480 frame, while maintaining the original 16:9 picture aspect ratio. The audio stream is copied from the source as is.

using PrimoSoftware.AVBlocks;
using System.Collections.Generic;

namespace PadVideo
{
    class Program
    {
        static void Main(string[] args)
        {
            Library.Initialize();

            // start with two identical input and output configurations
            var inputInfo = new MediaInfo() {
                InputFile = "AndrewConnolly_2014.mp4"
            };

            var outputInfo = new MediaInfo() {
                InputFile = "AndrewConnolly_2014.mp4"
            };

            if (inputInfo.Load() && outputInfo.Load()) {
                
                // create input socket
                var inputSocket = MediaSocket.FromMediaInfo(inputInfo);

                // create output socket
                var outputSocket = MediaSocket.FromMediaInfo(outputInfo);
                outputSocket.File = "AndrewConnolly_2014_640x480_Padded.mp4";

                // set the frame width and height, and the display ratio
                var outVideoStream = outputSocket.Pins[0].StreamInfo as VideoStreamInfo;

                // set the new frame width and height to 640 x 480
                outVideoStream.FrameWidth = 640;
                outVideoStream.FrameHeight = 480;

                // set the display ratio to 4:3
                outVideoStream.DisplayRatioWidth = 4;
                outVideoStream.DisplayRatioHeight = 3;

                // set Pad.Top and Pad.Bottom
                var outVideoPin = outputSocket.Pins[0];
                outVideoPin.Params = new Dictionary<string, object>() {
                    // The input video is 16:9, 854 x 480,  
                    // to squeeze it in 640 x 480 and keep the 16:9 display ratio, 
                    // the height has to be: 640 * 9 / 16
                    
                    // Pad by (480 - 640 * 9 / 16) / 2 pixels on each side.
                    { Param.Video.Pad.Top,    (480 - 640 * 9 / 16) / 2 },
                    { Param.Video.Pad.Bottom, (480 - 640 * 9 / 16) / 2 }, 

                    // The padding will trigger downscaling, 
					// so we also set the interpolation method,
                    // for best result when downscaling, use InterpolationMethod.Super
                    { Param.Video.Resize.InterpolationMethod, InterpolationMethod.Super }
                };

                // create a transcoder and run it
                using (var transcoder = new Transcoder()) {
                    // set inputs and outputs
                    transcoder.Inputs.Add(inputSocket);
                    transcoder.Outputs.Add(outputSocket);

                    // process
                    if (transcoder.Open())
                    {
                        transcoder.Run();
                        transcoder.Close();
                    }
                }
            }

            Library.Shutdown();
        }
    }
}

How to run

Follow the steps to create a C# console application in Visual Studio, but in Program.cs use the code from this article.

Download the AndrewConnolly_2014.mp4 MPEG4 file from the Internet Archive and save it in bin/x64/Debug/net6.0 under the project’s directory.

Run the application in Visual Studio. Wait a few seconds for the Transcoder to finish. The converted file AndrewConnolly_2014_640x480_Padded.mp4 will be in the bin/x64/Debug/net6.0 directory.