Pad Video

This article explains how you can 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.

Code

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.

Windows

#include <primo/avblocks/avb.h>
#include <primo/platform/reference++.h>

// link with AVBlocks64.lib
#pragma comment(lib, "./avblocks/lib/x64/AVBlocks64.lib")

using namespace primo::codecs;
using namespace primo::avblocks;

int main(int argc, const char * argv[]) {
    Library::initialize();

    // start with two identical input and output configurations
    ref<MediaInfo> inputInfo (Library::createMediaInfo());
    inputInfo->setInputFile(L"AndrewConnolly_2014.mp4");

    ref<MediaInfo> outputInfo (Library::createMediaInfo());
    outputInfo->setInputFile(L"AndrewConnolly_2014.mp4");

    if (inputInfo->load() && outputInfo->load()) {
        using namespace Param::Video;

        // create input socket
        ref<MediaSocket> inputSocket (Library::createMediaSocket(inputInfo.get()));

        // create output socket
        ref<MediaSocket> outputSocket (Library::createMediaSocket(outputInfo.get()));
        outputSocket->setFile(L"AndrewConnolly_2014_640x480_Padded.mp4");

        // get output video pin
        auto outVideoPin = outputSocket->pins()->at(0);

        auto outVideoStream = static_cast<VideoStreamInfo*>(outVideoPin->streamInfo()); 
        
        // set the new frame width and height to 640 x 480
        outVideoStream->setFrameWidth(640);
        outVideoStream->setFrameHeight(480);

        // set the display ratio to 4:3
        outVideoStream->setDisplayRatioWidth(4);
        outVideoStream->setDisplayRatioHeight(3);

        // set Pad.Top and Pad.Bottom
        ref<ParameterList> outVidePinsParams (Library::createParameterList()); 

        // 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.
        ref<IntParameter> padTop (Library::createIntParameter());
        padTop->setName(Pad::Top);
        padTop->setValue((480 - 640 * 9 / 16) / 2);

        outVidePinsParams->add(padTop.get());

        ref<IntParameter> padBotttom (Library::createIntParameter());
        padBotttom->setName(Pad::Bottom);
        padBotttom->setValue((480 - 640 * 9 / 16) / 2);

        outVidePinsParams->add(padBotttom.get());

        // The padding will trigger downscaling, 
        // so we also set the interpolation method,
        // for best result when downscaling, use InterpolationMethod::Super
        ref<IntParameter> interpolationMethod (Library::createIntParameter()); 
        interpolationMethod->setName(Resize::InterpolationMethod);
        interpolationMethod->setValue(InterpolationMethod::Super);

        outVidePinsParams->add(interpolationMethod.get());

        outVideoPin->setParams(outVidePinsParams.get());

        // create a Transcoder
        ref<Transcoder> transcoder (Library::createTranscoder());

        // add input and output sockets
        transcoder->inputs()->add(inputSocket.get());
        transcoder->outputs()->add(outputSocket.get());

        // process
        if (transcoder->open()) {
            transcoder->run();
            transcoder->close();
        }
    }

    Library::shutdown();

    return 0;
}

How to run

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

Download the AndrewConnolly_2014.mp4 MPEG4 file from the Internet Archive and save it in the project 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 project directory.